How a 50-Year-Old Patient With Mucinous CRC Achieved Tumor-Free Status Through Targeted Treatment

A 50-year-old man initially presented with a mucinous adenocarcinoma of the right colon (pT3N0, M0, stage IIa) in March 2017. Genetic testing confirmed Lynch syndrome (MSH2 gene frameshift mutation), aligning with a significant family history of early-onset colon cancer. His initial surgery achieved an R0 resection, and the KRAS, NRAS, and BRAF genes were all wild-type. Fourteen months later, surveillance imaging detected peritoneal and nodal recurrence, with lesions measuring up to 10 cm alongside rising carcinoembryonic antigen (CEA) levels.

First-line XELOX plus bevacizumab was stopped after three cycles due to severe gastrointestinal toxicity. Given his microsatellite instability (MSI) status, he began off-label immunotherapy with nivolumab (240 mg every two weeks) in August 2018. Early assessments revealed stable disease with a 10% increase in tumor size but normalization of CEA. Subsequent follow-up scans demonstrated minor tumor shrinkage (up to 15%) and continued clinical stability. Throughout therapy, he maintained an active lifestyle and resumed full-time work.

Ultimately, the decision was made to proceed with cytoreductive surgery. Postoperative histological examination of peritoneal deposits showed copious mucin with intense fibroinflammatory changes—but crucially, no viable tumor cells remained. Flow-cytometry analysis of the fresh surgical specimen highlighted a robust immune infiltrate, especially B-cells and T-lymphocytes, with minimal PD-1 expression—likely reflecting immunomodulation by nivolumab.

The patient recovered swiftly from surgery, after which nivolumab was discontinued. At nine months post-surgery, repeat imaging and blood tests, including CEA, showed no signs of recurrence. He has fully regained his pre-disease level of activity and continues regular follow-up.

This case exemplifies the potential power of immunotherapy in MSI-high/Lynch syndrome-driven colorectal cancers, particularly for those with challenging peritoneal metastases. By integrating targeted systemic treatment, surgical expertise, and close multidisciplinary teamwork, advanced patients can achieve striking clinical responses and durable remissions.