Estimates of Risks LONG-TERM LOW DOSE EFFECTS OF IONIZING RADIATION

Estimates of Risks LONG-TERM LOW DOSE EFFECTS OF IONIZING RADIATION

Low Level Radiation Exposure Single exposure of 10 rad or less Larger exposures delivered over periods of days or longer (low dose rates)

STOCHASTIC EFFECTS: In low dose ranges hereditary effects, carcinogenesis. NON-STOCHASTIC EFFECTS: Various somatic effects, including erythema, epilation, cataracts, impaired fertility, etc.

RISKS OF LOW-LEVEL RADIATION EXPOSURE Genetic Effects Induction of Cancer Effects on the embryo

CANCER INDUCTION

GENERAL CONCEPTS Considerable time may elapse between radiation exposure and cancer development. In human beings the length of this latent period may be 10, 20, 30 or even 40 years.

Variable Radiation Sensitivity Cancer can be induced in almost all body tissues but they vary considerably in their sensitivity.

Latent Effects 4. Whole body exposure produces more solid tumors than leukemias. Solid tumors also have longer latent periods and periods of expression.

Radiation Induced Cancers Cancers induced by radiation are indistinguishable from those that occur spontaneously, hence their existence must be inferred on the basis of statistical excess.

Radiation cancer is difficult to demonstrate at high doses and essentially impossible to quantify at low doses even with large populations.

High Background of Spontaneous Cancers Reason: The observed number of cancers in the control population is so large with respect to the number of cancers induced by radiation that the radiation effects become undetectable.

DOSE RATE EFFECTS

Thyroid and Breast Cancers The incidence of radiation-induced human breast and thyroid cancer is such that the total cancer risk is greater for women than for men. For other cancers, the risks are about equal.

Example of Linear-, Non- Threshold Estimation of Risks at Low Doses 100 rad to breast 1000 women Observed = 40 cancers Expected [Background] = 22 Excess = 18

100 rad 1000 women 18 cancers 1 rad 100,000 women 18 cancers LINEAR EXTRAPOLATION OF RISK ESTIMATION

LINEAR EXTRAPOLATION OF RISK ESTIMATION 100 rad 1000 women 1 rad 100,000 women Expected = 22 Expected = 2,200 Excess = 18 Excess = 18 22 vs. 40 2200 vs. 2218 vs. 22

Age Dependency Age is a major factor in the risk of radiationinduced cancer (breast, lung, and leukemia in A-bomb survivors; in utero irradiation).

R.J. Hall, Radiobiology for the Radiologist 5 th ed. Lippincott Williams & Williams, Philadelphia, PA 2000, p. 150

TREATMENT OF HYPERTHYROID DISEASE IN HUMANS WITH 131 I RADIOIODINE (NA 131 I) Thyroid Gland Dose ~ 50-100 Gy No significant increase in thyroid cancer or leukemia compared to hyperthyroid control cohort.

EXPRESSION OF RADIOSENSITIVITY Absolute Risk Breast and thyroid are well ahead of bone marrow and lung Relative Risk The sequence is probably: Thyroid, bone marrow, lung, and breast.

Somatic Effects Approximately 450 cancer deaths/million/rem Natural incidence of cancer deaths is 200,000/million

Hall, R.J., Radiobiology for the Radiologist 5 th Ed. Lippincott Williams & Williams, Philadelphia, PA 2000, p. 157

The radiation-induced cancers among A-bomb survivors (estimated average radiation dose = 28 rads) was approximately percent of the cancers naturally occurring in that population. a. 5.1% b. 51% c. 510%

Radiation induced cancer deaths in the Japanese A-Bomb survivor cohort (1950-1990) a,c % Ca s Radiation Total CA Deaths b Radiation Induced Induced Solid Tumors 7,578 334 4.4% Leukemia 249 87 35% Combined 7,827 b 421 5.4% a. 86,572 individuals are included in the Life Span Study of survivors (whole body dose received by this group was >10 rads with an average dose of 28 rads). b. By 1991, approximately 38,600 had died; approximately 20% from cancer. Calculations estimate that 1.1% of the deaths in this population resulted from the radiation exposure c. From UNSCEAR report to the United Nations in 2000.

