Calculation of effective dose from measurements of secondary neutronspectra and scattered photon dose from dynamic MLC IMRT for 6 MV,15 MV, and 18 MV beam energies

Rebecca M. Howella�

Emory University School of Medicine, Department of Radiation Oncology, 1365 Clifton Road AT200,Atlanta, Georgia 30322

Nolan E. Hertel, Zhonglu Wang, and Jesson HutchinsonGeorgia Institute of Technology, 900 Atlantic Drive, Atlanta, Georgia 30332

Gary D. FullertonUniv. Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, Texas 78229

�Received 29 April 2005; revised 1 November 2005; accepted for publication 1 November 2005;published 17 January 2006�

Effective doses were calculated from the delivery of 6 MV, 15 MV, and 18 MV conventional andintensity-modulated radiation therapy �IMRT� prostate treatment plans. ICRP-60 tissue weightingfactors were used for the calculations. Photon doses were measured in phantom for all beamenergies. Neutron spectra were measured for 15 MV and 18 MV and ICRP-74 quality conversionfactors used to calculate ambient dose equivalents. The ambient dose equivalents were corrected foreach tissue using neutron depth dose data from the literature. The depth corrected neutron doseswere then used as a measure of the neutron component of the ICRP protection quantity, organequivalent dose. IMRT resulted in an increased photon dose to many organs. However, the IMRTtreatments resulted in an overall decrease in effective dose compared to conventional radiotherapy.This decrease correlates to the ability of an intensity-modulated field to minimize dose to criticalnormal structures in close proximity to the treatment volume. In a comparison of the three beamenergies used for the IMRT treatments, 6 MV resulted in the lowest effective dose, while 18 MVresulted in the highest effective dose. This is attributed to the large neutron contribution for 18 MVcompared to no neutron contribution for 6 MV. © 2006 American Association of Physicists inMedicine. �DOI: 10.1118/1.2140119�

Key words: secondary neutrons, IMRT, effective dose

I. INTRODUCTION

In this work, the effective dose �including photon and neu-tron components� from intensity-modulated radiation therapy�IMRT� and conventional radiotherapy for three differentbeam energies are evaluated. The impact of 6 MV, 15 MV,and 18 MV x-ray beam energies are compared. Although theneutron radiation weighting factors are not fully applicable atsome of the organ doses, the effective dose still represents auseful measure for comparing radiation delivery methods.The data presented here are an extension of previous re-search by these authors.1

Typical prostate treatments were used in the investigationsince this site is often treated with high-energy photons. Ex-ternal beam radiotherapy for prostate cancer is typically de-livered using two separate treatment plans, with a total doseof 75.6 Gy. The initial treatment plan delivers 45 Gy in 25fractions, 1.8 Gy per fraction. The planning tumor volume�PTV� for the initial 45 Gy generally includes the prostate,seminal vesicles, and a margin to allow for organ motion anddaily setup variation. The second treatment plan �boost plan�typically delivers 30.6 Gy in 17 fractions, 1.8 Gy per frac-tion. The boost PTV is smaller than the initial PTV and gen-

erally includes the prostate and a reduced margin.

360 Med. Phys. 33 „2…, February 2006 0094-2405/2006/33

Effective doses presented in this work are for measure-ments made during the delivery of 45 Gy from the initialtreatment plans. Data were not extrapolated to the higherdoses of 75.6 Gy because no data were collected for boosttreatment plans. The multileaf collimator �MLC� shaping forboost plans results in smaller treatment areas. These smallertreatment areas result in decreased dose to nearby criticalnormal structures, such as the bladder and rectum, and mayaffect the neutron spectra and scatter/leakage photon dose todistant organs. Future work will include an investigation ofeffective dose from the delivery of prostate boost treatmentplans.

II. METHODS AND MATERIALS

Measurements were performed at the Emory Clinic, De-partment of Radiation Oncology, in Atlanta, GA. The 6 MVand 18 MV data were measured on a Varian Trilogy linac.The 15 MV data were measured on a Varian 23EX linac�with trilogy upgrades�. Both accelerators are equipped withMillenium 120 Leaf MLC. X-ray and neutron data were col-lected separately. These measurements are described in detail

in the following sections.

360„2…/360/9/$23.00 © 2006 Am. Assoc. Phys. Med.

361 Howell et al.: Effective Dose from IMRT 361

A. Organ delineation and treatment planning

Anatomical data for this work were based on a computedtomography �CT� scan of the ART male dosimetry phantomconstructed of tissue equivalent material �ICRU-44�.2 Thephantom is transected horizontally in 2.5 cm slices. Eachslice has a 1.5 cm2 grid pattern of holes plugged with softtissue-equivalent and lung-equivalent pins which can be re-placed by thermoluminescent dosimeter �TLD� holders.Bony landmarks and air cavities in the phantom were used tocontour the following organs: Prostate, bladder, seminalvesicles, rectum, brain, salivary glands, spinal cord, thyroid,esophagus, lungs, breasts, heart, liver, stomach, spleen, pan-creas, kidneys, gonads, colon �ascending transverse and de-scending�, and the femoral heads. All organ contours wereapproximated and based on their presumed location relativeto bony anatomy and air cavities. Contouring was completedby radiation oncologists and surgeons well versed in CTanatomy. These contours were used to create a map of theTLD-holder locations corresponding to organ locations.Table I gives details of the center location of each organincluding the distance from isocenter �along the longitudinalaxis� to the center of the organ and depth from the phantomsurface to the center of organ.

