Pembrolizumab therapy for MSI-H colon cancer in a nonagenarian patient: a case report

A 94-year-old female patient underwent a robotic single-port right hemicolectomy for colon cancer in April 2025 and was subsequently observed without adjuvant chemotherapy due to her extreme age.While routine follow-up showed no recurrence, the patient developed symptoms 7 months postoperatively.Diagnostic imaging revealed multiple distant metastases, and subsequent molecular profiling identified the tumor as microsatellite instability-high (MSI-H).Considering her age and performance status, pembrolizumab monotherapy was initiated, which was exceptionally well-tolerated without any significant immune-related adverse events.Per RECIST 1.1 criteria, follow-up imaging showed complete resolution of target lesions and marked regression of non-target lesions, with a best overall response of PR reflecting near-complete tumor resolution.This case underscores that advanced age alone should not be a limiting factor for biomarker-driven immunotherapy, providing valuable real-world evidence that pembrolizumab can safely induce a profound tumor response in nonagenarian patients with recurrent MSI-H colon cancer.