Beyond the image: achieving complete pathologic response in MSI-high metastatic colon cancer despite persistent PET activity

Background: Microsatellite instability-high (MSI-H) colorectal cancer, presents high tumor mutational burden and is highly responsive to immune checkpoint inhibitors (ICIs). ICIs have transformed metastatic colorectal cancer (mCRC) treatment. However, the role of partial or discontinued immunotherapy and the interpretation of PET imaging in this setting remain challenging. Objective: We aim to report a case of MSI-H mCRC with liver metastasis demonstrating complete pathological response after partial immunotherapy despite persistent PET activity, highlighting integrative approaches of immunotherapy and surgery. Case presentation: A 49-year-old female with MSI-H left colon adenocarcinoma and a single liver metastasis underwent laparoscopic subtotal colectomy, followed by chemotherapy (FOLFOX plus bevacizumab) combined with pembrolizumab. Immunotherapy was discontinued after four cycles due to limited resources. PET-CT after chemotherapy showed persistent metabolic activity in liver lesions. She underwent surgical segmentectomy with negative margins. Final pathology showed no residual tumor. Discussion: Partial immunotherapy can induce profound pathological response in MSI-H mCRC. Persistent PET positivity may reflect immune-related inflammation rather than viable tumor, complicating response assessment. Combined immunotherapy and surgery improve survival in MSI-H metastatic disease. Emerging evidence supports tailored immunotherapy duration and multidisciplinary management. Conclusion: Partial immunotherapy in MSI-H metastatic colon cancer can achieve complete pathological remission despite PET persistence. Surgery remains vital for definitive response assessment and cure. Multimodal strategies integrating systemic immunotherapy and local treatment optimize outcomes for MSI-H mCRC patients.