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Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Medicine 1111
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Perinatal Risk Factors forChildhood Leukemia
BY
ESTELLE NAUMBURG
ACTA UNIVERSITATIS UPSALIENSISUPPSALA 2002
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Dissertation for the Degree of Doctor of Philosophy, Faculty of Medicine in Pediatrics presented at Uppsala University in 2002
ABSTRACT Naumburg, E. 2002. Perinatal Risk Factors for Childhood Leukemia. Acta Universitatis Upsaliensis Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine 1111. 44 pp. Uppsala. ISBN 91 –554 –5205-1 The aim of the studies described in this thesis was to assess the association between certain perinatal factors and the risk of childhood lymphatic and myeloid leukemia and infant leukemia.
The five studies presented were all conducted in Sweden as population-based case-control studies. All cases were born and diagnosed between 1973-89 with leukemia up to the age of 16 years. A control was individually matched to each case. As Down’s syndrome entails a major risk for childhood leukemia, children with Down’s syndrome were excluded. The studies comprised a total of 652 cases, 47 of whom were diagnosed before the age of one year. Exposure data were extracted blindly from antenatal, obstetric, pediatric and other standardized medical records.
No association was found between prenatal exposure to ultrasound or diagnostic x-ray and childhood lymphatic or myeloid leukemia. Infant leukemia was associated with prenatal exposure to x-ray. A history of maternal lower genital tract infection significantly increased the risk of childhood leukemia, especially among children diagnosed at four years or older or in infancy. Factors such as young maternal age, and mothers working with children or in the health sector were associated with infant leukemia. Resuscitation with 100% oxygen with a face-mask and bag directly postpartum was associated with an increased risk of childhood lymphatic leukemia. The oxygen-related risk further increased if the manual ventilation lasted for three minutes or more. There was no association between lymphatic or infant leukemia and supplementary oxygen later in the neonatal period or other birth-related factors. Low Apgar scores at one and five minutes were associated with a non-significantly increased risk of lymphatic leukemia, and were significantly associated with infant leukemia.
Previously reported relations between childhood leukemia and exposures such as maternal diagnostic x-ray and birth related factors could not be confirmed by these studies. However, the present studies indicate that events during pregnancy or during the neonatal period are associated with increased risks of childhood and infant leukemia. These events can either be non-specific, such as exposure to maternal lower genital tract infection, or specific, such as the use of supplementary oxygen directly postpartum. Key words: Childhood leukemia, infant leukemia, leukemia epidemiology, supplementary oxygen, perinatal risk factors, perinatal infection, prenatal ultrasound, prenatal x-ray. Estelle Naumburg, Department of Woman’s and Children’s Health, Section for Pediatrics, Uppsala University Children’s Hospital, SE-751 85 Uppsala, Sweden © Estelle Naumburg 2002 ISSN 0282-7476 ISBN 91-554-5205-1 Printed in Sweden by Uppsala University, Tryck&Medier, Uppsala 2002
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”There is a theory which states that if ever anyone discovers exactly what the universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.” (Douglas Adams, The restaurant at the end of the universe) To Jan, Karl Adam, Ylva and Camilla
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List of publications The present work is based on the following papers, which will be referred to by their Roman numerals.
I. Prenatal Ultrasound Examinations and Risk of Childhood Leukaemia. Naumburg, E., Bellocco, R., Cnattingius, S., Hall, P., Ekbom, A. BMJ 2000; 320:282-283.
II. Intrauterine Exposure to Diagnostic X-ray and Risk of Childhood
Leukemia Subtype. Naumburg, E., Bellocco, R., Cnattingius, S., Hall, P., Boice JR, J. D., Ekbom, A. Radiation Research, 2002; 157:1.
III. Perinatal exposure to infection and risk of childhood leukemia.
Naumburg, E., Bellocco, R., Cnattingius, S., Jonzon, A., Ekbom, A. Medical and Pediatric Oncology, in press.
IV. Supplementary oxygen and risk of childhood leukemia.
Naumburg, E., Bellocco, R., Cnattingius, S., Jonzon, A., Ekbom, A. Submitted manuscript.
V. Characteristics of infants with leukemia. A Swedish case-control
population-based study. Naumburg, E., Bellocco, R., Cnattingius, S., Jonzon, A., Ekbom, A. Submitted manuscript.
Reprints were made with permission by the publishers
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Contents Introduction ........................................................................................................................ 7
General introduction....................................................................................................... 7 Leukemia .................................................................................................................... 7 The developing immune system of the fetus............................................................... 8
Environmental risk factors.............................................................................................. 9 Ultrasound .................................................................................................................. 9 Diagnostic X-ray......................................................................................................... 9 Infections .................................................................................................................. 10 Oxygen ..................................................................................................................... 11 Environmental risk factors and infant leukemia........................................................ 11
Objectives......................................................................................................................... 13 Material and Methods....................................................................................................... 14
National registration number ........................................................................................ 14 Data Registries sources................................................................................................. 14
The Swedish Cancer Registry................................................................................... 14 The Swedish Medical Birth Registry........................................................................ 14 The Registry of Causes of Death .............................................................................. 14 Hospital archives ...................................................................................................... 15
Subjects and methods ................................................................................................... 15 Selection of cases ..................................................................................................... 15 Selection of controls ................................................................................................. 15 The total study group................................................................................................ 15 Exposure data ........................................................................................................... 17
Analysis ........................................................................................................................ 22 Statistical methods.................................................................................................... 22
Results .............................................................................................................................. 23 Prenatal Ultrasound Examinations and Risk of Childhood Leukemia (I) ..................... 23 Intrauterine Exposure to Diagnostic X-ray and Risk of Childhood Leukemia (II+V) .. 23 Exposure to infection and risk of childhood and infant leukemia (III+V) .................... 24 Exposure to supplementary oxygen and risk of childhood lymphatic and infant leukemia (IV+V).......................................................................................................................... 25 Perinatal characteristics related to infant leukemia (V) ................................................ 27
General Discussion........................................................................................................... 29 Methodological considerations, strengths and weaknesses of the studies..................... 29
Inclusions and size of study...................................................................................... 29 Retrieval and specificity of exposure information .................................................... 30 Potential confounders ............................................................................................... 31 Potential biases ......................................................................................................... 31
Findings and Implications............................................................................................. 31 Age at diagnosis........................................................................................................ 33 Clinical implications................................................................................................. 34 Implications for the future ........................................................................................ 35
Conclusions ...................................................................................................................... 36 Acknowledgements........................................................................................................... 37 References ........................................................................................................................ 39
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Abbreviations ALL acute lymphatic leukemia AML acute myeloid leukemia CML chronic myeloid leukemia ROP retinopathy of prematurity BPD bronchopulmonary dysplasia ICD International Classification of Diseases SGA small for gestational age SD standard deviation AGA appropriate for gestational age LGA large for gestational age OR odds ratio CI confidence interval LGTI lower genital tract infection
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Introduction General introduction Hematological malignancies, such as leukemia, arise when a transformation occurs in a cell from the hematopoietic or lymphoid tissue and this cell divides without control. Recent advances in the treatment of childhood leukemia have not been matched by similar advances in the understanding of its etiology. Childhood leukemia is a very heterogeneous disease, both clinically and regarding its cellular origins and molecular pathogenesis (1), and the underlying causes of the disease may therefore be complex. The age-specific incidence curve has a distinctive shape, with a marked peak at two to four years of age (2-6), indicating that exposures in utero and/or early in life may be important determinants of the disease. The aim of the present studies was to assess the impact of certain perinatal exposures on the risk for later development of infant or childhood leukemia.