Treatment plans were created using Eclipse Helios with a5 mm calculation grid. The PTV included the prostate andseminal vesicles and a margin. The conventional plans allused four-field beam arrangements with gantry angles of

TABLE I. Details of center location of each organ: Distance from isocenteralong the longitudinal axis to center of organ and depth from phantomsurface to center of organ. These data were determined using measurementtools in Eclipse and from measurements of estimated organ locations in theART phantom.

Organ

Distance from isocenter�along longitudinal axis�

�cm�

Organ depth inphantom

�surface to centerof organ�

�cm�

Esophagus 46 13.5Breast 46 2

Thyroid 60.6 2Lung 45 12.5

Stomach 32 10.5Liver 31.5 8

Gonads 4 1Colon �excluding rectum� 18 9.5

Rectum 3.5 14Femoral heads 8 11.5

Bladder 2.3 8.5Salivary glands 67.5 6

Spleen 32.5 9Heart 41 9.5

Pancreas 30 11Kidney 27 12.5Brain 75.88 13Cord 43.6 16

270°, 0°, 90°, and 180° with static MLC conformed to the

Medical Physics, Vol. 33, No. 2, February 2006

PTV. The IMRT plans used five-field beam arrangementswith gantry angles of 225°, 285°, 0°, 75°, and 135° withoptimized dynamic MLC.

Although the prescribed photon dose was the same forboth types of therapy, the beam-on time required to deliver45 Gy from the IMRT treatment was greater than that for theconventional treatment. The beam-on time was also longerfor lower beam energies. The total number of monitor units�MU�, required to deliver the 18 MV IMRT and the conven-tional plans were 11,500 and 5375, respectively. The totalnumber of MU required to deliver the 15 MV IMRT and theconventional plans were 11,900 and 5700, respectively. Thetotal number of MU required to deliver the 6 MV IMRT andthe conventional plans were 14,275 and 7125, respectively.Dose volume histograms �DVHs� and axial slice isodoselines were compared for all treatment plans. The high-energyconventional plans were superior to the 6 MV conventionalplan due to decreased dose to subcutaneous tissues. TheIMRT plans were all optimized to achieve comparable spar-ing of the bladder, rectum, and femoral heads. Little differ-ence was seen in doses to subcutaneous tissues between theIMRT plans.

B. Neutron component of equivalent dose

Neutron measurements were performed using a197Au-based Bonner sphere system. This measurement sys-tem consists of 197Au activation foils placed on the surface ofa special holder �developed by Sweezy et al.�3 with differentsizes of neutron moderating spheres. The holder is fabricatedout of an aluminum rod on a polyethylene base. The Bonnersphere measurement apparatus uses seven levels of modera-tion: The bare holder and the holder inside 2, 3, 5, 8, 10, and12 in. polyethylene moderating spheres �Fig. 1�.

198Au atoms which are formed following neutron capture,decay in a �-� cascade with emission of 411 keV �-rays�T1/2=2.7 days�. Following irradiation, the 411 keV gamma-ray photopeak was measured for each foil using a high-purity

FIG. 1. The 197Au foil Bonner system used to measure the neutron spectra:�a� Set of Bonner spheres, �b� Au-foil holder inside the 10 in. Bonnersphere, and �c� Close-up of Au-foil holder and orientation of Au activationfoil.

germanium detector. The activity at the end of irradiation,

362 Howell et al.: Effective Dose from IMRT 362

A�tr�, was calculated according to Eq. �1�. The average pro-

duction rate per unit mass of target �Bq · s−1g−1�, P, was thencalculated according to Eq. �2�:

A�tr� =C�

�P��e−��tc−tr� − e−��ts−tr��, �1�

P =�A�tr�

1 − e−��tr−to�1

m, �2�

where A�tr� is the activity at end of irradiation, C is themeasured counts, � is the decay constant for 198Au, � is thedetector efficiency, tc is the beginning of counting time, tr isthe end of irradiation time, ts is the end of counting time,P�=0.96, the probability of emission of 411 keV gamma, to

is the beginning of irradiation time, and m is the mass of the197Au foil �g�.

An appropriate neutron spectrum was determined for eachset of Bonner sphere data via mathematical deconvolution�unfolding�. The unfolding process was previously describedby these authors in an earlier publication and is discussed inmore detail in the literature.1,3–6 The resulting activated foildata were unfolded with MXD�FC33 code �one of the un-folding programs in the PTB/UMG package�.7 It was neces-sary to calculate a response matrix for the gold foil measure-ment system for use in the unfolding. A response matrix forthe gold foil-based Bonner sphere system was calculated us-ing MCNP5 �Ref. 8� for each level of moderation. The re-sponse matrix was verified by unfolding 197Au-Foil Bonnersphere system data from irradiations with a neutron sourcewith a known spectrum, 252Cf, at the Georgia Institute ofTechnology Neely Nuclear Research Center �NNRC�, At-lanta, GA calibration facility. Results of the response matrixverification are shown in Fig. 2.

