The incidence of secondary pelvic tumors after previous (chemo)radiation for rectal cancer.

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491 Background: The aim of this study was to analyze the association between radiotherapy (RT) for rectal cancer and the development of second primary tumors. Methods: Data on all surgically treated, non-metastasized primary rectal cancer patients diagnosed between 1989 and 2007 were retrieved from a population-based cancer registry and retrospectively reviewed. To estimate the cumulative incidence of a second tumor, Fine and Gray’s competing risk model was used with death as a competing event. Standardized incidence ratios (SIR’s) were calculated for comparison with the incidence of primary tumors in the general population, taking in account sex, age and calendar year. Results: The cohort consisted of 29,214 patients of which 15,454 patients had undergone (chemo)RT. Median follow-up was 6.2 years (range 0-24). 3655 patients were diagnosed with at least one second primary tumor of which 808 patients had pelvic tumors. The SIR for any second tumor was 1.14 (95% confidence interval [CI] 1.10-1.17), resulting in 23.3/10,000 excess cases per year. RT reduced the cumulative incidence of second pelvic tumors compared to patients who received no RT (SHR 0.70, 95% CI 0.61-0.81). Second pelvic tumors were more common in patients who underwent post-operative RT than in patients who underwent pre-operative RT (SHR 1.37, 95% CI 1.10-1.70). Organ-specific analyses showed that second prostate tumors were less common in patients who received RT compared to patients who received no RT (SHR = 0.51, 95% CI 0.43-0.62). RT also reduced the risk for a second primary tumor in the rectum(sigmoid) compared to patients who did not receive RT (SHR 0.59 95% CI 0.37-0.94). Patients who received post-operative RT had higher chances of developing a second rectum(sigmoid) tumor then patients who received pre-operative RT (SHR 2.25, 95% CI 1.07-4.73). Patients without RT had worse overall survival than patients who received RT (hazard ratio 1.22, 95% CI 1.19-1.26). Conclusions: In this nationwide study, patients with previous rectal cancer had a slightly increased chance of developing another primary tumor compared with the general population. We found a protective effect of RT on the development of secondary pelvic tumors, predominantly for prostate cancer.