Leukemia
History Leukemia has been recognized since 1845, when reports were published on a patient who had died of a disease whith “purulent matter in the blood” and another deceased patient who had shown an increased number of colorless cells in the blood (7, 8). Twenty years later it was suggested that the bone marrow was the site of hematopoiesis (9), and the diagnosis of leukemia became possible through bone marrow puncture.
Anti-leukemia treatment with anti-metabolite therapy was initiated in 1948 (10), and escalating doses have improved the survival rates from nil to 50-80 % of today (2, 11). The knowledge in both the treatment strategies and the classification of leukemia has grown rapidly over the last decades. However, the etiological mechanisms of the disease still remain unknown.
Leukemia in Children Malignant diseases in childhood are rare and account for approximately 1-2% of all human cancers (12). In Sweden the incidence of childhood cancer is 250/100,000 and approximately one out of 450 children will develop a malignant disease before the age of fifteen. Leukemia is the major pediatric malignancy, and accounts for about one third of all neoplasms in childhood (2-4, 13). The estimated incidence of childhood leukemia in Sweden is 4.4/100,000 children per year (0-15 years at diagnosis), with a male: female ratio of 1:13 (2-4). The incidence of childhood leukemia increased in developed countries during the first part of the 20th century, but in the last 20 years of the century the annual
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incidence leveled off. The incidence peak of childhood leukemia occurs between the ages of two and four years.
Leukemia subtypes Acute lymphatic leukemia (ALL) is the most common type of leukemia and accounts for 75-80% of all cases of childhood leukemia. The yearly incidence in Nordic countries between 1985 and 1994 was 3.9/100,000 children, which meant that 75 children in Sweden were affected by the disease every year. In the 1960s the children survived three to four months, whereas today total remission is achieved in more than 90% of the cases and long-term disease-free survival is approximately 70% (2).
Acute myeloid leukemia (AML) is the second major type, and accounts for 15-20% of all leukemia cases in childhood. The annual incidence of AML in the Nordic countries is 0.7/100,000. Among children with Down’s syndrome the risk of developing leukemia is increased 10- to 30-fold and these children account for 10% of all cases of myeloid leukemia in Sweden (2). The remission rate of AML in recent years has been 92%, children with Down’s syndrome included, but among otherwise normal children the 5-year survival is less than 50% (14).
Other forms of leukemia represent less than 5% of all leukemia cases, and the most common type is chronic myeloid leukemia (CML).
Infant leukemia Infants less than one year of age at diagnosis account for only 3% of all cases of childhood leukemia in the western world (2, 15), and the disease in infancy is clinically and biologically different from childhood leukemia at older ages (16). AML is the predominating type of leukemia in infancy (17). A specific rearrangement of chromosome 11 is present in nearly 80% of infants with leukemia (16) and the survival rate in infant leukemia is poor, but improves with increasing age at diagnosis of the infant (18).
The developing immune system of the fetus The human immunological response is a complex humoral, cellular, and enzymatic system in which the interaction between the organ and the environment is essential. The immunological development of the embryo starts early in pregnancy with an immunologically tolerant environment and an embryo-maternal interaction (19). Exposures in utero and in early life are part of the immunological programming of the fetal immune system. Microorganisms in the amniotic cavity or maternal compartment may reach the human fetus and stimulate the biosynthesis of proinflammatory cytokines (20). The fetus will thus develop an acute-phase response (21), similar to that observed in adult patients with systemic inflammatory response syndrome (22). The timing and pattern of this programming is important. Early exposures, by maternal immune protection through transplacental immunoglobins, dose limitation, and breast milk, will
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modulate and prime the immunological response of the infant for future exposures. Thus, a biological “norm” for many common or endemic infections is encountered around birth (23). However, in several respects the immune system of the newborn infant is not fully adapted for postnatal life (24). Many potential immunological mechanisms, for example cytokine activity, may induce proliferative stress in lymphoid stem cells in the bone marrow, of a sufficient degree to convert normal cells to leukemia via mutations (23). Environmental risk factors Acute leukemia, like other cancers, is a clonal disorder driven by mutations. However, only a small proportion of the cases are associated with inherited predisposing genetic syndromes (such as Down’s syndrome) (23). Decades of research have been focused on numerous environmental exposures during pregnancy and their possible link to childhood leukemia, including pesticides (25-27) and traffic air pollution (28, 29). Perinatal and birth-related factors (30-36) and endogenous factors (37) have also been investigated as being possibly associated with childhood leukemia. At present, the only established risk factors of leukemia are ionizing radiation, chemotherapy and chromosomal anomalies, and these account for about 3-5% of all cases (38, 39).
Ultrasound Obstetric ultrasound is today an integral part of antenatal care and is performed routinely in all pregnancies. Over the last two decades higher amplitude pulses, higher power and higher time-average intensities of ultrasonic waves have been introduced (40). Low doses of therapeutic ultrasound have been shown to cause significant membrane changes in vitro that may have undesirable side effects on embryogenesis and later on prenatal and postnatal development (41). Epidemiological studies have focused on different outcomes in order to assess the risks following exposure to ultrasound. The observed association between exposure to ultrasound and an increased incidence of non-right-handedness (42) indicates that the fetus may be sensitive to the energies used in ultrasound. Previous studies have not revealed any association between ultrasound exposure in utero and childhood malignancy (33, 43-47). However, some of these studies have been hampered by low statistical power and/or based on interviews with the parents retrospectively.
Diagnostic X-ray Ionizing radiation is a known risk factor for leukemia at all ages (48, 49), but the existence of a causal relation between prenatal exposure to diagnostic x-ray and childhood leukemia still remains controversial (50-52). Radiation-related excess occurrence of leukemia has not been observed among atomic bomb survivors exposed in utero (53-55), or among children exposed to electromagnetic fields (56, 57) or living in areas around nuclear power plants (58), or after the Chernobyl
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reactor accident(59). As early as in 1956 Stewart et al (60) claimed that prenatal radiation was associated with a subsequently increased risk of leukemia and solid tumors during childhood. This report was followed by others (48, 49, 52, 61-69), and contributed to major changes in medical practice. Major case-control studies, and meta-analyses, have consistently shown a moderate risk increase in childhood leukemia following prenatal radiation, (48, 64, 67, 69), but most cohort studies have not supported this association (48, 51, 66). Twins are more likely to be exposed to intrauterine radiation, but studies on twins have also been contradictory (70-73).
Early case-control studies were criticized for selection bias (74), and on the grounds that no adjustments were made for potential confounders such as concomitant disease in the mother and/or the fetus, or recall bias, as exposure information was based on interviews of parents of affected children. Some of these concerns were reduced, however, in studies where information on exposure was derived from medical records (75). A dose-response relationship between increasing number of x-ray films and risk of childhood cancer has only been shown in one study (69).
Besides being hampered by low statistical power, most previous studies have often dealt with childhood leukemia as one entity. Swedish studies on prenatal exposure to diagnostic radiation and risk of childhood leukemia have so far been confined to twin cohorts and have covered the period from 1936 to 1967.
Infections The possibility of an association between childhood leukemia and infection was proposed as early as 60 years ago (76). This led to spatial and/or time clustering studies of acute leukemia and initiated searches for tumor viruses. The role of Epstein-Barr virus in neoplastic tissue is well known, and lymphomas are one group of tumors associated with this virus (77). A virus associated with childhood leukemia has not yet been identified, but the role of JC virus, a member of the polyomavirus family, has been discussed (78).
