The TAB-EOAO study: Trends in use and benefit of adjuvant chemotherapy between early onset and average onset locally advanced colon cancer.

105 Background: Despite the declining incidence rates (IR) in average onset colon cancer (AO-CC), IR for early onset colon cancer (EO-CC), defined as patients (pts) younger than 50-years-old at diagnosis, has been increasing. Treatment for high risk locally advanced colon cancer (CC) is surgical resection followed by adjuvant chemotherapy (AC). Data indicates a more permissive chemotherapy use in EO-CC. There is limited data about treatment and survival trends in EO-CC. Thus, we aimed to assess the use and benefit of AC between in EO vs AO CC in locally advanced disease. Methods: CC diagnosed between 2000-2020 were identified in the Surveillance, Epidemiology, and End Results Program (SEER) Database. Eligible pts were adults (≥18), Stage II or III at diagnosis. Stage II disease was further stratified as low risk (pT1-3) and high risk (pT4). Logistic regression methods were used to assess Odds Ratio (OR) of AC between EO-CC and AO-CC; Cox models were used to compare overall survival (OS) and Cancer-specific survival (CSS) between both groups during 2000-2010 and 2011-2020. Results: Our study included 136,676 pts (AO-CC= 121,606; EO-CC =15,070). The median age for EO-CC was 44 years versus 71 years for AO-CC population. The use of AC increased over time for both the EO-CC (63% vs 66%) and AO-CC (33% vs 40%). Moreover, pts with AO-CC received less AC than EO-CC in the entire study time (see table). In pts with AO-CC, AC improved CSS (HR=0.69 for 2000-2010, HR=0.50 2011-2020). However, among those with EO-CC, AC improved CSS in 2011-2020 (HR=0.76) but did not improve CSS in 2000-2010 (HR=1.02). Conclusions: Over the past 20 years EO-CC has been managed aggressively, especially for stage II low risk patients where current guidelines do not support the use of AC. In this real world data set we show that for EO-CC AC did not improve CSS in the decade 2000- 2010 but did improve CSS in the 2011- 2020 period. Further analysis may clarify stage specific benefits of AC. There clearly remains a need for better risk stratification, including the use of ctDNA, to identify the subpopulations of EO-CC patients that will best benefit from AC. Multivariable logistic regression model of chemotherapy by timeframe with interaction term: Age* stage. 2000-2010 2011-2020 AO-CC vs EO-CC OR 95% CI OR 95% CI Stage II low risk 0.23 (0.21, 0.26) 0.28 (0.24, 0.32) Stage II high risk 0.28 (0.22, 0.36) 0.28 (0.21, 0.36) Stage III 0.36 (0.32, 0.4) 0.30 (0.26, 0.34)