The ICRU operational quantity, ambient dose equivalent,H*�10� was calculated using the measured neutron fluences,

the unfolded spectra, and the Schumacher and Siebert ana-

Medical Physics, Vol. 33, No. 2, February 2006

lytical function, Eq. �3�.9–11 Ambient dose equivalent, H*�10�is defined by the ICRU as the dose equivalent that would beproduced by the corresponding expanded and aligned field inthe ICRU sphere at a depth of 10 mm, on the radius oppos-ing the direction of the aligned field:10,11

H * �10��

= �10∧�� 1.110

1 + �0.08379 + 0.1428x�2�+ � 2.174

1 + exp�11.76 − 2.464x��+ � 0.1150

1 + exp�219.4 − 30.68x���� , �3�

where H*�10� is the ambient dose equivalent, � is the flu-ence, and x is the log10 of the average energy of neutrons ina given energy bin. The ambient dose equivalents were cor-rected for tissue depth using neutron depth dose data re-ported by d’Ericco et al.12 to account for falloff in neutrondose with depth in tissue. The corrections were determinedfor the average neutron energy of 0.5 MeV �see Table II� anddepth from phantom surface to the center of each organ �seeTable I�.

The depth corrected neutron doses were then used as ameasure of the neutron component of the ICRP protectionquantity, organ equivalent dose, HT.10,11 The ICRP definesHT as the absorbed dose averaged over the tissue or organ, T,for irradiation in a field consisting of different radiations, thevalues of absorbed dose for each radiation type is weightedby the appropriate radiation weighting factor, WR, and usedto obtain the organ equivalent dose.10,11,13

HT = wRDT,R. �4�

The aforementioned neutron measurements were per-formed �separately� for both 15 MV and 18 MV IMRT and

FIG. 2. Comparison of the known spectrum and the un-folded data from measurements with the 197Au foilBonner Sphere system. Measurements were made for a252CF source at Georgia Institute of Technology NNRCcalibration facility.

conventional treatment plans at two different locations: In-

363 Howell et al.: Effective Dose from IMRT 363

side the treatment field at the isocenter and outside the treat-ment field at 40 cm superior to the isocenter. To performthese measurements, the ART phantom was placed on thetreatment couch. External fiducial marks placed during theCT simulation �these marks represent the location of the iso-center used for treatment planning� and room lasers wereused to properly align the phantom on the treatment couch.Sectional slices were removed to allow placement of the197Au foil holder and moderator at the isocenter for infieldmeasurements. A photograph of the experimental setup forthe in-field measurements is shown in Fig. 3. The setup wassimilar for out-of-field measurements, except that sectionalslices in the pelvis were put back in place and sectional sliceswere removed from the upper chest region to allow place-ment of the Au foil holder and moderator 40 cm superior tothe isocenter �along the longitudinal axis�. A photograph ofthe experimental setup for out-of-field measurements isshown in Fig. 4.

It was impossible to measure the spectra for each organlocation due to the size of the measurement apparatus. Thefollowing assumptions were therefore employed for data col-lection: The neutron spectra at locations close to the iso-center would be similar to the neutron spectrum measured atthe isocenter, and that the neutron spectra at locations fartherfrom the isocenter �but still in the patient plane� would besimilar to the neutron spectrum measured at 40 cm from theisocenter. The above assumption was based on measurements

FIG. 3. The setup for in-field measurements at isocenter with 10 in. Bonner

TABLE II. Summary of the 15 MV and 18 MV conventional �conv� and IMphotoneutron energy, measured at isocenter �iso� and 40 cm superior to isoc

18 MV iso 18

Conv IMRT Conv

� per Gy �ncm−1 Gy−1� 1.58E7 3.43E7 1.43EH*�10� per Gy �mSv Gy−1� 3.13 6.58 2.30Avg. neutron energy �MeV� 0.48 0.53 0.52

sphere.

Medical Physics, Vol. 33, No. 2, February 2006

with 197Au foils irradiated with an equivalent amount ofmoderation at isocenter and 20, 40, and 60 cm superior to theisocenter along the longitudinal axis of the treatment couch.The foils were all irradiated with 5000 MU. The counts fromeach foil were determined and corrected for decay. Therewas approximately a 10% difference between measurementsat the isocenter and 20 cm, at 20 cm and 40 cm, and at40 cm and 60 cm. If the 197Au foils are exposed in a mod-erator to the same beam and the resulting foil activities aresimilar, then it is plausible that the neutrons have similarenergy distributions regardless of location. So, ambient doseequivalents measured at the isocenter were applied to organsin close proximity, �20 cm from the isocenter �bladder, co-lon, gonads, and femoral heads�. Ambient dose equivalentsmeasured at 40 cm superior to the isocenter were utilized forall other organs. As stated above, the neutron component ofequivalent dose for each organ was then approximated bycorrecting the ambient dose equivalents for depth in tissue.

C. X-ray component of equivalent dose

The x-ray dose to each organ was determined by TLDmeasurement, DVH data, or from a combination of the bothTLD measured and DVH calculated data. For organs farfrom the treatment volume, a large dose was required todetect measurable scatter and or leakage radiation; 45 Gywas delivered for consistency with the neutron measure-

eutron ambient dose equivalents, H*�10� and neutron fluences, �, average�40�.