The hypothesis proposed by Greaves (23, 79-81), as well as other authors (82, 83), is that pediatric acute lymphatic leukemia is initiated prenatally, at a very early stage of fetal hematopoiesis, and that a second event occurs at a time of maximum stress on lymphocyte precursor proliferation. Leukemia then develops as an unusual result of an exposure to common infection in childhood (1, 23, 79, 80).
High rates of leukemia in children have also been associated with features of community isolation and population growth (84-98). Some studies indicate a relation between leukemia in children and hygiene conditions or socio-economic indicators (1, 80, 99-102). Childhood leukemia has also been reported in
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association with infectious agents such as mycoplasma pneumoniae (103) and in clusters (104).
Oxygen Of all newborn infants, 5% require some degree of basic life support at birth (105). Neonatal resuscitation is related to several characteristics of the newborn infant, such as birth weight, gestational age and asphyxia, which in turn have been associated with childhood leukemia. Exposure to supplementary oxygen has been found in one study to be an independent risk factor for childhood leukemia (34), but the exposure data were crude, and information was lacking on the duration, dose, and underlying reason for the treatment.
Hypoxia-reoxygenation injury is known to cause a number of conditions in the prematurely born infant (retinopathy of prematurity (ROP), bronchopulmonary dysplasia (BPD), nectrotizing enterocolitis, patent ductus arteriosus and possibly periventricular leucomalacia) (106), and it is suggested that these may represent different forms of the “oxygen radical disease of neonatology” (107). The etiology of hyperoxic toxicity involves excess production of reactive oxygen species (free radicals), possibly from by-products of metabolism of the endothelial and epithelial cells and/or generated by inflammatory cells (108). These species can cause damage to biomolecules (108, 109), including DNA, and thus be mutagenic and carcinogenic.
Several enzymes protect the cell from the toxic effects of oxygen-derived species and thus act as antioxidants. Oxidative DNA damage and antioxidant enzymes have been studied in children with leukemia, and levels of these antioxidant enzymes in lymphocytes are reported to be lower in leukemia patients than among controls (110). Premature infants have a reduced intracellular defense against oxygen radicals (111), but the total radical trapping capacity of the antioxidants in plasma in healthy newborns has been shown by others to be equal to or even higher than that in adults (112).
Environmental risk factors and infant leukemia The findings of the specific chromosomal rearrangement in a majority of infants with leukemia (16) and the high concordance rate for leukemia among identical twins have lead to speculations as to whether events in utero may influence the infant leukaemia rates (17, 32, 113). Owing to the rarity of the disease, epidemiological studies focusing only on infants with leukemia are few and are based on interviews with or questionnaires answered by affected parents (32, 35), a method which may introduce recall bias.
Significant associations have been found between infant leukemia and maternal smoking and alcohol and marijuana use (17), high birth weight, or being a later-born child (32). Females are more common among infants with leukemia (32), in contrast to childhood leukemia, where males are predominant. Congenital
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leukemia (diagnosed before the age of one week), a subgroup of infant leukemia, has been reported in association with isolated congenital anomalies (such as Down’s syndrome and trisomies 9 and 13) (17).
A high relative risk of infant leukemia has been associated with previous maternal fetal loss, as recorded in many parts of the world (18). Maternal exposure to naturally occurring DNA topoisomerase II inhibitors (through diet and medication), leading to fetal chromosome damage, may lead to fetal loss or infant leukemia. DNA topoisomerase II inhibitors are known to be associated with the MLL gene abnormalities observed in treatment-related AML occurring in children and adults following therapy for a primary cancer. Infant leukemia (both AML and ALL) is associated with abnormalities involving the MLL gene (114).
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Objectives The overall objective of this investigation was to increase the understanding of some risk factors associated with childhood leukemia. The specific aims of the studies were to assess:
the impact of prenatal ultrasound on the risk of childhood leukemia, in separate analyses for lymphatic and myeloid leukemia;
the association between prenatal exposure to diagnostic x-ray and the risk of childhood leukemia, in separate analyses for lymphatic and myeloid leukemia;
the association between prenatal exposure to infectious diseases, through maternal infections, and exposure to infections during infancy and the risk of childhood leukemia, in separate analyses for lymphatic and myeloid leukaemia;
the association between supplementary oxygen treatment and other perinatal factors, and the risk of childhood lymphatic leukaemia;
the association between maternal exposures during the entire pregnancy and postnatal exposures of the neonate, and the risk of childhood infant leukemia.
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Material and Methods National registration number All Swedish residents are assigned a unique 10-digit national registration number, which is used for official authorities, including population-based registers and archives. This number makes it possible to identify individuals and collect certain information within registers, and also to link information between different registries. Data Registries sources
The Swedish Cancer Registry The Swedish Cancer Registry was established in 1958. The registry receives reports about all newly diagnosed malignant tumors from both the physician who has made the diagnosis and the pathologist or cytologist who has confirmed it. The register is coded according to the seventh International Classification of Diseases (ICD 7) (117). Almost 97% of all patients with a malignant tumor are reported to the Swedish Cancer Registry (115).
The Swedish Medical Birth Registry The Swedish Medical Birth Registry was established in 1973 and includes data on more than 99% of all births in Sweden (116). A standardized set of medical records is used by all antenatal care clinics, delivery units, and at the examination of the newborn infant. Copies of the form are sent to the register, which is held by the National Board of Health and Welfare. For all Swedish births, medical information on maternal demographic data, the reproductive history, maternal smoking habits, the pregnancy and delivery, and neonatal factors are included on the form. The registration starts at the first visit to the antenatal clinic and is completed when the mother and newborn infant are discharged from the hospital.
The Registry of Causes of Death The Registry of Causes of Death provides information on all deceased persons registered in the population register in Sweden at the time of death, irrespective of whether the death occurs in Sweden, or abroad. The register keeps information including the cause and date of and age at death. The information is obtained from the medical death certificate and coded according to the International Classification of Diseases, Injuries, and Causes of Death (ICD). The attending physician or the coroner is responsible for completing and filing the medical certificate.
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Hospital archives In each Swedish hospital medical records are kept in archives. Subjects and methods
Selection of cases The five studies presented were all conducted in Sweden as population-based case-control studies. The cases were all infants born alive from 1973 through 1989 and diagnosed with leukemia between 1973 and 1989 (I-V). The cases were identified through the National Cancer Registry. A total of 752 children with childhood leukemia (ICD-7 and ICD-8 codes 2040, 2041, 2043, 2044, 2047, 2049, 2060, 2069, 2070, 2072, 2079, 2046, 2050, 2051, 2059) (117, 118) were included as cases in the studies (I-IV). Within this group, 58 infants were diagnosed (infant leukemia) before the age of one year (V).
Selection of controls For each case, a control of the same sex and born in the same year and month was randomly selected though the Swedish Medical Birth Registry (I-IV). The control was to be alive and without leukemia on the date of diagnosis of the matched case. By linkage to the Swedish Cancer Registry, the Swedish Medical Birth Registry, and the Registry of Causes of Death using the unique 10-digit national registration number, the living and non-leukemia status of the control was confirmed. In the study of infant leukemia (V), the total number of controls was included, without individual linkage to the different cases.
The total study group Out of 752 children with leukemia, medical records from the pregnancy and birth were retrieved for 623 cases with lymphatic leukemia and 88 cases with myeloid leukemia (94 %) (Table 1). Of the 58 infants with leukemia, medical records of 53 cases were retrieved (91 %) (Table 3). Among the missing infants, one was diagnosed at one month of age, one at three months, and one at nine months. Two infants were diagnosed at eleven months of age.
