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McGill University Health CentreCentre de Santé de l’Université McGill563-613B L#3 MDCE Sep24”04

Health Physics563-613B

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Risk and Dose Limits

Michael EvansLecture 3 (Sep. 24, 2004)


RISK

Hazard: anything that can cause harm ( e.g., chemicals, electricity, working from ladders, etc..

Risk: is the chance, high or low, that somebody will be harmed by the hazard

American Cancer Society

www.cancer.org


Cause-consequenceAnalysis



Cause-consequence analysis revolves around decision trees and the assumption that truly independent variables contribute to occurrences and outcomes. That is, what independent things must conspire together to bring about an event, and having occurred, what are the

possible outcomes.


RISKFault Tree

A "fault" tree is effectively a statement of what events have to conspire together to bring about an undesired outcome. Traditionally these have been drawn top-down and therefore the undesired event known as the "top event". Because of the logical hierarchy of the items, it can be sometimes seen as a form of time sequence going from the bottom towards the top of the page.




RISK : Differing Perceptions






Nuclear Industry Advertisement @ 1970s

Many different types of radiation may have different effects on cancer rates.Experiments with humans cannot be set up so effects are not well known.Different age and sex groups vary in sensitivity.A single big dose has a greater net effect over time than the same dose spread out over small increments.Extrapolation from high doses: we don't know how to extrapolate to low doses.




RISK: Perception

Risk is the probability of consequence.The perception of risk is determined by how the individual views its probability and its severity


RISK: Perceptionand Decision Tables


RISK PERCEPTION

Smoking ( 45,000 deaths per year - Canada)

Smoke Alarms (368,000 home fires US, 16,975 injuries and $5.5 billion in property damage)

Fire Extinguishers (85% of fire-related deaths occur in home fires)

Radon (from 7,000 to 30,000 deaths per year -USEPA)

Driving (Nearly 3,000 people died and 217,614 were injured Canada)




RISK

1. Understanding Risk (Fischoff et al.)

2. Assessing risk involves decisions.

3. No approach is comprehensive - there are always biases.

4. Personal beliefs affect our interpretation of fact.

5. determining factors in risk decisions relate to the definition of the problem ( consequences, uncertainties…).

6. Decisions may be influenced through subtleties in the wording of the problem or the question.


Data Interpretation:Radon


Radon- Canadian recommendations

Although provincial and territorial governments have jurisdiction over the health effects of background radiation, Health Canada has recommended that the guideline for exposure to radon gas should be 800 becquerels per cubic metre as the annual average concentration in a normal living area. The guideline is an upper limit, and Health Canada recommends taking action to reduce radon levels in your home if they exceed the limit. Because there is some risk at any level of radon exposure, homeowners may want to reduce their exposure to radon, regardless of levels.


Radon- US EPA recommendations

Radon is a radioactive gas and has been identified as a leading cause of lung cancer, second only to cigarette smoking in the United States. EPA's most recent health risk assessment estimates that 21,000 lung cancer deaths each year are due to radon.Test your home for radon, and have it fixed if it is at or above EPA's Action Level of 4 picocuries per liter. You may want to take action if the levels are in the range of 2-4 picocuries per liter. Generally, levels can be brought below 2 pCi/l fairly simply.


Health Effects : Radiation

Possible health effects from exposure to radiation may include cancer, birth defects in future generations and cataracts. These effects have been observed in studies of medical radiologists, uranium miners, radium dial painters, radiotherapy patients and atomic bomb survivors

Observations are based on doses that are much higher than workers who are occupationally exposed. The Linear-non-threshold theory has not been reasonably proven down to low doses, and there have been no strong cause-effect studies showing relationships between low doses and health effects.

Still prudent to assume that there are some health effects at lower doses



Genetic damage may be caused by low doses although no direct evidence of this exists.

Acute radiation exposure may cause both prompt and delayed effects.

Chronic exposure ( small doses delivered over long time periods) may cause delayed effects but not prompt effects.



Prompt effects: observable after large doses in a short period of time. 450 cSv in one hour will cause vomiting and diarrhea in a few hours; loss of hair fever and weight loss within a few weeks: 50 % chance of death within 60 days without medical treatment (LD 50-60).

Delayed effects such as cancer may occur years after exposure to radiation.

Genetic effects can occur when there is damage to the genetic material. Effects may show up in future generations although to date there have been no clear observations indicating genetic effects caused by radiation in human populations.


