The aim of this study was to analyze the association between radiation therapy (RT) for rectal cancer and the development of second tumors.

Data on all surgically treated non-metastatic primary rectal cancer patients diagnosed between 1989 and 2007 were retrieved from the Netherlands population-based cancer registry. Fine and Gray's competing risk model was used for estimation of the cumulative incidence of second tumors. Multivariable analysis was conducted using Cox regression.

The cohort consisted of 29 027 patients of which 15 467 patients had undergone RT. Median follow-up was 7.7 years (range 0-27). Among all 4398 patients who were diagnosed with a second primary tumor, 1030 had one or more pelvic tumors. The standardized incidence risk for any second tumor was 1.16 (95% confidence interval [CI] 1.12-1.19), resulting in 27.7/10 000 excess cancer cases per year in patients treated for rectal cancer compared with the general population. RT reduced the cumulative incidence of second pelvic tumors compared with patients who did not receive RT (subhazard ratio [SHR] 0.77, CI 0.68-0.88). Second prostate tumors were less common in patients who received RT (SHR 0.54, CI 0.46-0.64), gynecological tumors were more frequently observed in patients who received RT (SHR 1.49, CI 1.11-2.00).

Patients with previous rectal cancer had a marginally increased risk of a second tumor compared with the general population. Gynecological tumors occurred more often in females who received RT, but this did not result in an overall increased risk for a second cancer. RT even seemed to have a protective effect on the development of other second pelvic tumors, pre-dominantly for prostate cancer. These findings are highly important and can contribute to improved patient counseling.