In the largest study of its kind, researchers led by Dr. Peter Inskip of NCI's Division of Cancer Epidemiology and Genetics estimated the relationship between exposure to radiation during childhood cancer treatment and adult breast cancer risk in 6,647 women from the Childhood Cancer Survivor Study (CCSS). The results were published July 20 in the Journal of Clinical Oncology.

Overall, women who received any radiation therapy for their initial cancer were 2.7 times as likely to develop breast cancer. The risk increased linearly with increasing radiation dose to the area where the tumor developed in the breast, reaching 11-fold for doses in the highest category of exposure (40 Gray), relative to no radiation.

The women had been diagnosed with their primary cancer as children, between 1970 and 1986. One hundred twenty developed breast cancer by the end of 2001 and gave consent to have their medical records reviewed. The researchers matched each of the 120 women with 4 women in a control group from the CCSS who had not developed breast cancer. Medical physicists estimated radiation doses to the breast and ovaries.

Interestingly, women who had received radiation to the breast as well as a sterilizing radiation dose to the ovaries (which stops ovarian hormone production) had a sharply reduced risk of radiation-related breast cancer. "A radiation dose to the breast can cause damage that may potentially develop into cancer, but whether that damage does develop into cancer can be influenced by hormonal stimulation from the ovaries," explained Dr. Inskip.

The authors noted that modern radiation therapy for most types of childhood cancer uses substantially lower radiation doses than protocols used between 1970 and 1986. In addition, said Dr. Inskip, "we're highlighting the complications of treatment here, but we should remember that there has been remarkable progress in treating childhood cancer. Many children who might have died from their cancer are living due to advances in treatments."