As Down’s syndrome is a major risk factor for childhood leukemia, especially myeloid leukemia (34, 119, 120), children with Down’s syndrome were excluded. All children with Down’s syndrome were cases: 11 cases with lymphatic leukemia and 10 cases with myeloid leukemia (I-IV) (Table 1). Within the study group of infants, six infants had Down’s syndrome; one case with lymphatic leukemia and five cases with myeloid leukemia (V) (Table 3). Of the initial 760 children selected as controls, medical records of 708 (93 %) were retrieved. Cases and controls lacking a matched partner were excluded from the analyses (I-IV). Thus, the first studies (I, III-IV) encompassed a total of 652
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cases (578 with lymphatic leukemia and 74 with myeloid leukemia) with an equal number of controls (Table 1). Twenty-nine children were born in twin pregnancies. Fifteen of them were cases (I-IV) and in one set of twins the case and control were siblings. Children born in twin pregnancies were excluded in the second study (II). With these exclusions, 624 cases (552 with lymphatic leukemia and 72 with myeloid leukemia) and an equal number of controls were left for analysis (II) (Table 2). After exclusions of infants with Down’s syndrome, the study-base of infants included 47 cases (35 with lymphatic leukemia and 12 with myeloid leukemia) and 708 controls (Table 3).
In all there were 670 boys (51 %) and 634 girls (49 %) in the total study-base (I, III-IV). Of the 652 cases, 48 % had developed leukemia before the age of five years (I, III-IV). There were 646 boys (52 %) and 602 girls (48 %) in the second study (II). Of the 624 cases, 48 % had developed leukemia before the age of four years. In the study-base of infants (V) there was a total of 28 boys and 19 girls among the cases, and 361 boys and 347 girls among the controls. Five cases (2 lymphatic and 3 myeloid leukemia) were diagnosed during their first week of life (congenital leukemia), 14 cases (8 lymphatic and 6 myeloid leukemia) were diagnosed between one week and 4 months of age, 8 cases (all lymphatic) were diagnosed between the ages of four and six months, and twenty cases (17 lymphatic and 3 myeloid leukemia) were diagnosed after the age of six months. Thirty-eight (83%) of the cases with infant leukemia died of their disease. Table 1. The study-base setting (I, III-IV)
Exclusions Lymphatic cases/controls
Myeloid cases/controls
Total cases/controls
Original study-base 658/667 94/93 752/760 Retrieved medical records 623/620 88/89 711/708 Excluded cases with DS* 11/0 10/0 21/0 Excluded controls to DS cases* 0/10 0/9 0/19 Excluded (no match was found)
34/27 4/5 38/32
Exclusion of extra controls 0/5 0/0 0/5 Children included 578/578 74/74 652/652 *DS= Down’s syndrome Table 2. The study-base setting, exclusion of twins (II)
Exclusions Lymphatic cases/controls
Myeloid cases/controls
Total cases/controls
Original study-base 578/578 74/74 652/652 Exclusions of twins 13/14 2/0 15/14 Exclusions of subjects matched to twin
13/12 0/2 13/14
Children included 552/552 72/72 624/624
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Table 3. The study-base of infants with leukemia (V)
Exclusions Cases/Controls Original study-base 58/760 Retrieved medical records 53/708 Excluded infants with Down’s syndrome 6/0 Lymphatic leukemia 35/0 Myeloid leukemia 12/0 Infants included in the study 47/708
Exposure data Exposure data were obtained from antenatal, obstetric, pediatric and other standardized medical records located at each hospital where the birth of the case or control took place. The data were extracted blindly with regard to case or control status and were collected in a uniform manner by the author.
The following information about the mother was retrieved: age, medical and reproductive history (parity, years of infertility, previous fetal loss), occupation (working with children, working in health sector, or working with animals), daily smoking habits, pregnancy complications, and mode of delivery (non-instrumental and instrumental vaginal delivery, emergency and elective Cesarean section) (I-V). Information on the newborn infant was retrieved as follows: sex, gestational age at birth, birth weight and length, and Apgar scores at one and five minutes (I-V).
Gestational age was estimated from the commencement of the last menstrual period and stratified into preterm (< 36 completed weeks), term (37-42 weeks), and postterm births (>43 weeks). Small for gestational age (SGA) was defined as more than 2 standard deviations (SD) below the mean birth weight for gestational age according to the Swedish birth weight curve, appropriate for gestational age (AGA) was defined as a birth weight between –2 SD and +2 SD, and large for gestational age (LGA) was defined as a birth weight more than 2 SD above the mean (121).
Information on time of exposure was obtained by calculating the completed gestational week and trimester of pregnancy. The gestational period was divided into three time periods (trimesters): the first trimester of pregnancy, from the first gestational week to the 14th week; the second trimester, from the 14th to the 28th week of pregnancy; and the third trimester, from the 28th gestational week onwards.
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Specific exposure information
Paper I Information on occurrence of exposure to diagnostic ultrasound, time of exposure during pregnancy and number of exposures was retrieved (Table 4).
Paper II Exposure to diagnostic x-ray was divided into abdominal (pelvimetry or x-ray examination in order to determine the fetal position) and other exposures (e.g. dental intraoral, sinoid, chest, non-abdominal fluoroscopy, and skeleton). The number of diagnostic x-rays carried out during pregnancy was calculated, and the trimesters of pregnancy when the examinations were performed was noted. Up to 1980 pelvimetry was performed in a uniform way in Sweden, but thereafter technical improvements, resulting in a lower radiation dose, occurred at different times in different parts of the country (122). Exposure to abdominal x-ray was therefore stratified by year of performance (Table 4). Paper III Exposure to maternal infection was divided into: local infections i.e., lower genital tract infections and urinary tract infections; and general infections referred to as “other infections” (such as pneumonia, gastroenteritis, sinusitis, common cold). This stratification was based on two reasons: a) lower genital tract infections are common among pregnant women (123), and b) maternal bacterial vaginosis increases cytokines in both maternal and newborn fluids (124-126), a change which has been linked to several adverse neonatal outcomes (127). The mode of treatment of infection was also recorded and categorized as either local (local treatment in the vagina with econazole, clotrimazole, or gentian violet) or general treatment (such as orally administered antibiotics). The number of infections during pregnancy and the trimesters of pregnancy when the infections occurred were recorded. Regarding the exposure to infection, newborn infants were categorized as exposed or unexposed (Table 4). Paper IV Exposure to supplementary oxygen was recorded as resuscitation with 100% oxygen postpartum and the duration of ventilation with a face-mask and bag immediately postpartum (divided into two groups: 0-2 complete minutes and 3 minutes or more). The occurrence of supplementary oxygen treatment during the first two weeks after birth (divided into groups according to percentage of supplementary oxygen (21-40%, 50-60%, >70%), and the duration of this treatment (≤ 24 hours and ≥25 hours) were recorded. Information on pulmonary distress and congenital malformations was also recorded (Table 4).
The Apgar score was used as a marker for asphyxia and was grouped in two different ways: 1) According to the World Health Organization International
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Classification of Diseases (10th revision), which classifies severe birth asphyxia as an Apgar score at one minute of three or less (128); and 2) according to the definition of severe asphyxia as an Apgar score at five minutes of six or less (Table 4). Paper V Information on the following maternal exposures was retrieved: infection (stratification as in study III) and diagnostic x-ray (stratification as in study II). Information on exposure to the newborn infant to resuscitation with 100% oxygen immediately postpartum, and on neonatal exposure to diagnostic x-ray, was recorded (Table 4).
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Table 4. Summary of subjects, exposure variables and covariates in each study
Study Cases Exposure variables Covariates I All leukemia
cases 1973-89, 652 cases (578 lymphatic, 74 myeloid).
