studies for all cancers. Matanoski and colleagues (1987) reported higher overall mortality and higher cancer mortality in radiologists compared to other specialists with lower expected exposures.

A survey of the health of radiologic technologists (Boice and others 1992) gathered information on risk factors including smoking status, reproductive history, use of oral contraceptives, personal exposure to radiographs, height, weight, use of hair dye, and postmenopausal estrogens, and family and personal medical history of cancer. Members of the study population (n = 143,517, registered for more than 2 years with the American Registry of Radiologic Technologists, ARRT) were predominantly female and white. Personal dosimetric information was available for 64% of all the registered technologists, but only 34% of the breast cancer cases and 35% of the controls. Cases and controls were generally older and more likely to have stopped work before computerized records of dosimetry information were begun in 1979. Occupational exposure was estimated through the number of years worked as a technologist obtained from questionnaire data.

A cohort study using the ARRT database (Doody and others 1998) reported SMRs and RRs adjusted for age, calendar year of follow-up, and gender. No significant excess mortality among radiological technologists was observed for lung cancer, breast cancer, or leukemia. The SMR for all malignant neoplasms exhibited a significant trend with the number of years certified (p < .001), as it did for breast cancer. In the absence of complete personal dosimetry information, accurate estimates of risk due to exposures to ionizing radiation are not possible.

Yoshinaga and colleagues (1999) reported results from a retrospective cohort study of radiological technologists in Japan. External comparisons were also made with all workers and with professional and technical workers to address the issue of the healthy worker effect. The study used all Japanese men as the external comparison group; the SMR for all cancers in this study was 0.81 (95% CI 0.73, 0.95). Although elevated SMRs were observed for cancers of the colon, skin, lymphoma, multiple myeloma, and leukemia, none was statistically significant. The SMR for leukemia was significant in comparison to the total workforce as the reference group (SMR = 1.99; 95% CI 1.09, 3.33) and also for professional and technical workers as the reference group (SMR = 1.82; 95% CI 1.00, 3.06). No quantitative information on dosimetry was given in the report, nor was there an internal comparison, thus limiting the usefulness of the report for the estimation of risk.

Since 1990, a number of studies of radiologists have been published that utilized measurements of individual exposure (Andersson and others 1991). Andersson and colleagues (1991) studied the cancer risk among staff at two radiotherapy departments in Denmark. The average cumulative radiation dose was 18.4 mSv, although 63% of the persons had doses <5 mSv. The expected number of cancers was estimated using cancer incidence rates from the Danish Cancer Registry. The overall relative risk was 1.07 (95% CI 0.91, 1.25) for all cancers, and no significant dose-response was observed. The risks for cancers that are considered radiation sensitive were not elevated.

Berrington and colleagues (2001) reported the results of 100 years of follow-up of British radiologists who registered with a radiological society between 1897 and 1979 and who were followed until January 1, 1997. A progressive increase was observed in the SMRs for cancer with number of years since first registration. It appears that excess risk of cancer mortality in the period more than 40 years after first registration is likely a long-term effect of radiation exposure for radiologists registering between 1921 and 1954. Radiologists whose first registration was after 1954 demonstrated no increase in cancer mortality, possibly because of their lower overall radiation exposure.

SUMMARY

Epidemiologic studies of radiation workers and other persons exposed to ionizing radiation in the workplace started in the late 1950s with the study of British radiologists. Since then, numerous studies have considered the mortality and cancer incidence of various occupationally exposed groups in medicine, industry, defense, research, and aviation.

Studies of occupationally exposed groups are, in principle, well suited for the direct estimation of the effects of low doses and low dose rates of ionizing radiation. Potentially, the most informative studies at present are those of nuclear industry workers (including the workers of Mayak in the former USSR), for whom individual real-time estimates of doses have been collected since the 1940s with the use of personal dosimeters. More than 1 million workers have been employed in this industry since its beginning. However, studies of individual worker cohorts are limited in their ability to estimate precisely the potentially small risks associated with low levels of exposure. Risk estimates from these studies are variable, ranging from no risk to risks an order of magnitude or more than those seen in atomic bomb survivors.

Combined analyses of data from multiple cohorts offer an opportunity to increase the sensitivity of such studies and provide direct estimates of the effects of long-term, low-dose, low-LET radiation. The most comprehensive and precise estimates to date are those derived from the U.K. National Registry of Radiation Workers and the three-country study (Canada-United Kingdom-United States), which have provided estimates of leukemia and all cancer risks. Although the estimates are lower than the linear estimates obtained from studies of atomic bomb survivors, they are compatible with a range of possibilities, from a reduction of risk at low doses to risks twice those upon which current radiation protection recommendations are based. Overall, there



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