Immunotherapy for MSI-H colorectal cancer with isolated peritoneal metastases.

187 Background: NCCN guidelines recommend immunotherapy for MSI-H colon cancer (CC) with isolated peritoneal metastases (iPM); however, limited data support its benefit. This study aimed to evaluate the impact of immunotherapy in this population. Methods: Patients diagnosed with MSI-H CC with iPM from 2016-2019 were selected from the NCDB. Patients who received immunotherapy were compared to those who did not. Overall survival was assessed using Kaplan-Meier analysis and multivariate Cox proportional hazard regression. The trend of immunotherapy utilization from 2016-2020 was analyzed using the Cochran-Armitage test. Results: We analyzed a cohort of 444 patients, with 161 (36%) receiving immunotherapy. Patients in the immunotherapy group were, on average, younger (63 vs. 71 years; P<0.001), with balanced gender distribution between groups. Among those who received immunotherapy, 70% were treated as a first-line agent, and 30% after chemotherapy. Immunotherapy was more commonly administered in the adjuvant setting (66%) compared to neoadjuvant (7%) or without surgery (27%). Immunotherapy was associated with significantly longer median survival (33.6 vs. 14.9 months; P<0.001). In multivariate analysis, immunotherapy was associated with improved overall survival, whether given as a first-line agent (HR 0.54, 95% CI 0.39-0.74; P<0.001) or second-line after chemotherapy (HR 0.59, 95% CI 0.37-0.94; P=0.03). Additionally, primary tumor resection (HR 0.44, 95% CI 0.32-0.60; P<0.001), but not metastatectomy (HR 0.96, 95% CI 0.71-1.29; P=0.8), was associated with prolonged survival. Patients who received both immunotherapy and primary tumor resection (n=118) had a median survival of 43.8 months. From 2016 to 2020, the utilization of immunotherapy in this cohort increased from 29% to 37% (χ^2=0.23, P=0.63). Conclusions: Utilization of immunotherapy remains low in MSI-H CRC with iPM and did not change significantly during the study period. Immunotherapy is associated with improved overall survival and should be considered for appropriate candidates. The combination of primary tumor resection and immunotherapy yielded the best overall survival.