Prenatal ultrasound Maternal age, high birth weight, and multiple birth
II All leukemia cases 1973-89, twins excluded. 624 cases (552 lymphatic, 72 myeloid).
Prenatal abdominal x-ray, other prenatal x-ray
Maternal age, parity, Cesarean delivery, maternal smoking, gestational age at birth, birth weight, and multiple birth
III All leukemia cases 1973-89, 652 cases (578 lymphatic, 74 myeloid).
Maternal lower genital tract infection, maternal urinary tract infection, other maternal infections, treatment, neonatal infection
Maternal age, parity, mode of delivery (vaginal or Cesarean), maternal smoking, time elapsed from rupture of membranes to delivery, gestational age at birth, birth weight, birth weight for gestational age, type of birth (single or multiple)
IV All lymphatic leukemia cases 1973-89, (578 cases)
Resuscitation with oxygen, neonatal supplementary oxygen treatment, asphyxia, birth characteristics (gestational age at birth, birth weight, and multiple birth), pulmonary distress, congenital malformations
Maternal age, parity, maternal smoking
V All infant leukemia cases 1973-89, (47 cases)
Maternal characteristics: age, medical and reproductive history, occupation, smoking, habits, diagnostic x-ray, infection. Newborn characteristics: sex, gestational age, birth weight, Apgar score, supplementary oxygen, x-ray
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Table 5. Characteristics of covariates in cases and controls (I, III-IV)
Total Lymphatic Myeloid Cases Controls Cases Controls Cases Controls Maternal age (years) <20 29 19 23 16 6 3 20-29 405 433 359 382 46 51 30-34 158 145 141 131 17 14 ≥35 60 55 55 49 5 6
Parity 1st 264 258 233 225 31 33 2nd-3rd 327 326 291 291 36 35 4th or more 33 32 28 31 5 1 missing 28 36 26 31 2 5
Gestational age (weeks) ≤36 36 36 32 32 4 4 ≥37 611 612 541 542 70 70 missing 5 4 5 4 0 0
Birth weight (grams) <2500 27 33 23 27 4 6 2500-4499 596 597 528 530 68 67 >4500 29 21 27 20 2 1 missing 0 1 0 1 0 0
Cesarean section no 577 577 520 509 57 68 yes 71 66 56 61 15 5 missing 4 9 2 8 2 1
Maternal smoking no 320 331 284 293 36 38 yes 192 194 163 171 29 23 missing 140 127 131 114 9 13
Multiple birth no 636 638 564 564 72 74 yes 15 14 13 14 2 0 missing 1 0 1 0 0 0
LGAa no 596 597 526 526 70 71 yes 35 36 33 33 2 3 missing 21 19 19 19 2 0
SGAb no 611 617 543 546 68 71 yes 20 16 16 13 4 3 missing 21 19 19 19 2 0
Time elapsedc 0-23 hours 587 573 523 506 64 67 >=24 hours 9 7 8 5 1 2 missing 56 72 47 67 9 5 aLarge for gestational age bSmall for gestational age cTime elapsed from rupture of membranes to delivery
22
Analysis As the etiological relevance of specific intrauterine exposures may differ between the two major types of childhood leukemia (34, 120), lymphatic and myeloid, these types were analyzed as separate outcomes (I-III). To study the impact of the exposures on leukemia diagnosed at different ages, cases diagnosed at ages of 0-2, 2-4 and >4 years were analyzed separately (II-IV). Age at diagnosis was also analyzed in groups among infant leukemia cases (V): 1) those with leukemia present at birth (congenital) or diagnosed up to the age of six months and 2) those diagnosed at six months of age or older.
Statistical methods Conditional logistic regression was performed to study the association between childhood lymphatic and/or myeloid leukemia, and the perinatal exposure (I-IV) (129). Maximum-likelihood estimates of the odds ratio (OR) and 95% confidence intervals (CI) were obtained, taking into account potential confounding factors. The statistical software Stata (130) was used to fit the conditional logistic model to our 1:1 matched case-control studies (I-IV). In studies III and IV we combined observations with the same matching criteria and therefore our 1:1 matched-pair case-control study was conducted as a k1i: k2i matched-pair case-control study, where k1i and k2i represent the number of cases and controls for the I-stratum. A final model included adjustments for potential confounders (Table 4). Model building was based on both statistical and subject matter considerations. Characteristics of covariates are presented in Table 5. Logistic regression modeling was used to study the crude association between infant leukemia and perinatal exposures (V). The statistical software Stata (130) was used to fit the logistic model to our case-control study.
23
Results Prenatal Ultrasound Examinations and Risk of Childhood Leukemia (I) There was no association between prenatal exposure to ultrasound and risk of childhood leukemia, either lymphatic (OR= 0.85; 95% CI 0.62-1.17) or myeloid (OR=1.00; 95% CI 0.42-2.40). Neither the increasing numbers of ultrasound examinations nor the trimester of exposure altered the results. The risk was slightly but not significantly increased in cases with myeloid leukemia examined during the second trimester (OR=1.42; 95% CI 0.88-2.29). Adjustments for potential confounding did not alter the results. Intrauterine Exposure to Diagnostic X-ray and Risk of Childhood Leukemia (II+V) Overall, 24% of the mothers of children in the total study-base who developed leukemia (23% of the lymphatic and 33% or the myeloid cases) were x-rayed during pregnancy, in comparison with 23% of the mothers who gave birth to healthy control children (II). Prenatal x-ray exposure was not associated with a statistical significantly increased risk of childhood leukemia overall (OR=1.11; 95% CI 0.83-1.47) or of lymphatic leukemia (OR=1.04; 95% CI 0.77-1.40), or of myeloid leukemia (OR=1.49; 95% CI 0.48-4.72) (adjusted model) (II). There was no significant association between the risk of leukemia (either lymphatic or myeloid) and the type of x-ray (Table 6) or the number of x-ray examinations or the trimester of pregnancy when x-ray was performed (II). Neither abdominal x-ray examinations carried out before 1980 nor those performed in 1980 or later were associated with an increased risk of childhood lymphatic or myeloid leukemia. There was no difference in the risk estimates when different ages at diagnosis of lymphatic leukemia, i.e., 0-2, 2-4 or >4 years, were analyzed as separate outcomes.
There was a significant association between perinatal exposure to maternal abdominal x-ray and infant leukemia (OR= 2.36; 95% CI 1.12-4.96) (Table 6). The association was more pronounced among infants six months of age or older at diagnosis (OR= 2.61; 95% CI 1.18-5.76) than among those younger at diagnosis (OR= 1.35; 95% CI 0.16-11.40). Most of the x-ray examinations were performed in the third trimester of pregnancy. There was no association between neonatal exposure to diagnostic x-ray and infant leukemia (data not shown).
24
Table 6. Risk of lymphatic and myeloid childhood leukemia and of infant leukemia following prenatal abdominal x-ray.
Leukemia Type of x-ray Cases (624) Controls (624) Adjusted ORa 95% CI Lymphatic unexposed 449 450 1.00 - abdominal 55 54 1.01 0.68-1.51 other x-ray 31 28 1.07 0.62-2.83 unknown typeb 17 20 - - Myeloid unexposed 54 61 1.00 - abdominal 13 7 1.74 0.53-5.74 other x-ray 3 4 1.04 0.20-5.33 unknown typeb 2 0 - - Infant cases
(47) Infant controls (689)
Univariate OR 95% CI
unexposed 35 574 1.00 - Infant leukemia abdominal 10 70 2.36 1.12-4.96 other 2 39 0.84 0.20-3.64 unknown typeb 0 3 - aOR was calculated by means of conditional logistic regression bUnknown type of x-ray Exposure to infection and risk of childhood and infant leukemia (III+V) Prenatal exposure to maternal infection was not associated with an increased risk of childhood leukemia overall (OR=1.25; 95% CI 0.95-1.64), or of childhood lymphatic leukemia (OR=1.22; 95% CI 0.92-1.63) or childhood myeloid leukemia (OR=1.44; 95% CI 0.62-3.38) (III). There was no association between childhood leukemia (lymphatic or myeloid) and either number of infections, trimester of pregnancy when the infection occurred, or age at diagnosis (III).