Cancer Induction

-Radiation can damage cell chromosomes causing the cell to undergo uncontrolled growth (malignancy).

- Radiation may suppress the bodies’ natural immune system causing a reduce resistance to existing viruses which can multiply and damage cells

- Radiation may activate viruses dormant in the body which then attack cell control mechanisms causing rapid growth


Effects

Deterministic effects : describe organ killing or dysfunction that is thought to have a certain threshold dose above which the effect is almost certain to appear.

Stochastic effects : describe random effects which can be predicted for,populations and not for individuals. They are usually applied to low dose & dose-rate situations.

Stochastic effects may occur in the individual and are termed somatic ( cancer induction ). Stochastic effects which are passed on to future generations in the germinal cells are hereditary or genetic.


Risks of RadiationDose(mSv) vs Possible Effects

10 000 instantaneous Immediate illness and possible death if untreated.

1 000 instantaneous Illness (nausea) but death unlikely.1 000 chronic Long term cancer induction in one in

20 - 25 persons.

50 No evidence of consequences - NEW yearly limit CNSC.

10 Natural background level for some populations - India.

2.2 Average annual dose received by all Canadians. No observable effects.

1 Yearly limit to the public with respect to CNSC licensed facilities.


Risks of Radiation

Dose ( mSv) Possible Effects

5000 - 8000 to gonads Permanent sterility in males and females

200 to testes Temporary reduction in sperm count

No effect on sexual function

< 50 whole body No observed effect on fertility sexual function


Population groups

Japanese A-bomb- acute victims and survivors

Patients exposed to radiation for medical reasons - TB using lung and thymus irradiation- ankylosing spondilitis- secondary cancers from radiation therapy (Hodgkins, contralateral breast…)- Thorotrast and Radium 224 studies

Workers- uranium miners- radium dial painters- U.S. Nuclear Navy Shipyard (NNS) workers


Population groups

Japanese A-bomb-dosimetry is unsure-single ethnic population makes worldwide extrapolation difficult-high dose & dose-rate

Patients exposed to radiation for medical reasons -data is often anecdotal-genetic effects are not reliably recorded

Workers-other factors such as cigarette smoking, toxins in

the work environment may confound results


Population data from lab studies


Linear No-Threshold Theory(LNT)

Extrapolation from the high dose region down to the low dose region causes problems in estimating the effects of low-level radiation


Model Fitting

No Threshold

Classic LNT (Linear No-Threshold Theory)

Parabolic limiting

Hormesis




Central Requirements for Radiation Protection

ICRP 26:

1 No practice shall be adopted unless its introduction produces a net benefit

2 All exposures shall be kept as low as reasonably achievable, economic and social factors being taken into account (ALARA)

3 The dose equivalent to individuals shall not exceed the limits recommended for the appropriate circumstances by the Commission

Benefits Detriment


Central Requirements for Radiation Protection

ICRP 60 § 115 - 129 :

Justification (net benefit)

Optimization (ALARA)

Dose Limits

Potential exposures

Benefits Detriment


Basic Units

Exposure» Total charge of ions of one sign produced in air» C/kg, 1 R = 1 esu/cm3 air = 2.58 x 10-4 C/kg

Absorbed dose» Energy absorbed per unit mass» 1 Gy = 1 J/kg = 100 rad

– 1 cGy ≈ 1 R (numerically)


Equivalent dose

• Not all radiation produces the same endpoint– 1 mGy of photons or 1 mGy of neutrons

– radiation weighting factors used to modify the absorbed dose to express the same levels of risk between radiations

– units: 1 Sv = 1 J/kg = 100 rem

n


Effective dose

Not all tissues have the same risks» 1 mSv on lungs or 1 mSv on the large intestine

» tissue weighting factors used to modify the equivalent dose to express the same levels of risk between tissues

» units: 1 Sv = 1 J/kg = 100 rem


Estimates of Cancer Incidencefrom Low-Level Radiation

Number of additional cancers estimated to occur in 1 million people after exposure of each to 10 mSv of radiation

SourceBEIR, 1980 160 - 450

ICRP, 1977 200

UNSCEAR, 1977 150 - 350

Average for this example - 300


Estimates of Cancer Incidencefrom Low-Level Radiation

American Cancer Society estimates that 25% of all adults in the 20 - 65 year group will develop cancer from all causes ( smoking, food, drugs, lifestyle …).

