Survival benefits of immunotherapy combined with chemoradiotherapy in metastatic colorectal cancer: an analysis based on data from the SEER database.

Objectives: Chemoradiotherapy (CRT) alone shows limited efficacy in improving outcomes for metastatic colorectal cancer (mCRC) patients. This study aimed to comprehensively evaluate the survival benefits of combining CRT with immunotherapy (CRT + IMT) in this population. Methods: Data from the SEER database were extracted for mCRC patients diagnosed between 2010-2015 and 2018-2020. Survival outcomes were assessed using Kaplan-Meier analysis, log-rank test, Cox regression, and propensity score matching (PSM) to minimize baseline differences between treatment groups, and multivariable Cox models were constructed based on variables selected by LASSO regression. Results: A total of 2,451 patients were identified, with 1,888 receiving CRT alone and 563 CRT + IMT. After 1:2 PSM, 1,594 remained. Kaplan-Meier curves showed CRT + IMT significantly improved overall survival (OS, p<0.0001) and cancer-specific survival (CSS, p<0.0001). Multivariable Cox confirmed CRT + IMT as an independent predictor (OS HR=0.54, 95 % CI: 0.44-0.65; CSS HR=0.54, 95 % CI: 0.44-0.66). Favorable factors included distal colon/rectal tumors, surgery, and combined therapy, while older age (>60), advanced stage, N2, and metastasis predicted worse survival. The subgroup analyses revealed a more pronounced benefit in specific cohorts, and significant interactions were observed for factors such as histologic grade and nodal stage. Conclusions: The combination of CRT and immunotherapy confers a significant survival advantage over CRT alone in patients with mCRC, supporting its potential role in optimized treatment strategies.