LAUSR

649 Background: This study sought to compare adverse events (AEs) of older and younger adults with lower gastrointestinal (GI) malignancies treated on NRG studies. Methods: Data from six NRG trials (RTOG 9811/0012/0247/0529/0822 & NSABP R-04), testing combined modality therapy (radiation and chemotherapy) in patients with anal or rectal cancer, were collected to test the hypothesis that older age was associated with increase in acute ( ≤ 90 days from treatment start) AEs. AEs were defined as GI, Genitourinary (GU), hematologic, or skin. AEs and compliance with protocol-directed therapy were compared between patients aged ≥ 70 years and < 70 years. Categorical variables were compared across age groups using the chi-square test. The association of age on AEs was evaluated using a covariate-adjusted logistic regression model, with odds ratio (OR) reported. To adjust for multiple comparisons, a p-value < 0.01 was considered statistically significant. Results: Data from 2525 patients were collected (43% female, 72% rectal cancer). There were 380 patients ≥ 70 years old (15%). Older patients were more likely to have worse baseline performance status (PS 1 or 2) (23% vs. 16%, p <0.01), but otherwise baseline characteristics were similar. Older patients were less likely to have completed their chemotherapy (78% vs. 87%, p < 0.01), but had similar median RT duration. On univariate analysis, patients ≥ 70 were more likely to experience grade ≥ 3 GI AEs (36% vs. 23%, OR 1.82, p < 0.001), and less likely to experience ≥ 3 skin AEs (8% vs. 14%, OR 0.56, p = 0.002). There was no difference between GU or hematologic AEs. On multivariable analysis, age ≥ 70 was associated with grade ≥ 3 GI AE (OR 1.80, 95% CI: 1.40, 2.31; p < 0.001) after adjusting for gender, PS, T stage, disease site, RT duration, and chemotherapy completion. Conclusions: Older patients with curable lower GI cancers who underwent combined-modality therapy were less likely to complete chemotherapy and were more likely to experience serious GI toxicity, whereas younger patients had higher rates of serious skin AEs.