A population of 10, 000 not occupationally exposed to radiation will express 2,500 cases of cancer.

If all 10, 000 were exposed to 10 mSv we estimate 3 extra cases to develop. (Cancer rate has increased from 25% to 25.03 %).

A lifetime occupational dose of 100 mSv might increase chances to 25.3% and 1000 mSv to 28%. (This assumes a simple linear model).


Risks and Collective Dose

Collective dose is the sum of all dose received by all workers in a defined population.

If 100 workers out of a population of 200 each receive a dose of 2 mSv, the average dose per worker is 1 mSv and the collective dose is 200 person- mSv.

The total additional risk is assumed to depend upon the collective dose.


Risks and Collective Dose

For radiation protection the risk is assumed to be proportional to the amount of dose - not the rate at which it is received.

For a given collective dose the risk is assumed to be the same even if a large number of people share the dose.

Therefore spreading out the dose on a large population may reduce the individual risk, but not that of the whole population.


Estimates of Collective Dose

Source Average individual dose (U.S.)

mSv / yearNatural background 1.0Releases - mining, milling etc. 0.05Medical 0.54Nuclear fallout 0.08Nuclear Energy 0.003Radon Gas & Daughters 1.98

Total 3.652

The average individual in the population receives about 3.6 mSv from natural and man-made environment.


Risk and dose limits

The risk associated with 1 mSv/year is calculated to build up to 1 extra cancer death in 100,000 persons per year after sustaining many such years of exposure. (BEIR, AAPM 18)

Canada 2004

New Cases of Cancer: 458/100,000

Cancer Deaths : 215 / 100,00

Or a 1/1 million chance per 0.1 mSv/year


Comparing Health Risks

Health Risk Days of life expectancy lost

Smoking 20/day 2,370 (6.5 years)Overweight 20% 985 (2.7 years)All accidents 435 (1.2 years)Auto accidents 200Alcohol 130Home accidents 95Drowning 41Natural background radiation 8Diagnostic X-rays 6Catastrophes (earthquakes…) 3.510 mSv occupational 1


Cancer death probabilityICRP 60 C.2

Simple additive model.

The excess probability rate after a single dose, D, is assumed to be proportional to the dose, but first after a minimum latency period and over a ‘plateau” period of time.


Cancer death probabilityICRP 60 C.2

Simple multiplicative model.

The excess probability after a single dose, D, is also assumed to be proportional to the background rate of cancer death , B (u).

Age (u)


Mortality AttributesICRP 60 pp153 & C.8

Choice of dose limits:

Lifetime death probability.Time lost due to deathReduction of life expectancyAnnual distribution of death probabilityIncrease in age specific mortality

C.8 Change in the total conditional death probability rate after an exposure of 50 mSv per year from age 18 to age 65 years.


ICRP 60 (§ 89)Probability Co-efficients for stochastic effects

Exposed Population Detriment

Adult WholeWorkers Population

Fatal cancer 4%/Sv 5%/Sv

Non-fatalcancer 0.8%/Sv 1.0%/Sv

Severe hereditaryEffects 0.8%/Sv 1.3%/Sv

Total 5.6%/sv 7.3%/SvValues between populations differ due to differences in age distributions.


Historical dose limits

Maximum permissible doses have been applied to four population groups:

Occupationally exposed persons

Members of the public near radiation sources

The whole population (average dose)

The developing fetus


Historical dose limits USA

Occupational Maximum Permissible Dose

1934 200 mrem (2 mSv) / day ICRP

1936 100 mrem (1 mSv) / day NCRP

1948 300 mrem (3 mSv) / week NCRP

1957 * 100 mrem (1 mSv) / week NCRP

* similar to current value


ICRP 60 (§ 194)Recommended dose limits

Application Occupational Public

Effective dose 20 mSv/y 1 mSv/yaveraged over definedperiods of 5 years(not to exceed 50 mSv inany single year).

Annual equivalent doseLens of eye 150 mSv 15 mSvSkin 500 mSv 50 mSvHands and feet 500 mSv

Effective dose (E): sum of weighted equivalent doses in all the tissues and organs of the body..Equivalent dose (H): average dose in tissue or organ multiplied by the radiation weighting factor.


