In a study of adults who survived at least 5 years after being treated for solid cancers that are routinely treated with radiation therapy, 9 percent developed a second solid cancer over an average follow-up time of 12 years. About 8 percent of those cancers appear to be related to radiation therapy. Results from the study, led by Dr. Amy Berrington de González of NCI’s Division of Cancer Epidemiology and Genetics, were published online March 29 in Lancet Oncology.
In the first comprehensive analysis of its kind, Dr. Berrington de González and her colleagues examined data from nearly 650,000 adult patients recorded in nine of the Surveillance, Epidemiology, and End Results (SEER) registries between 1973 and 2002. Depending on their initial cancer type, between 23 percent and 79 percent of the patients received radiation therapy during treatment.
The researchers calculated the relative risk of developing a second cancer for patients who received radiation therapy during their initial treatment compared with patients who did not. The relative risk of developing a second cancer associated with radiation varied with type of first cancer and was highest in survivors of testicular seminoma. After adjusting for factors such as age, time since initial diagnosis, and year of diagnosis, the researchers estimated that about 3,300 of the 60,271 second cancers observed over the study period could be attributed to radiation therapy.
The study also described for the first time the overall absolute risk of second cancers related to radiation therapy: an estimated 5 of every 1,000 patients treated with radiation therapy who survive for 15 years would be anticipated to develop a radiation-related cancer. “That number can be used by doctors to convey the message to patients that the absolute risk of developing a second cancer related to radiotherapy is quite small,” explained Dr. Berrington de González.
Further research will be needed to determine the risks from newer radiation therapy technologies, such as intensity-modulated radiation therapy (IMRT), which expose normal tissues to different patterns of radiation than older techniques. Previous studies have raised concerns “that the second cancer risk might be higher in patients treated with IMRT. We will need to study this exposure in the future to assess that risk,” concluded Dr. Berrington de González.