Radiation Therapy for Initial Cancer Causes Relatively Few Second Cancers

Researchers from the National Cancer Institute report that radiation therapy for a first cancer is unlikely to lead to a second cancer diagnosis later in life. The investigative team conducted a retrospective review of data from the Surveillance, Epidemiology, and End Results cancer registry for nearly 650,000 adults who received a cancer diagnosis between 1973 and 2002 and survived at least 5 years. Approximately 50% of patients received radiation, and only 8% of these (n = 3266) developed a second malignancy considered attributable to the therapy (95% confidence interval [CI], 7%-9%). All ~650,000 patients were at least 20 years of age at diagnosis and had 1 of 15 tumor types commonly treated with radiation: rectal, lung, breast, prostate, cervical, testicular, endometrial, brain, central nervous system, ocular and orbital, laryngeal, salivary gland, oropharyngeal, and soft tissue tumors. An estimated 60,000 patients, including 42,000 given radiotherapy, developed a second primary cancer, suggesting that the overall rate of second primary cancers is low. The authors said at 15 years follow-up, 5 excess cancers occurred per 1000 patients whose initial cancers were treated with radiotherapy (median follow-up, 12 years). Among patients who received radiation, secondary cancer risk varied according to the initial type of cancer. The relative risk was lowest for women treated for breast cancer, at 1.10 (95% CI, 1.07-1.13). Men treated for testicular cancer had the highest relative risk, at 1.43 (95% CI, 1.13-1.84). Factors associated with increased risk of a second cancer included a younger age at diagnosis of the first cancer and greater radiation exposure (>5 Gy) during therapy. The risk increased as the patients aged. The authors ruled out second cancers that occurred with 5 years of radiation therapy as being treatment related. They noted that patients often have more follow-up care in the first 5 years after treatment and were concerned that including cancers diagnosed during this time frame might bias results. The investigators did not assess whether the patients had other known risk factors for cancer, such as smoking history, hormonal therapy, obesity, and exposure to carcinogenic chemotherapy drugs. They did a subanalysis of patients who did not receive chemotherapy, however, and found consistent results. Most patients in the study received radiotherapy prior to widespread use of intensity-modulated radiotherapy, but the authors believe studies will continue to show that the benefits of radiotherapy outweigh the risk of second cancers. Berrington de Gonzalez A, et al. Proportion of second cancers attributable to radiotherapy treatment in adults: A cohort study in the US SEER cancer registries. Lancet Oncol. 2011.