Health Risks of Ionizing Radiation: - Clark University

Preconceptional exposures are discussed in detail in
section 10 and appendix C.
3.7 Radiologists’ occupational exposures
In contrast to estimates of the controlled exposure
received by patients, exposure estimates for
radiologists are uncertain and often based on the
number of years a radiologist is certified. Exposures
have decreased dramatically over time, so that a
although radiologist in the 1920s or 30s might have
been exposed to 1 Gy per year, exposures today
are about a thousand times less (0.5 mGy per year;
Berrington et al. 2001).
Doody et al. (1998) and Mohan et al. (2003)
conducted a cohort study of radiologists in the US.
Mortality was lower, overall, than in the general
population, indicating a healthy worker effect.
Comparisons between radiologists first employed in
the 1940s or 50s and radiologists entering the field
more recently are useful because, as noted above,
exposure has declined over time. Radiologists in
the 1950s were likely exposed to 50-100 mGy per
year (Berrington et al. 2001). Mohan et al. found
increased risks of breast cancer (RR 2.92, 1.22-
7.00) and leukemia (RR 1.64, 0.42-6.31) making
such comparisons 39 . This study also found that the
risk of breast cancer or leukemia was related to the
number of years working in the field before 1950.
Sigurdson et al. (2003) found that radiologists who
were employed in the US between 1926 and 1982
were at elevated risk of all solid tumors, breast
cancer, melanoma, and thyroid cancer. Although
the eligibility requirements for the study were that
radiologists must have been certified in the US
between 1926 and 1982, thus allowing for earlier
employees who may have had higher exposures,
78% of the cohort was first certified in 1960 or
later. This implies that the doses that radiologists
received were low, although there are no direct
dose estimates available. Berrington et al. (2001)
looked at occupational exposure in the UK and
found again that there was a significant risk only for
those radiologists who had been registered for over
40 years. In conclusion, although risks associated
Medical Exposures 31
with historical exposures can be quantified, the
risks associated with the low doses that radiologists
currently receive would be too small to be detected
by epidemiological methods (Brenner and Hall
2003).
3.8 Conclusions
There are several areas of medical radiation
epidemiology that have a particularly strong body
of evidence associating low doses and risk. Several
important characteristics of these risks have been
described in the studies discussed above:
• There is a wide body of research that shows
that radiation therapy for non-cancer disease
can cause cancer in patients, although these
are typically high-dose exposures. Radiation
therapy varies according to the disease being
treated, the parts of the body that are exposed,
and the cancer sites that might respond.
Leukemia and cancers of the pancreas, stomach,
small intestine, liver, gall bladder, kidney, brain,
central nervous system, connective tissue,
breast, and thyroid have all been associated
with therapeutic exposure. Davis et al. (1989),
Boice et al. (1991) and Little and Boice (1999)
show some of the most compelling evidence for
the risks of low dose and fractionated exposure
where fluoroscopy patients who were exposed
to small doses over time show risks similar to
those observed in the atomic bomb survivors.
• Comparisons of medical exposure and atomic
bomb studies seem to show that some cell-killing
and risk attenuation effects may occur with
fractionated doses (Little et al. 1999), but there
is also evidence suggesting that radiosensitive
tissues such as the breast may be more sensitive
to fractionated doses (Boice et al. 1991, Doody
et al. 2000, Davis et al. 1999).
• Gibson et al. (1972), Preston Martin et al. (1988),
Preston-Martin et al. (1988), and Doody et al.
(2000) have found risks of leukemia, parotid
gland tumors and breast cancer associated
39 The breast cancer risk estimate compares first employment before 1940 to after 1960; the leukemia risk estimate
compares first employment before or after 1950.