Trends in second primary cancer incidence by primary colorectal cancer location.

e15707 Background: Patients with primary colorectal cancer (CRC) are at an increased risk of developing a second primary cancer (SPC) at any site. In the United States, incidence patterns of primary CRC are shifting, with a notable rise in early-onset CRC and a decline in late-onset CRC. However, trends in SPC incidence remain poorly understood. We analyzed SPC incidence, stratified by the anatomical location and stage at diagnosis of primary CRC. Methods: CRC cases diagnosed between 2000-2018 were identified using SEER.Stat 8.4.4. Patients developing a SPC with a latency period of > 6 months, and at a different location from the primary were identified. Incidence rates were calculated as the number of SPC cases in a given year divided by person-years at risk. Patients were categorized into four groups based on the location of primary CRC 1: cecum, ascending colon and hepatic flexure, 2: transverse colon, 3: splenic flexure, descending colon and sigmoid colon, and 4: rectosigmoid junction, and rectum. Stages at diagnosis of primary CRC were classified as early (in situ, localized) or late (regional, distant). Annual percentage change (APC) and average APC (AAPC) were estimated using Jointpoint regression and weighted Bayesian Information Criteria (P < 0.05). Results: 40,022 (10.21%) patients developed a SPC over the study period. The overall incidence of SPC decreased from 2000 to 2009 (APC: -0.61 (-2.70,-0.10)), while it increased from 2009 to 2018 (APC: 0.58 (0.05,2.69)). For early-stage primaries, overall SPC incidence increased from 2009 to 2018 (APC: 0.90 (0.42,2.31)), following a period of stability from 2000 to 2009 (APC: -0.41%, (-1.95,0.03)). Among anatomical groups, early-stage rectosigmoid junction and rectum showed a consistent increase in SPC incidence from 2000 to 2018 (AAPC: 0.66 (0.06,1.26)). For late-stage primaries, a decline in incidence of SPC was observed from 2000 to 2010 (APC: -0.68(-3.39,-0.09)), followed by a stable trend from 2010 to 2018 (APC: 0.66 (-0.16,3.39)). Splenic flexure, descending colon and sigmoid colon demonstrated a notable decline from 2000 to 2004 (APC: -5.4 (-13.4,-0.80)), with a continued decline overall from 2000 to 2018 (AAPC: -0.79(1.15,-0.43)). Late-stage rectosigmoid junction and rectum showed a decrease in SPC incidence from 2000 to 2018 (AAPC: -0.79(-1.15,-0.43)). Conclusions: The incidence of second primary cancers following primary colorectal cancer demonstrates conflicting trends across different stages of diagnosis and anatomical sites. These trends are influenced by improved screening practices, shifting demographics, lifestyle modifications, and potential genetic predispositions. These findings highlight the importance of targeted surveillance and prevention strategies for specific anatomical sites and stages of CRC.