“Old” Canadian Dose Limits

Atomic Radiation Control Act Amended SOR/85–355

whole bodygonads, bone marrow

publicmSv/year

atomic radiation workermSv/quarter mSv/year

30 5

bone, skin, thyroid 150 30

extremities (hands, feet)

15lungs, eyes & single organs

10 mSv to abdomen for balance0.6 mSv per two weeks

pregnant ARW

30

50

300

380 750

80 150

*

‡

* effective dose‡ equivalent dose


NRC Dose Limits


Workers Limits

Why are workers limits 20x higher?– monitored by Health Canada– most do not actually receive this limit– public includes children– occupational hazard– benefit of working– radiation workers are a small fraction of the total

population and the additional health burden on society is acceptable


Occupational exposures

This can all seem a bit abstract but…


Occupational exposures

Radiation is invisible to our senses of sight, feel, hear, taste, smell.


ALARA / ALARP

As Low As Reasonably Achievable / Practicable( economic and social factors being taken into

account)

The key word is “reasonable”. In radiation safety there are three basic principles.

Distance : 1/d 2

Time : 1/t

Shielding : e -x


ALARA / ALARP

Different safety conditions require a judicious choice of these principles.

Brachy (sources) : Distance , Time , Shielding

Diagnostic : Distance , Shielding

Therapy : Shielding, Distance , Time

Shielding is often the intuitive choice but not always the best. It can give a false sense of security, slow down safe practices and even create

unwanted radiation in the case of - emitters.


Cost-Benefit AnalysisICRP 37 § 30

In the case of the introduction of a practice involving radiation exposures, the net benefit to society can be ideally expressed as:

B = V - (P + X + Y) where

B is the net benefit from the introduction of the practice,V is the gross benefit,P is all production costs excluding radiation protection costs (materials, salaries, infrastructure…),X is the cost of achieving a select level of radiation protection (shielding, detectors, salaries…),Y is the cost of the detriment due to the level of radiation protection (psychological and social factors …).

…UNITS?….



Optimizing

B = V - (P + X + Y)

Involves an interplay of production and detriment costs such that their sum

X()+ Y() is a minimum.The cost of protection and the cost of the detriment are functions of the level of protection (), - e.g. shielding thickness, ventilation rate, alternate options of protective equipment).Generally assumed that V and P are independent of .

§ 34 - optimization is an intuitive process



Concept of detriment developed to quantify all deleterious effects due to exposure to ionizing radiation. These effects do not include other harmful effects caused by conventional injuries (burns, crush, toxins …) that may be incurred in obtaining the selected level of radiation protection.

It is difficult to assign $ values to all variables ( V , the gross benefit and Y, the detriment include other elements such as stress).

Optimizing B = V - (P + X + Y) may vary from simple to complex.


Risk & Cost-Benefit AnalysisICRP and NCRP

ICRP Publication 45: Quantitative Bases for Developing a Unified Index of Harm, 1986.

ICRP Publication 83: Risk Estimation for Multifactorial Diseases, 2000.

ICRP Publication 37: Cost Benefit Analysis in the Optimization of Radiation Protection, 1983

NCRP 139: Risk-Based Classification of Radioactive and Hazardous Chemical Wastes (2002)

NCRP 126: Uncertainties in Fatal Cancer Risk Estimates Used in Radiation Protection (1997)

NCRP 115: Risk Estimates for Radiation Protection (1993)


Radiation Hormesis…its not the poison but the dose…

The A-bomb survivors are living longer than the controls despite the 400+ radiation induced cancer deaths. The increase in cancer deaths caused by radiation is only about 1% of all deaths among the survivors. In the U.S. the cancer death rate varies about 50% from the lowest cancer death rate - Utah - to the highest - Washington DC. A one percent increase in cancer deaths would not be detected in the US.


Radiation Hormesis…its not the poison but the dose…

There have been about 90,000 children and grandchildren of the A-bomb survivors and still no detectable increase in genetic mutations. Fruit flies and mice show genetic mutations but so far, no humans. All animals have repair mechanisms, humans seem to have better than average

At moderate doses, about 10 cSv, the A-bomb survivors had about 100 fewer cancers than the unexposed population - about 1900 cancers for the exposed group and about 2,000 for the controls. This doesn't prove radiation hormesis but it is inconsistent with the LNT model..


Hormesis

In 1925 the radium dial industry forbade painters from touching the brush to their tongue. No person who started work after 1925 ever had radium induced bone cancer! (Do you still think the LNT model makes sense?)