A significantly increased risk of childhood leukemia was associated with maternal lower genital tract infection (Table 7) (III). This association was restricted to cases with lymphatic leukemia (Table 7) (III). When the analyses were stratified by age at diagnosis of leukemia, the risk associated with maternal lower genital tract infection was confined to children diagnosed at four years of age or older (OR=2.06; 95% CI 1.12-3.80) (III) and to cases with infant leukemia (V) (Table 7). The association was most pronounced in infants of less than six months of age at diagnosis (OR= 10.0; 95% CI 2.31-43.38), compared to older infants (OR= 1.42; 95% CI 0.42-4.85) (V). There was no association between maternal urinary tract infection (OR=1.27; 95% CI 0.69-2.34) or other infections
25
(OR=1.13; 95% CI 0.83-1.55) and childhood leukemia or infant leukemia (data not shown) (III,V).
Almost all mothers with lower genital tract infection (mothers of 54 cases and mothers of 34 controls in the total study-group) had been treated locally in the vagina with econazole, clotrimazole or gentian violet (III). There was an association between local maternal treatment and childhood leukemia (OR=1.65; 95% CI 1.04-2.60) (III). Exposures to other treatments, such as orally administered antibiotics for other types of infection, were not associated with an increased risk of childhood leukemia (OR=1.16; 95% CI 0.73-1.84) (III).
There was no association between exposure to infections during the infant’s first two weeks of life and childhood leukemia (OR=1.01; 95% CI 0.50-2.04) (III). Table 7. Risk of childhood and infant leukemia following maternal lower genital tract infection (LGTI)
LGTI Cases (652) Controls (652) Total ORa 95% CI Unexposed 588 614 1.00 - All leukemia cases 64 38 1.78 1.17-2.72 Lymphatic leukemia 56 36 1.63 1.04-2.53 Myeloid leukemia 8 2 4.00 0.85-18.8 Infant cases (47) Infant controls (708) Univariate OR 95% CI Unexposed 41 669 1.00 - Infant leukemia 6 39 2.51 1.00-6.27 aOR was calculated by means of conditional logistic regression Exposure to supplementary oxygen and risk of childhood lymphatic and infant leukemia (IV+V) Resuscitation with 100% oxygen with face-mask ventilation directly postpartum was associated with an increased risk of childhood lymphatic leukemia, and the association was even more pronounced when the resuscitation lasted for three minutes or more (Table 8) (IV). The risk was mainly increased among cases diagnosed at four years of age or later (Table 8) (IV). Resuscitation with 100% oxygen directly postpartum was not associated with a risk of infant leukemia (Table 8) (V). Treatment with supplementary oxygen later in the neonatal period was not associated with lymphatic leukemia: 0-24 hours (OR= 0.77; 95% CI 0.34-1.76) and >25 hours, (OR=1.99; 95% CI 0.50-7.95); and <50% oxygen, (OR=0.93; 95% CI 0.43-2.00); 50-70% oxygen, (OR= 1.50; 95% CI 0.25-8.98) (IV). Low Apgar scores were not significantly associated with a risk of childhood lymphatic leukemia, but a low Apgar score at five minutes was associated with a six-fold increase in the risk of infant leukemia (Table 8) (V).
26
There were no associations between childhood lymphatic or infant leukemia and other birth-related factors, such as birth weight, gestational age, birth weight for gestational age, and mode of delivery (data not shown) (IV,V). Similarly, the risk of lymphatic leukemia showed no association with factors related to the child, such as pulmonary distress, congenital malformations or congenital heart malformations (data not shown) (V).
The association between resuscitation with 100% oxygen with a face-mask directly postpartum and childhood lymphatic leukemia became even more pronounced, albeit non-significantly (OR=10.7; 95%CI 0.82-132.22), when adjustments were made for an Apgar score at one minute of three or less (IV). Adjustment for other potential confounders (mother’s age, smoking, and parity) affected the results only marginally (IV).
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Table 8. Risk of childhood lymphatic leukemia and infant leukemia by Apgar score and resuscitation with 100% oxygen, and by minutes of exposure and age at diagnosis
Cases (578) Controls (578) ORa 95%CI 4-10 554 563 1.00 - 0-3 11 15 2.15 0.75-6.19
Apgar score at one minute
No data 13 10 - -
7-10 560 564 1.00 - 0-6 7 4 1.78 0.51-6.24
Apgar score at five minutes
No data 11 10 - -
Unexposed 558 570 1.00 - Exposed 20 7 2.87 1.21-6.82
Oxygen by mask
No data 0 1 - -
Unexposed 558 570 1.00 - Oxygen by mask (minutes) 0-2 6 3 2.00 0.50-8.00 3-10 14 4 3.54 1.16-10.80 No data 0 1 - - Oxygen by mask, age at diagnosis (years)
<2 2-4 >4
3 6 11
0 6 1
- 1.00 11.0
- 0.32-3.10 1.42-85.20
Infant
cases (47) Infant controls (708)
Univariate OR 95% CI
Oxygen by mask Unexposed 43 628 1.00 - Exposed 4 80 2.51 0.71-8.81 Apgar score one minute 5-10 45 690 1.00 - 0-4 2 7 4.38 0.88-21.7 five minutes 7-10 45 692 1.00 - 0-6 2 5 6.15 1.16 –32.6 a OR was calculated by means of conditional logistic regression Perinatal characteristics related to infant leukemia (V) Infant leukemia showed an association with maternal abdominal x-ray (Table 6), maternal lower genital tract infection (Table 7), and a low Apgar score (Table 8) (V). We also found that a young age of the mother (less than 20 years) was associated with an almost three-fold increase in the risk of infant leukemia (OR=2.36; 95% CI 0.98-7.05) (V). No association was found between maternal
28
daily smoking during pregnancy, infertility (more than one year), previous fetal losses (one or more), or parity and infant leukemia (data not shown) (V). The risk of infant leukemia was also increased in children of mothers who worked with children in daycare (OR=2.54; 95% CI 1.13-5.73) or worked in the health sector (OR=2.51; 95% CI 1.09-5.80) (V).
29
General Discussion The results of the studies summarized in this thesis indicate that the extensive use of prenatal ultrasound in Swedish antenatal care, and the restricted used of diagnostic x-ray, are not associated with an increased risk of childhood leukemia overall. However, it is not inconceivable that childhood leukemia may be related to certain gestational and neonatal events that possibly induce proliferative stress or mutations in lymphoid stem cells of the developing fetal immune system. The exposures are either non-specific, such as exposure to maternal lower genital tract infection, or specific, such as administration of supplementary oxygen directly postpartum. Methodological considerations, strengths and weaknesses of the studies
Inclusions and size of study The case-control studies (I-V) are based on the population of Sweden and include all childhood leukemia cases born and diagnosed from 1973 through 1989. This approach enabled us to identify a substantial number of cases (I-IV), which increased the statistical power. As in many other studies on infant leukemia, the size of our study (V) is a matter of concern, only 47 cases, and the risk estimates need to be interpreted with caution. Chance cannot be ruled out. The size of the total study group (I-IV) also allowed us to some extent, to assess the change in risk over time (II) and to make separate analyses with regard to leukemia subtypes (I-IV).
