[Complete remission induced by immunotherapy in peritoneal carcinomatosis due to colorectal cancer].

We present the cases of two patients who underwent surgery for right-sided colon adenocarcinoma stage III and, after receiving adjuvant therapy, developed early peritoneal recurrence. In both cases, the molecular phenotype was MSI-H/dMMR, which led to treatment with immunotherapy, resulting in a complete clinical response. The first patient, a 51-year-old woman, was diagnosed with moderately differentiated adenocarcinoma (pT-3N1bM0). Immunohistochemistry revealed loss of MLH1 and PMS2. Six months after completing adjuvant treatment with CAPOX, she developed peritoneal recurrence. She started combination therapy with ipilimumab and nivolumab, achieving complete metabolic and imaging response as confirmed by PET-CT at 12 months. The second case concerns an 81-year-old woman with adenocarcinoma (pT3N2Mx) and loss of expression of MLH1 and PMS2. She presented with peritoneal relapse five months after completing adjuvant therapy. She started treatment with pembrolizumab, achieving a sustained complete response at 12 months. Currently, 22 months after starting treatment, the patient is in optimal functional status (ECOG 0). She continues with pembrolizumab 800 mg every 42 days as a maintenance regimen. Both cases illustrate the remarkable clinical benefit of immunotherapy in patients with early peritoneal recurrence of MSI-H/dMMR colon cancer, which is usually associated with poor prognosis. These cases highlight the need to consider the molecular profile in therapeutic planning and reinforce the emerging value of immunotherapy as a cornerstone in the management of these cases.