Most of our internal radiation dose comes from natural potassium-40. An adult has about 4,000 Bq (decays per second) or about 20 million per hour! Billions of our cells are irradiated every hour - do you think one more gamma ray may cause cancer?


Galactic Cosmic Radiation


Cosmic Rays

Cosmic ray exposure increases with altitude» air flights are closer to the cosmic ray source

– 5 µSv/hr for commercial air flights– airline flight crews receive 1 mSv/y


Risk reduction

Therac 25 Radiotherapy MachineThe Therac 25 was a radiation therapy system that intermittently gave the wrong radiation doses over a period of three years (1985-87) due to errors in the software controlling its operation. The major problems with the system have been attributed to poor interface design and software failure. Six accidents, three of them fatal, have been attributed to failures in Therac 25.The basic issue involved the replacement of hardware interlocks used in previous models with a software-only system.




Risk reduction

State



www.smi.stanford.edu/.../ humanerrortalk.html

Electron mode involved a low-power electron beam, while photon (X-ray) mode involved a high power electron beam (3 orders of magnitude more powerful), but with a metal plate between it and the patient, to generate the X-rays. The electron beam must be in low-power mode if the plate is not present, and in earlier designs (Therac 6 and Therac 20) there was a mechanical device which physically ensured this. This hardware interlock wasremoved from the Therac 25 which was left to rely on a (faulty) software interlock.


Risk reduction

The software was poorly designed and specified, and much of it was imported as-is from the previous models despitechanges in requirements. The problem was compounded by a complex user interface. In some cases, if the operatortried to enter certain control sequences (either in error or as shortcuts), the machine would operate incorrectly, usingthe high-power beam with no plate. It would then report an error, which it would normally do when no treatment hadbeen delivered, often leading operators to repeat the process.


Fetal Irradiation



Risk of childhood cancer from fetal irradiation R Doll and R Wakeford 1997

The excess relative risk obtained from combining the results of these studies has high statistical significance and suggests that, in the past, a radiographic examination of the abdomen of a pregnant woman produced a proportional increase in risk of about 40%. The excess absolute risk coefficient at this level of exposure is approximately 6% per gray, although the exact value of this risk coefficient remains uncertain.


Radiation & Pregnancy

Effect NaturalFrequency

AdditionalCases After

10 mSvSpontaneous

Abortion3000 to 5000 none

CongenitalAnomalies

600 to 800 none

MentalRetardation

~ 80 none

ChildhoodCancer

20 5

LifetimeFatal Cancer

~ 2500 15

GeneticDefects

~ 2500 2

per10,000births orconceptions


Fetal Irradiation

ICRP 60 pp 90

The effects of antenatal exposure depend on the time of the exposure relative to conception.When the number of cells is small the damage is most likely to be an undetectable death.Exposure in the first three weeks is unlikely to result in deterministic or stochastic effects in the live-born child.During major organogenesis (> 3 weeks) malformations may be formed. These effects are deterministic in character with a threshold in man, estimated from animal experiments, to be about 0.1 Gy ( 100 mSv).


Fetal Irradiation


“10 day/28 day rule”: obsolete


Radon in the early 20th C

“Weak Discouraged Men!Now Bubble Over with Joyous Vitality through the Use of Glands and Radium “




are needed to see this picture. Radioactive

toothpaste


Hormesis and radon today!Radon therapy effective for - but not limited to - the following immune system conditions and symptoms:• Ankylosing Spondylitis (AS)• Diabetes Type I & II• Migraine Headaches• Arthritis (OA, RA, JRA, etc.)• Eczema• Mobility• Asthma• Emphysema• Multiple Sclerosis (MS)• Behcet's• Fibromyalgia• Post Polio Syndrome (PPS)• Bronchitis• Gout• Prostate (BPH)• Bursitis• Hayfever• Psoriasis• Cancer (Breast)• High Blood Pressure• Scleroderma• Carpal Tunnel• Inflammation• Sinus• Chronic Pain• Lupus (SLE)• Ulcerative Colitis• Circulation




Risky business…

..the (HPS) has concluded that estimates of risk should be limited to individuals receiving a dose of 5 rem (50 mSv) in one year or a lifetime dose of 10 rem (100 mSv) in addition to natural background. Below these doses, risk estimates should not be used: expressions of risk should only be qualitative emphasizing the inability to detect any increased health detriment (I.e., zero health effects is the most likely outcome).

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