40 cm 15 MV iso 15 MV 40 cm

IMRT Conv IMRT Conv IMRT

3.04E7 9.44E6 1.92E7 6.88E6 1.66E75.38 1.85 3.59 1.04 2.860.4 0.47 0.5 0.59 0.44

FIG. 4. The setup for out-of-field measurements at 40 cm superior to iso-

RT nenter

MV

7

center with 10 in. Bonner sphere.

364 Howell et al.: Effective Dose from IMRT 364

ments. However, 45 Gy would saturate TLDs in close prox-imity to the treatment volume. Thus, DVH data were used todetermine the dose to the bladder, rectum, and femoralheads. The dose to the colon was determined by using avolume weighted average of the mean dose to the rectumreported on the DVH and the mean dose determined from theTLD measurements for the ascending, transverse, and de-scending colon.

Harshaw TLD-700 1�1�6 mm3 rods �LiF 99.93% 7Li�are insensitive to neutrons and were used for this work. Atotal of 116 TLDs were loaded into the phantom at variousorgan locations previously defined by the organ map: Brain,salivary glands, spinal cord, thyroid, esophagus, lungs,breasts, heart, liver, stomach, spleen, pancreas, kidneys, co-lon �ascending, transverse, and descending�. In addition, 4TLDs were placed on the phantom exterior at locations rep-resenting the gonads. Another 10 TLDs were placed on dif-ferent regions of the phantom surface to estimate skin dose.Some organs, such as the salivary glands, are very small andcould only accommodate a few TLDs, whereas large organs,such as the liver and lungs, could accommodate a largernumber of TLDs. It is difficult to assess error in the TLDdata because of the variability in the number of TLDs as-signed to each organ. Additionally, in large organs, such asthe colon, some TLDs were closer to the treatment volumewhile others were farther away. All TLDs used in this workwere from the same batch and were always read and an-nealed as a group. Reproducibility error within this batch ofTLDs was less than 3.5%.

After the phantom was loaded with TLDs, it was then setup on the treatment couch using external fiducial marks androom lasers. This experimental setup was used for the deliv-ery of IMRT and conventional treatment plans for 6 MV,15 MV, and 18 MV �6 sets of measured data, 130 TLD read-ings for each�. A total of 45 Gy was delivered to the iso-center for each measurement. Gonad TLDs on the phantomexterior were removed following the delivery of 1.8 Gy �dueto their close proximity to the treatment fields�. The remain-ing 43.2 Gy was subsequently delivered.

After irradiation, TLDs were preheated �in an annealingoven� for 10 min at 100 C and then read using a HarshawTLD reader, model 3500. TLDs were heated at a rate of5 C per second to a reading temperature of 300 C. TLDswere annealed for reuse at 400 C for 1 h, followed by 100 Cfor 2 h.

The mean TLD reading for each organ was converted todose using a single nC-to-Gy calibration factor. The nC-to-Gy calibration factor was determined for the TLD batchby exposing TLDs to known doses on the linac using 6 MVphotons. TLD response linearity was determined by exposingthe TLDs with a series of different doses: 5, 10, 20, and40 cGy �10 measurements for each dose�. TLDs were readusing the protocol described above. All TLDs used for thecalibration were from the same batch that was used in theexperimental data collection. The percent error for the cali-bration TLDs was between 1.5 and 3.4%. It was thus as-

sumed that the error for this batch of TLDs was less than

Medical Physics, Vol. 33, No. 2, February 2006

3.5% �as stated above�. A calibration curve was fitted, givinga straight line �R2 value=0.998� for the response of theTLDs to the different doses.

The radiation weighting factor is unity for photons. As aresult, the x-ray dose is simply equal to the x-ray componentof equivalent dose for each organ.

D. Effective dose calculation

The equivalent dose �Eq. �4�� was calculated for each or-gan by summing the x-ray and neutron components. Tissueweighting factors, WT, from ICRP-60 were used to calculatethe effective dose, E, as follows:13

E = WT · HT. �5�

The following organs have a designated WT values:esophagus, breast, thyroid, lung, stomach, liver, skin, gonads,colon, bone marrow, and bladder.13 ICRP publication 60gives a weight of 0.05 to the “remainder organs.” The ICRPdefines the remainder organs as adrenals, brain, small intes-tine, kidney, muscle, pancreas, spleen, thymus, extrathoracicairways, and uterus.13,10 However, not all of these organscould be delineated based on the visible landmarks in thephantom. For the purpose of this investigation, the remainderorgans were taken as the average of the data for all organsthat were contoured, but did not have designated WT values�brain, salivary glands, spinal cord, heart, spleen, pancreas,and kidneys�. Bone marrow dose was calculated based on theactive red bone marrow distribution in a normal 40-years-oldmale.14 For the purpose of this study, the components of thebone marrow dose were approximated using data measuredfor the brain, breast, heart, spinal cord, and femoral heads toapproximate the head, upper limb girdle, sternum/ribs, verte-brae, and sacrum/lower girdle, respectively. The data wereweighted according to active marrow content. The percent-ages of active bone marrow in the head, upper limb girdle�scapulae, clavicles, humeral heads�, sternum, ribs, vertebrae,sacrum, and lower girdle �pelvis, femoral head� are 13.1%,8.3%, 2.3%, 7.9%, 3.4%, 14.1%, 10.9%, 13.9, and 26.1%,respectively.14