Earlier Epidemiology Studies [most] of these studies was considered by the Committee to constitute reliable evidence at present for use in risk estimation, for various reasons, including inadequate sample size in some instances, inadequate statistical analysis, and unconfirmed results. BEIR 1980, p. 138

Populations exposed to very low levels of irradiation DOE s Hanford Facility Portsmouth Naval Nuclear Shipyard Tri-state study of leukemia deaths Utah residents exposed to fallout Project Smoky Three-Mile Island

Project Smoky 1957 3,153 Nevada atmospheric A-bomb test Military personnel Dose estimate Most under 5 rem Leukemia Observed 8 cases expected 2-4 cases

BEIR V the possibility that there may be no risks from exposure comparable to external natural background radiation cannot be ruled out. Page 181

GENETIC MUTATION

Estimation of Genetic Effects Conclusions are largely mousebound. How are mouse numbers converted to human numbers? Some evidence of similarity from in vitro studies of imitation and chromosome changes.

GENETIC EFFECTS Must be estimated on the basis of data from animal experiments because NO Conclusive Human Data Exists.

Effects for Which No Relationship with A-Bomb Exposure Has Been Shown 1. Increased birth defects in the F 1 generation. 2. Increased F 1 mortality. 3. Infertility. 4. Accelerated aging. 5. Altered immune function. 6. Diseases other than neoplasm.

EFFECTS ON THE EMBRYO

Radiation Effects on the Embryo Radiation dose Dose-rate Stage of gestation

Classical Triad of Effects of Radiation on the Embryo Growth retardation Embryonic, fetal or neonatal death Congenital malformation

Hall, R.J., Radiobiology for the Radiologist 5 th Ed. Lippincott Williams & Williams, Philadelphia, PA 2000, p. 187

Embryo is Radiosensitive Embryonic cells have high rates of cell division and cell differentiation. Composed of relatively few cells.

NCRP Report 54, page 6 Animal experiments have shown that irradiation during the pre-implantation period generally produces an all-or-none effect, i.e., either very early embryonic death (pre- or immediately post-implantation) is caused, or there is apparent normalcy (including growth rate, fertility, and longevity) of survivors.

Hall, R.J., Radiobiology for the Radiologist 5 th Ed. Lippincott Williams & Williams, Philadelphia, PA 2000, p. 187

Radiation and Congenital Malformations Radiation produces no unique abnormalities. High rate of spontaneous abnormality (4-6%).

NCRP Report 54, page 7 Noting that the excess risk of adverse effects arising from doses below 10 rad probably is not statistically detectable in experiments involving manageable numbers of animals, one must decide on a level above which scheduling is indicated. Doses below 5 rad to the human embryo-fetus are considered by many to represent an acceptable risk when compared to the potential medical benefit of the examination to the patient.

10 Day Rule It must be emphasized that both the ICRP and NCRP recommended application of the 10-day rule only to those studies that do not contribute to management of current disease. It, therefore, follows that studies which do contribute to diagnosis or treatment of current disease should be performed in fertile women without regard to stage of the menstrual cycle. The ACR supports the American College of Obstetricians and Gynecologists 1977 Guidelines for Diagnostic X- ray Examinations of Fertile Women.

Hall, R.J., Radiobiology for the Radiologist 5 th Ed. Lippincott Williams & Williams, Philadelphia, PA 2000, p. 188

Hall, R.J., Radiobiology for the Radiologist 5 th Ed. Lippincott Williams & Williams, Philadelphia, PA 2000, p. 187

Health Effects of Exposure to Low Levels of Ionizing Radiation BEIR V National Research Council. National Academy Press, Washington D.C., 1990, P. 357.

Fetal Irradiation Therefore, one concludes that from preconception to birth there is no period during which a radiological examination of the lower abdomen and pelvis of a woman of reproductive capacity can be conducted with no risk.