Relationship between Radiation Exposure and Risk of Second Primary Cancers among Atomic Bomb Survivors

Radiation exposure is related to risk of numerous types of cancer, but relatively little is known about its impact on risk of multiple primary cancers. Using follow-up data through 2002 from 77,752 Japanese atomic bomb survivors, we identified 14,048 participants diagnosed with a first primary cancer, of whom 1,088 were diagnosed with a second primary cancer. Relationships between radiation exposure and risks of first and second primary cancers were quantified using Poisson regression. There was a similar linear dose-response relationship between radiation exposure and risks of both first and second primary solid tumors [excess relative risk (ERR) per Gray = 0.65, 95% confidence interval (CI): 0.57–0.74 and ERR/Gy=0.56, 95% CI: 0.33–0.80, respectively] and risk of both first and second primary leukemias (ERR/Gy=2.65, 95% CI: 1.78–3.78 and ERR/Gy=3.65, 95% CI: 0.96–10.70, respectively). Background incidence rates were higher for second solid cancers, compared to first solid cancers, until about age 70 years for men and 80 years for women (p<0.0001), but radiation-related ERRs did not differ between first and second primary solid cancers (p=0.70). Radiation dose was most strongly related to risk of solid tumors that are radiation sensitive including second primary lung, colon, female breast, thyroid, and bladder cancers. Radiation exposure confers equally high relative risks of second primary cancers as first primary cancers. Radiation is a potent carcinogen and those with substantial exposures who are diagnosed with a first primary cancer should be carefully screened for second primary cancers, particularly for cancers that are radiation sensitive.