III. RESULTS AND DISCUSSION

A. Neutron component of equivalent dose

The unfolded neutron spectra for 15 MV and 18 MV con-ventional and IMRT treatment plans delivered at the iso-center and measured at the isocenter and 40 cm superior tothe isocenter are shown in Fig. 5. The spectra data were usedto calculate the average neutron energy for conventional andIMRT at both measurement locations for 15 MV and 18 MV,see Table II. The average neutron energy varied form0.4 MeV to 0.59 MeV, with a mean value of approximately0.5 MeV. Swanson15 reported that photoneutron spectra arerelatively independent of primary electron-beam energy,ranging from 0.5 to 0.8 MeV for 10–18 MV x-ray beamsafter filtration through the accelerator head. More recent datafrom d’Errico et al.12 are in good agreement with Swanson’s

reported energy values. The average energy of 0.5 MeV is

365 Howell et al.: Effective Dose from IMRT 365

also consistent with the average energy range of0.2 to 2 MeV reported in NCRP-79 for neutrons transportedthrough the accelerator head.16

The neutron fluence and ambient dose equivalents for15 MV and 18 MV conventional and IMRT treatments mea-sured the isocenter and 40 cm superior to the isocenter aregiven in Table II. The 18 MV neutron fluences are nearlytwice the 15 MV neutron fluences. The data show an in-crease in the secondary neutron dose for high-energy IMRTin comparison to conventional radiotherapy for the sametreatment energy. The ambient dose equivalent for the18 MV conventional treatment measured at 40 cm from theisocenter is 2.3 mSv per photon Gy. This is consistent withdata in the literature. Neutron doses from conventional radio-therapy measured at 50 cm from isocenter for an 18 MVbeam ranging from 0.8 to 2.5 mSv/Gy are reported inNCRP-79 �however, these are for the AECL Saturneaccelerators�.16 A neutron dose of 2.3 mSv/Gy is reportedfor an 18 MV beam measured at 50 cm from isocenter for a10�10 conventional field delivered using a Varian accelera-tor by Vanhavere et al.17

In Table III, neutron fluences per MU measured at theisocenter and 40 cm superior to the isocenter are comparedto neutron fluences per MU reported by Kry et al.18 mea-sured at the isocenter and at the 30 cm superior to the iso-

TABLE III. Neutron fluences per MU measured at isoto neutron fluences per MU reported in the literature

Howella

18 MV 15 MV

Iso 40 cm Iso 40 cm

1.30E5 1.20E5 7.20E4 6.30E4

aFrom current study.bFrom Kry et al. �Ref. 18�. These data were reported

isocenter with 15 MV and 18 MV beams from Varian lina

Medical Physics, Vol. 33, No. 2, February 2006

center. Fluences agree to within 20%, which is quite goodgiven the different measurement techniques and other vari-ability between machines.

Table IV gives the neutron component of equivalent dosefor each organ in Sv per photon Gy delivered to isocenter.These data were determined by applying a depth dose cor-rection based on average beam energy of 0.5 MeV. Thesevalues are lower than those reported by Kry et al.,18 with thedissimilarity being attributed to using a lower average energyfor depth dose corrections. The depth dose falls off muchfaster for lower neutron energies. Poor agreement is seenbetween the neutron organ data and that reported by Vanha-vere et al. �no organ depth information is provided, making atrue comparison of the data unrealistic�.17

B. X-ray component of equivalent dose

Table V gives the x-ray component of equivalent dose foreach organ in units of Sv per photon Gy delivered to theisocenter. The doses to the bladder, colon, and bone marroware all greater for the conventional plans. This is primarily aresult of the beam intensity optimization which results in fulldose coverage of the PTV, while minimizing the dose tocritical normal structures, i.e., bladder, rectum, and femoralheads �largest component of bone marrow�. The gonads

FIG. 5. The unfolded neutron spectra for 15 MV and18 MV conventional and IMRT delivered at isocenterand measured at isocenter and 40 cm superior toisocenter.

r and 40 cm superior to isocenter �iso� are compared

Kryb

18 MV 15 MV

Iso 30 cm Iso 30 cm

1.20E5 1.04E5 7.50E4 5.40E4

measurements at isocenter and at 30 cm superior to

cente.

for

cs.

366 Howell et al.: Effective Dose from IMRT 366

which are not in the field, but very near the PTV also receiveless dose from the IMRT treatment plans. The field edges ofindividual IMRT beamlets are farther away from the gonadsthan the static conventional MLC field borders, resulting in alower gonadal dose.

X-ray dose to the patient, but far outside the treatmentfield �peripheral dose�, is a function of several factors includ-ing distance from field edge, field size, beam energy, anddepth.18–21 The sources of dose outside the treatment beam

TABLE IV. Neutron component of organ equivalentAmbient dose equivalents measured at isocenter wevolume �bladder, colon, gonads, and bone marrow�isocenter were applied to all other organs. The neutroapproximated by correcting the ambient dose equivaledose data from d’Errico et al. �Ref. 12�.

