Risk of second primary cancers after treatment for locoregional rectal cancer.

Abstract

3578 Background: The risk of second primary colorectal cancers among rectal cancer patients has been described, but little is known about the risk of non-colorectal malignancies that may occur in the field of radiation. We attempted to quantify the risk, using data from the large population-based California Cancer Registry (CCR). Methods: We analyzed the CCR data for surgically-treated locoregional rectal cancer cases, diagnosed during the period 1988–2009. We excluded cases with second primary tumor (SPT) diagnosed within 12 months of initial diagnosis . Radiation treatment used was external beam radiation therapy. Standardized incidence ratios (SIR) with 95% confidence intervals (CI) were calculated to evaluate risk as compared to the underlying population after matching for age, sex, ethnicity, and time. Results: Of the study cohort of 13,418 rectal cancer cases, 1572 cases of SPTs were observed . The SIR was increased for small intestine cancer among cases receiving radiation treatment (4 cases observed vs. 1.01 cases expected; SIR=3.94, 95% CI 1.07-10.10) but not among cases lacking radiation treatment (4 observed vs. 4.45 expected; SIR=0.90, 5% CI 0.24-2.30). Among females treated with radiation, the SIR was increased for uterine cancer (12 observed vs. 5.59 expected; SIR=2.15, 95% CI 1.11 to 3.75) but not among cases lacking radiation therapy (23 observed vs. 26.17 expected; SIR=0.88, 95% CI 0.56-1.32). Among males receiving radiation treatment, the SIR for prostate cancer was decreased (23 observed vs. 69.78 expected; SIR=0.33; 95% CI 0.21 to 0.49) but of borderline significance among males lacking radiation therapy (243 observed vs. 276.97 expected; SIR=0.88, 95% CI 0.77-0.99). No significant differences were observed for cancers of the vagina, cervix, ovary, kidney, bladder, penis, testes, or leukemia based on prior radiation treatment for rectal cancer. Conclusions: Patients receiving pelvic radiation for treatment of rectal cancer have a subsequently higher than expected incidence of small intestine and uterine cancer. The incidence of prostate cancer appears to fall after pelvic radiation. These unexpected findings suggest complex relationships associated with radiation treatment for rectal cancer and SPT risk.