The children included in the studies were born and diagnosed within the same period of time, which implied that a follow up period was lacking for some children. However, the age distribution of cases with lymphatic leukemia in our study-base is about the same as that of the total number of cases up to the age of 16 years reported in a Scandinavian population between 1985 and 1994 (2) (Fig. 1). In our total study group 48% of the cases were diagnosed before the age of five years, compared to 50% in the overall Scandinavian population (2).
30
Figure 1. Percentage age distribution (years) of all childhood lymphatic leukemia cases (578 cases) in the present study group and in a Scandinavian childhood population (1687 cases, between 1985 and 1994 ) (NOPHO 99).
Retrieval and specificity of exposure information The records used at antenatal and delivery centers in Sweden are standardized and include information on the mother’s demographic and medical history, before and during pregnancy, on the delivery, and on the newborn infant (116). Thus, exposure data obtained prospectively from the charts have high specificity. The exposure data were collected blindly and in a uniform way and the risk of recall or selection bias was thereby reduced. Eleven percent of the cases and controls (I-IV), and nine percent of the infant cases (diagnosed among all ages in infancy) and seven percent of their controls (V) had to be excluded because the medical records of the subject or the matching partner were not found. These exclusions should, however, be non-differential between cases and controls with regard to exposure.
When exposure information is retrieved from medical charts there is always a risk that some exposure information or details about a certain type of exposure might not have been documented or that some exposure might have been underestimated (e.g., clinically mild maternal infections), which would limit the studies (I-V). Moreover, on some occasions the specificity can be questioned, as the information is based on clinical diagnosis without appropriate tests (IV). However, the data extraction was performed blindly with regard to the case and control status and the routine at the maternity centers makes it unlikely that underreporting is more than marginal. Therefore any misclassification should be non-differential between cases and controls.
The proportion of control subjects exposed to abdominal x-ray was about 11% (II), which is similar to the figures reported in most other studies (131), and the proportion exposed to ultrasound was about 33% (I), which probably mirrors the frequency of the study period.
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Age at diagnosis
perc
enta
ge o
f tot
al
num
ber o
f cas
es
NOPHO
Present Study-base(Lymphatic)
31
The prevalence of the maternal lower genital tract infection occurring most commonly during pregnancy (vaginal candidosis) is reported to range between 10 and 30% (123). The overall rate of exposure to maternal lower genital tract infection of eight percent in our study (III) suggests that only the most severely infected mothers were identified. The apparently low sensitivity for exposure to maternal lower genital tract infection among all cases with childhood leukemia may therefore result in an underestimation of the number of cases that are possibly attributable to genital infections. Almost all mothers with lower genital tract infection were treated locally in the vagina, but the type of medication varied over time. The impact of the treatments on childhood leukemia is not known. However, the difference in the type of treatment over time and the possible underestimation of the number of exposed mothers may suggest that exposure to prenatal maternal lower genital infection is associated with childhood leukemia and that for the mothers in this study (III) the treatment might have been insufficiant.
The rate of exposure of two percent to resuscitation with supplementary oxygen (IV) is lower than is reported from medical practice today (5%) (105). This suggests an underestimation of the possible attributable fraction to childhood leukemia cases of today.
Potential confounders Down’s syndrome is a well-known risk factor for childhood leukemia (34, 119, 120), and neonates with Down’s syndrome are often diagnosed with a non-malignant leukocytosis, which must be distinguished from leukemia (19). For this reason children with Down’s syndrome were excluded from the studies (I-V).
We also adjusted for other potential confounding factors and factors that might influence the causal pathway (such as maternal age, parity, maternal smoking, mode of delivery (vaginal or Cesarean), time elapsed from rupture of the membranes to delivery, gestational age at birth, birth weight, and multiple birth) (I -IV).
Potential biases The etiology of childhood leukemia is complex and several risk factors may act as intermediates. In our studies it was possible to some extent to adjust for interaction between specific perinatal factors (i.e., a low Apgar score at birth and resuscitation with oxygen, IV). Findings and Implications The results of the study of prenatal exposure to ultrasound (I) and diagnostic x-ray (II) are reassuring. No association was found between these exposures and lymphatic or myeloid leukemia. The risk did not increase with increasing numbers
32
of examinations (I,II), with time of exposure (I,II), or with type of x-ray (II). However, prenatal exposure to abdominal x-ray was associated with leukemia in infants who were more than six months of age at diagnosis (V).
Early studies of prenatal x-ray exposure concerned exposures during the 1950s, 1960s and 1970s and dealt with a different spectrum of doses (122, 132, 133). The doses have gradually decreased over time. Thus, the increased risk of childhood leukemia following diagnostic x-ray that was found in earlier studies in the 1950s and 1960s could have been an effect of the higher doses, compared to those used in the 1970s and 80s. In today’s modern era of diagnostic radiology, with lower doses and less frequent use, the fraction of children who develop leukemia possibly attributable to diagnostic x-rays must be almost negligible, and may be restricted only to infant leukemia.
We found an association between infant leukemia and both prenatal exposure to maternal lower genital tract infection and a young maternal age (V). Teenage pregnant women are more often diagnosed with vaginal chlamydial infections as well as other genital tract infections (134-136). We also found an association between childhood lymphatic leukaemia and maternal lower genital infection (III). Maternal bacterial vaginosis is known to increase the levels of various cytokines in both maternal and newborn fluids (124-126). This increase has been linked to several adverse neonatal outcomes (such as intracranial hemorrhage and periventricular leukomalacia, precursors of cerebral palsy and bronchopulmonary dysplasia) (127, 137-145). Animal and in vivo experiments have indicated that increased levels of cytokines such as TNF-alpha are associated with the neuronal cell death that precedes periventricular leukomalacia and hemorrhage (146). It is likely that the local vaginal immune system is upgraded following maternal lower genital tract infection, even if the woman is treated for the infection, and the impact of this alteration in the maternal immune system on the developing immune system of the fetus is unknown.
Working among children in daycare centers or among patients in the health care system was associated with infant leukemia (V). Such occupations may serve as markers for exposure to infections during pregnancy, and maternal reinfection or a reactivated infection may lead to fetal damage (144). It may be speculated that repeated exposures to infection during pregnancy may upgrade the maternal immune system and thereby affect the fetus through cytokines, immunoglobulins, and other mediators.
Resuscitation with supplementary oxygen immediately postpartum was associated with an increased risk of childhood lymphatic leukemia, and the risk increased if the treatment continued for more than three minutes (IV). Hypoxia-reoxygenation, by the production of free radicals, causes a number of conditions in the preterm infant (e.g., retinopathy of prematurity, bronchopulmonary dysplasia, necrotizing enterocolitis, patent ductus arteriosus, and possibly periventricular leukomalacia) (106). Free oxygen radicals can cause damage to
33
biomolecules (109), including DNA, and thus be mutagenic and carcinogenic. Antioxidant enzymes protect cells from the toxic effects of oxygen-derived species, and patients with childhood leukemia have been shown to have lower levels of these antioxidant enzymes compared to controls (110). Preterm infants have a reduced intracellular defense against oxygen radicals (111), but the total radical trapping capacity of the antioxidants in plasma in healthy newborns has been found by other authors to be equal to or even higher than that in adults (112). The long-term effect of supplementary oxygen administered immediately postpartum on a newborn infant with hypoxia is unknown, but it might be a part of a carcinogenic process of leukemia.