Organ

Neutron�Sv per

15 MV

Conv

Esophagus 2.28E−05 6Breast 8.45E−04 2

Thyroid 8.45E−04 2Lung 2.99E−05 8

Stomach 5.76E−05 1Liver 1.32E−04 3Skin 1.04E−03 2

Remaindera 1.30E−04 3Gonads 1.85E−03 3Colon 1.34E−04 2

Bone marrow 8.56E−05 2Bladder 2.04E−04 3

aFor the purpose of this investigation, the remainder othat were contoured, but did not have designated WT

pancreas, and kidneys�.

TABLE V. X-ray component of organ equivalent dose

Organ

X-ray com�Sv per unit

6 Mv

Conv IMRT

Esophogas 3.68E−4 6.97E−4 2Breast 3.63E−4 7.58E−4 2

Thyroid 3.13E−4 7.25E−4 1Lung 4.00E−4 7.45E−4 2

Stomach 6.66E−4 1.05E−3 5Liver 7.52E−4 1.18E−3 5Skin 6.76E−4 1.19E−3 5

Remaindera 5.78E−4 1.01E−3 4Gonads 4.39E−2 2.08E−2 4Colon 7.06E−2 6.63E−2 6

Bone marrow 3.933E−01 1.611E−01 3.8Bladder 8.267E−01 6.800E−01 8.2

aFor the purpose of this investigation, the remainder othat were contoured, but did not have designated WT

pancreas, and kidneys�.

Medical Physics, Vol. 33, No. 2, February 2006

are scatter from the collimating jaws, scatter from the usefultreatment beam within the patient, and photon leakagethrough the head of the linac. Peripheral x-ray dose isstrongly dependent on the distance from the field edge. Thisdistance dependence of peripheral x-ray dose is seen in thedata reported herein. At large distances from the field edge,the major component of x-ray dose is head leakage. Thex-ray doses to lung, esophagus, breast, and thyroid �all�40 cm from isocenter� from IMRT are approximately

in Sv per unit photon Gy delivered to isocenter.plied to organs in close proximity to the treatmentbient dose equivalents measured 40 cm superior tomponent of equivalent dose for each organ was thenor the depth of each organ by applying neutron depth

ponent of organ equivalent dosehoton Gy delivered to isocenter�

18 MV

T Conv IMRT

−05 5.00E−05 1.17E−04−03 1.86E−03 4.34E−03−03 1.86E−03 4.34E−03−05 6.56E−05 1.54E−04−04 1.26E−04 2.96E−04−04 2.90E−04 6.80E−04−03 2.30E−03 5.38E−03−04 2.86E−04 7.92E−04−03 3.13E−03 6.58E−03−04 2.27E−04 4.78E−04−04 1.88E−04 4.40E−04−04 3.45E−04 7.26E−04

s were taken as the average of the data for all organses �brain, salivary glands, spinal cord, heart, spleen,

v per unit photon Gy delivered to isocenter.

nt of organ equivalent doseon Gy delivered to isocenter�

15 MV 18 MV

IMRT Conv IMRT

−4 4.90E−4 2.53E−4 5.55E−4−4 4.85E−4 2.15E−4 5.06E−4−4 4.36E−4 1.84E−4 4.71E−4−4 5.31E−4 2.82E−4 5.97E−4−4 8.15E−4 4.63E−4 8.86E−4−4 9.75E−4 5.28E−4 9.61E−4−4 9.84E−4 5.54E−4 1.00E−3−4 7.56E−4 4.04E−4 7.98E−4−2 1.72E−2 4.43E−2 1.59E−2−2 6.56E−2 6.77E−2 6.32E−2−01 1.678E−01 3.611E−01 1.689E−01−01 6.844E−01 7.978E−01 6.822E−01

s were taken as the average of the data for all organses �brain, salivary glands, spinal cord, heart, spleen,

dosere ap. Amn conts f

comunit p

IMR

.22E

.31E

.31E

.16E

.57E

.61E

.86E

.55E

.59E

.60E

.34E

.96E

rganvalu

in S

ponephot

Conv

.45E

.30E

.78E

.77E

.15E

.53E

.38E

.26E

.78E

.97E00E44E

rganvalu

8 M

367 Howell et al.: Effective Dose from IMRT 367

double the x-ray doses from conventional, scaling approxi-mately with MU. At distances from less than 40 cm from thefield edge the dose is distributed from scatter from inside thepatient and head leakage.20 Both the stomach and liver arebetween 30 and 40 cm from the isocenter. The x-ray dose tothese organs is greater from IMRT than conventional; themagnitude of the difference is approximately 1.6.

Peripheral x-ray doses are compared to 6 MV, 15 MV,and 18 MV data reported by Kry et al.,18 and to 18 MV datareported by Vanhavere et al.17 for prostate IMRT treatmentson Varian accelerators, Table VI. X-ray doses are comparedfor the following organ locations: liver, stomach, lung,esophagus, and thyroid. Kry et al.18 reported dose to the

TABLE VII. Organ equivalent doses �shaded in light grey� in units of Sv perand x-ray components of equivalent dose for each organ. The effective doseproduct of equivalent dose and the appropriate tissue weighting factor for a

Organ WT

�S

6 MV

Conv IMRT

Esophagus 0.05 3.68E−04 6.97E−04Breast 0.05 3.63E−04 7.58E−04

Thyroid 0.05 3.13E−04 7.25E−04Lung 0.12 4.00E−04 7.45E−04

Stomach 0.12 6.66E−04 1.05E−03Liver 0.05 7.52E−04 1.18E−03Skin 0.01 6.76E−04 1.19E−03

