Abstract
BACKGROUND: There is a lack of comprehensive reports on time trends in synchronous prostate and rectal cancers. To address this, we conducted the largest cohort study to date to assess these trends in a population-based setting. METHODS: We included all adult (ages 18-99) men with incident prostate cancer in the Swedish Cancer Register in 1993-2019. Age-standardized incidence rates (ASIRs) of prostate cancer per 100,000 male population per year were calculated and compared to the ASIR of synchronous (± 6 months from rectal cancer diagnosis) prostate cancer. Age-adjusted synchronous-to-general incidence rate ratios (IRRs) were predicted using Poisson regression. As a sensitivity analysis to assess the effect of incidental findings due to the anatomical proximity, we investigated synchronous prostate and non-sigmoid colon cancers. RESULTS: Among 238,252 prostate cancer cases, 594 were synchronous with rectal cancer. The incidence of synchronous prostate cancer increased over the study period, with mean ASIR rising from 418/100,000 (1993-2001) to 788/100,000 (year 2011-2019). The synchronous-to-general IRR increased from 1.92 (95% confidence interval (CI) 1.60-2.31) to 2.61 (95% CI 2.32-2.95) over the same periods. Prostate cancer was also more commonly diagnosed in conjunction with non-sigmoid colon cancer than in the overall male population, but no time trend was observed. INTERPRETATION: The incidence of synchronous prostate and rectal cancers has increased over the past 20 years in Sweden, with no signs of plateauing. Future studies are warranted to explore factors contributing to prostate cancer overdiagnosis and to optimize clinical management strategies for this complex patient group.