Birth-related factors, such as a high birth weight, a low Apgar score, and asphyxia have been linked to childhood leukemia (30-36). However, these findings could be a result of an interaction of several pregnancy and neonatal events. Several mechanisms may lead to a low Apgar score in the newborn infant, such as asphyxia related to loss of placental transfer; asphyxia related to preterm birth (due to low energy storage and an immature nervous system); asphyxia related to a high birth weight; and malformations (107). A low Apgar score is, in turn, related to resuscitation given directly postpartum. An infant suffering from birth asphyxia is usually in a poor condition at birth and will undergo resuscitation with pure oxygen. A low Apgar score, as a marker for asphyxia of the newborn, was found in our study (IV) to be non-significantly associated with childhood leukemia. However, when the effect of mask ventilation was adjusted for severe asphyxia the association between supplementary oxygen and lymphatic leukemia became more pronounced. Intrauterine exposure to maternal infection has been linked to low Apgar scores and other markers for asphyxia (147). The association between resuscitation with pure oxygen and childhood lymphatic leukemia was not altered after adjustment for maternal lower genital infection (OR=2.83; 95% CI 1.19-6.74).
Age at diagnosis The hetrogeneity of childhood leukemia indicates that the underlying etiological mechanism is complex and that several risk factors may act as intermediates (1). The incidence peak of childhood leukemia at two to four years of age has raised the hypothesis that early life exposures may be important in the etiology. According to Greaves’ hypothesis, acute lymphatic leukemia arises as a result of two independent mutations, the first developing in utero and the second during early childhood (23, 79, 80). The exposures we studied increased the risk of childhood leukemia mainly for children diagnosed at four years of age or older, namely maternal lower genital tract infection (OR=2.01, III) and resuscitation with oxygen directly postpartum (OR=11.00, IV). The reasons for the age-specific incidence peak in childhood leukemia cannot be explained by our findings. However, children who develop leukemia at older ages may have a history of several events, starting with exposures very early in life when the development and programming of the immune system take place. These events might increase
34
the risk of mutations, and as a rare response to an unidentified infection, leukemia could arise. Our findings may therefore be interpreted as supporting Greaves’ hypothesis.
Factors related to infections or exposure to diagnostic x-rays may increase the risk of infant leukemia and indicate that exposures in utero are on the causal pathway for infant leukemia, in line with speculations on the chromosomal fragility manifested as chromosomal rearrangement and a high concordance rate for leukemia among identical twins (17, 32).
Clinical implications Exposure to maternal lower genital tract infection and newborn resuscitation with oxygen were found to be associated with childhood leukemia at all ages (III-V). The overall rate of exposure to maternal lower genital tract infection and of newborn resuscitation with supplementary oxygen in our studies suggests an underestimation of the attributable fraction of cases with childhood leukemia of today. However, therapy and change of routines can affect the prevalence of these two exposures.
Extensive treatment of maternal lower genital infection has been shown to prevent women from early fetal loss, preterm rupture of membranes, and overall preterm birth (148), but its effect has not been studied regarding other outcomes, such as cerebral palsy and bronchopulmonary dysplasia. Although most of the women with lower genital tract infection in our study were treated for the condition (IV and V), the association with childhood leukemia remained, possibly for the reason that only the most severely infected mothers were identified and treated. The most pronounced increase in risk following prenatal exposure to maternal lower genital tract infection was observed for infants less than six months of age at diagnosis (OR= 10.0, V). This may indicate, though this is still speculative, that the local vaginal immune system in pregnant women with genital tract infection may increase the levels of cytokines in the fetus also, and cause cell damage and be directly carcinogenic. Extensive treatment of maternal lower genital tract infection may influence the rates of several neonatal outcomes, including childhood and infant leukemia.
It has been recommended in textbooks and guidelines that reoxygenation after birth asphyxia should be carried out with 100% oxygen. However, resuscitation with room air has been shown to be equally effective (105). The increased risk of several adverse outcomes in the neonate following treatment with 100% oxygen, and the long-term increased in the risk of childhood leukemia found in our study (IV) may influence the debate.
35
Implications for the future Exposures in utero and/or early in life may be important determinants of childhood leukemia, and lie in the causal pathway of the disease. The possibility of an etiological association between childhood leukemia and infection was proposed as long as 60 years ago (76). The findings of an association between maternal lower genital tract infection and childhood leukemia (III and V) indicate that it would be of interest to explore the inflammatory effects on the fetal bone marrow. Several birth-related factors have been associated with childhood leukemia (30-36), and are all related to resuscitation with oxygen. The association between resuscitation with oxygen and childhood leukemia observed in our study (IV) implies a new aspect of the birth-related risk factors presented by others. Further studies are warranted to confirm our findings.
36
Conclusions
− Prenatal exposure to ultrasound is not associated with childhood leukemia.
− Prenatal exposure to diagnostic x-ray is not associated with childhood
leukemia.
− Prenatal exposure to maternal lower genital infection is associated with childhood lymphatic leukemia.
− Supplementary oxygen treatment directly postpartum may be an
independent risk factor for childhood leukemia.
− Resuscitation with 100% oxygen seems to carry a dose-response relationship: the longer the duration of oxygen ventilation, the greater the risk of leukemia.
− Young maternal age, maternal lower genital tract infection and abdominal
x-ray are all associated with infant leukemia.
37
Acknowledgements This work would not have been accomplished without the support of generous collaborators, good friends and the best of families. I wish to express my sincere gratitude to all people who have generously contributed to the completion of this project, and I would like to extend special thanks to: Anders Ekbom, my main advisor, for your unfailing enthusiasm and friendship and for sharing your extensive knowledge in epidemiology, and for always being available for discussions. I also appreciate the patience with which you have observed the expansion of my family. Anders Jonzon, my advisor, for your support and friendship, for your encouragement and scientific guidance and for being a clinical mentor in pediatric cardiology. Sven Cnattingius, co-author and advisor, for sharing your knowledge in perinatal epidemiology and for your generosity and reliability. Rino Bellocco, co-author and statistical support, for all the work and effort you have put into this project. Per Hall, co-author, for sharing your knowledge in the field of ionizing radiation epidemiology and for your never-failing encouragement. John D. Boice Jr, co-author, for your valuable comments, for sharing your knowledge and for your thorough revision of the x-ray study. Ann Almqvist, friend and collaborator, for all your assistance and reliability. Hans-Olov Adami, creator and chairman of the Department of Medical Epidemiology Karolinska Institute (MEP), Stockholm, for providing a stimulating and creative working atmosphere. Torsten Tuvemo, chairman of the Department of Women’s and Children’s Health, section for pediatrics, Uppsala University, for providing an inspiring clinical and scientific environment. Colleagues and friends at the Akademiska Barnsjukhuset, Uppsala, for contributing to a good working atmosphere, and colleagues and friends at MEP, for all the jokes and for encouraging discussions in epidemiology. Britt Clausson, Marina Möller and Malin Bentz-Hellgren, for your encouragement and support and for your great sense of humor. Lars-Olof and Marianne Norée, my parents, for your support and never-failing network, and particularly my mother, the artist of the cover painting, which she created when she was carrying me in her womb, and Karin Naumburg, my mother-in-law, for love and support. And the most important people in my life, Jan Naumburg, my beloved husband, life companion, and best friend, and Karl Adam, Ylva and Camilla, our wonderful children.
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This work was supported by grants from Cancerfonden (the Swedish Cancer Society), number 3520-B94-01XAB (title "Perinatal risk factors for childhood leukemia"), and Statens strålskyddsinstitut (SSI) (the Swedish Radiation Protection Institute), number P 959.96 (title "Diagnostic radiology as a risk factor for childhood leukemia").
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