Remainder 0.05 6.22E−04 1.13E−03Gonads 0.2 4.39E−02 2.08E−02Colon 0.12 7.06E−02 6.63E−02

Bone marrow 0.12 1.58E−01 6.50E−02Bladder 0.05 8.27E−01 6.80E−01

Effective dose 7.78E−02 5.43E−02

TABLE VI. Comparison of peripheral x-ray doses mealiterature for 6 MV, 15 MV, and 18 MV. X-ray dosestomach, lung, esophagus, and thyroid. All data are i

Organ

Peripherorgan equivalent d

Howella

6 MV 15 MV 18 MV

Esophogas 2.20 1.85 2.17Thyroid 2.29 1.65 1.84

Lung 2.35 2.01 2.34Stomach 3.31 3.08 3.47

Liver 3.72 3.69 3.76

aData from current study.bFrom Kry et al. �Ref. 18�. Data reported dose to thethe center location data are used for comparison withcomparison with current work.cFrom Vanhavere et al. �Ref. 17�. Data reported for 1

Medical Physics, Vol. 33, No. 2, February 2006

center and edge positions for the liver, stomach, and lung;the center location data are used for comparison with currentwork.1 All data have been given in units of Sv per MU.Good agreement is seen with the Kry et al.18 data for theesophagus, thyroid, and lung doses. Stomach and liver dosesare of the same magnitude, but were slightly less than 50%of data reported by Kry et al.18 This inconsistency is attrib-uted to the difference in approximated organ locations of thestomach and liver reported here as 32 cm from isocenterwhile Kry et al.18 reported these organs to be closer to iso-center at a distance of 25 cm. All peripheral x-ray dosesreported by Vanhavere et al.,17 except thyroid, are two tofour times lower than values reported here or by Kry et al.18

photon Gy delivered to isocenter were calculated by summing the neutronaded in dark grey� were calculated, for each beam energy, by summing theans.

Organ equivalent doseunit photon Gy delivered to isocenter�

15 MV 18 MV

Conv IMRT Conv IMRT

2.68E−04 5.52E−04 3.03E−04 6.72E−041.07E−03 2.79E−03 2.07E−03 4.85E−031.02E−03 2.74E−03 2.04E−03 4.82E−033.07E−04 6.13E−04 3.48E−04 7.51E−045.73E−04 9.72E−04 5.89E−04 1.18E−036.85E−04 1.34E−03 8.18E−04 1.64E−031.58E−03 3.84E−03 2.85E−03 6.38E−035.78E−04 1.15E−03 7.06E−04 1.62E−034.97E−02 2.08E−02 4.74E−02 2.25E−026.98E−02 6.59E−02 6.79E−02 6.37E−021.52E−01 6.78E−02 1.45E−01 6.84E−028.25E−01 6.85E−01 7.98E−01 6.83E−017.81E−02 5.51E−02 7.54E−02 5.54E−02

in this study to peripheral x-ray doses reported in thecompared for the following organ locations: Liver,

ts of Sv per MU delivered.

ans x-ray component ofrom IMRT �Sv per monitor unit�

Kryb Vanhaverec

6 MV 15 MV 18 MV 18 MV

3.20 2.60 2.70 8.842.60 1.80 2.30 0.423.70 3.80 3.60 8.428.20 8.00 7.80 16.008.20 7.90 8.10 13.05

r and edge positions for the liver, stomach, and lung;ent work. Data reported for Varian linac are used for

V beam from Varian linac.

units �shll org

v per

sureds aren uni

al orgose f

centecurr

368 Howell et al.: Effective Dose from IMRT 368

C. Organ equivalent dose and effective dosecalculation

Organ equivalent doses are calculated by summing theneutron and x-ray components equivalent dose for each or-gan and are shown in Table VII. Organs far from the treat-ment volume received higher equivalent doses from IMRT.The effective doses are also given in the last row of TableVII. Effective doses were higher for conventional radio-therapy compared to IMRT for all beam energies. IMRTgreatly reduces dose to nearby organs, such as femoralheads, gonads, bladder, and colon, thereby yielding lowereffective doses compared to conventional radiotherapy.Among the IMRT plans, the 6 MV beam energy results inthe lowest effective dose.

IV. CONCLUSION

Although IMRT resulted in an increased photon dose tomany organs, the data reported herein show that IMRT re-sulted in an overall decrease in effective dose compared toconventional radiotherapy. This decrease correlates with theability of an intensity modulated field to minimize dose tocritical normal structures in close proximity to the treatmentvolume. An evaluation of the three beam energies used forIMRT, reveals that 6 MV resulted in the lowest effectivedose, while 18 MV resulted in the highest effective dose.This is attributed to the large neutron contribution for 18 MVcompared with no neutrons for 6 MV.

