Second Primary Cancers in Men With Prostate Cancer: An Increased Risk of Male Breast Cancer

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Abstract
Purpose: Evaluation of second primary cancers provides valuable insight about etiology and shared risk factors. Studies of second primary cancers following prostate cancer conclude that overall risk of second primary cancers decreases. However, risk of bladder cancer and kidney cancer increases. We examine the risk of common and rare second primary cancers following prostate cancer in a large population based cohort to identify possible common etiological factors. Materials and Methods: All prostate cancer cases in the Swedish Cancer Registry (135, 713) from 1958 to the end of 1996 constituted the study base. Risk (standardized incidence ratio) of second primary cancers was calculated as the ratio between observed and expected number of cancers. We used 2-tailed 95% confidence intervals (CI) to test significance. Results: An overall increased risk (standardized incidence ratio 1.17, 95% CI 1.15–1.19) of second primary cancers was found but was only seen in the first 6 months of followup (ratio 3.45, 3.32–3.57). The most interesting finding was an increased risk (ratio 2.01, 95% CI 1.44–2.74) of male breast cancer. Other tumor sites with increased risk were the small intestine (standardized incidence ratio 1.39, 95% CI 1.09–1.51), skin melanoma (ratio 1.33, 95% CI 1.16–1.51) and endocrine tumors (ratio 1.41, 95% CI 1.13–1.74). Conclusions: A small but increased risk of second primary cancers following prostate cancer was found, most likely due to increased surveillance during the first 6 months after diagnosis. However, following prostate cancer there is an increased risk of endocrine related second primary cancers such as male breast cancer and carcinoids in the small intestine. To our knowledge these associations have not been reported previously, and they warrant more study.