ACKNOWLEDGMENTS

This work was funded in part by a seed grant from theGeorgia Cancer Coalition: The authors would like to expresstheir gratitude to Eric Burgett from Georgia Institute of Tech-nology for his help with the detection and calibration equip-ment. The authors would also like to thank Cindy Bryant,M.D., and Daniel L. Howell, M.D. for the many hours theyspent contouring the phantom organs.

a�Work performed �in partial fulfillment for a Ph.D. in Medical Physics� atthe Department of Radiological Sciences, University of Texas HealthScience Center, San Antonio, TX; electronic mail:rebecca@radonc.emory.org

1R. M. Howell, M. S. Ferenci, N. E. Hertel, and G. D. Fullerton, “Inves-tigation of secondary neutron dose for high-energy IMRT,” Med. Phys.32�3�, 786–793 �2005�.

2International Commission on Radiation Units and Measurements, “Tissuesubstitutes in radiation dosimetry and measurement,” ICRU Report 44,ICRU, Bethesda, MD, 1989.

3J. Sweezy, N. E. Hertel, K. G. Veinot and, R. A. Karam, “Performance of

multisphere spectrometry systems,” Radiat. Prot. Dosim. 78,

Medical Physics, Vol. 33, No. 2, February 2006

263–272 �1998�.4J. J. Doroshenko, S. N. Kraitor, T. V. Kuznetsova, K. K. Kushnereva, andE. S. Leonov, “New methods for measuring neutron spectra with energyfrom 0.4 to 10 MeV by track and activation detectors,” Nucl. Technol.33, 296–304 �1977�.

5T. L. Johnson, Y. Lee, K. A. Lowry, and S. G. Gorbics, “Recent advancesin Bonner sphere neutron spectrometry,” Proceedings of the Topical Con-ference on Theory and Practices in Radiation Protection and Shielding,22–24 April 1987, Knoxsville, TN, Sponsored by the Radiation Protectionand Shielding Division of the American Nuclear Society, pp. 83–92.

6N. E. Hertel and J. W. Davidson, “The Response of Bonner spheres toneutrons from thermal energies to 17.3 MeV,” Nucl. Instrum. MethodsPhys. Res. A 238, 509–516 �1985�.

7M. Reginatto, “The “few-channel” unfolding programs in the UMG pack-age: MXD�FC33, GRV�FC33 and IQU�FC33,” UMG package, version3.3-release date: March 1, 2004.

8X-5 Monte Carlo Team, 2003, MCNP—A General Monte CarloN-particle transport code, Version 5, LA-UR-03-1987, Los Alamos Na-tional Laboratory, Los Alamos, NM, 2004.

9H. Schumacher and B. R. L. Siebert, “Quality factors and ambient doseequivalent for neutrons based on the new ICRP recommendations,” Ra-diat. Prot. Dosim. 40�2�, 85–89 �1992�.

10International Commission on Radiation Protection and Measurements,“ICRP Publication 74: Conversion coefficients for use in radiologicalprotection against external radiation,” Ann. ICRP 26�3�, �1996�.

11International Commission on Radiation Units and Measurements, “Con-version coefficients for use in radiological protection against external ra-diation,” ICRU Report 57, ICRU, Bethesda, MD, 1998.

12F. d’Errico, M. Luszik-Bhadra, R. Nath, B. R. L. Siebert, and U. Wolf,“Depth-dose equivalent and effective energies of photoneutrons generatedby 6–18 MV x-ray beams for radiotherapy,” Health Phys. 80�1�, 4–11�2001�.

13International Commission on Radiation Protection and Measurements,“ICRP Publication 60: 1990 Recommendations of the International Com-mission on Radiological Protection,” Ann. ICRP 21, 1–3 �1990�.

14R. E. Ellis, “The distribution of active bone marrow in the adult,” Phys.Med. Biol. 5, 255–258 �1961�.

15W. P. Swanson, “Estimate of the risk in radiation therapy due to unwantedneutrons,” Med. Phys. 7, 141–144 �1980�.

16National Council on Radiation Protection and Measurements, “NeutronContamination from Medical Electron Accelerators,” NCRP-79. Wash-ington, DC, 1984.

17F. Vanhavere, D. Huyskens, and L. Struelens, “Peripheral neutron andgamma doses in radiotherapy with an 18 MV linear accelerator,” Radiat.Prot. Dosim. 110�1–4�, 607–612 �2004�.

18S. F. Kry, M. Salehpour, D. S. Followill, M. Stovall, D. A. Kuban, R. A.White, and I. I. Rosen, “Out-of-field photon and neutron dose equivalentsfrom step-and shoot intensity-modulated radiation therapy,” Int. J. Radiat.Oncol., Biol., Phys. 62�4�, 1204–1216 �2005�.

19D. Followill, P. Geis, and A. Boyer, “Estimates of whole-body doseequivalent produced by beam intensity modulated conformal therapy,”Int. J. Radiat. Oncol., Biol., Phys. 38�3�, 667–672 �1997�.

20American Association of Physicists on Medicine, “Fetal dose from radio-therapy with photon beams: Report of AAPM Radiation Therapy Com-mittee Task Group No. 36,” Med. Phys. 22�1�, 63–82 �1995�.

21S. F. Kry, M. Salehpour, D. S. Followill, M. Stovall, D. A. Kuban, R. A.White, and I. I. Rosen, “The calculated risk of fatal secondary malignan-cies from intensity-modulated radiation therapy,” Int. J. Radiat. Oncol.,

Biol., Phys. 62�4�, 1195–1203 �2005�.