Introduction and importanceSarcomatoid-predominant biphasic peritoneal metastases is a rapidly progressing and deeply invasive variant of this disease with survival measured in months. Although cytoreductive surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC) is a standard of care for epithelioid peritoneal mesothelioma, the sarcomatoid variant is so aggressive, the standard treatment is not recommended. Immunotherapy has recently been utilized for pleural mesothelioma. Partial responses to immunotherapy may be combined with CRS to achieve a favorable outcome in sarcomatoid-predominant peritoneal mesothelioma.Case presentationA 39-year-old woman developed an expanding abdomen. A 10 cm pelvic mass was removed by hysterectomy. With an initial diagnosis of advanced ovarian cancer, she was treated with cisplatin plus paclitaxel. Disease progression led to a review of her original pathology and a repeat biopsy which showed biphasic peritoneal mesothelioma with sarcomatoid predominance. Treatment with Nivolumab was transiently beneficial. Repeat CT 8 months later showed partial bowel obstruction and necrotic expanding tumor masses that were partially calcified. CRS with HIPEC and normothermic long-term intraperitoneal pemetrexed (NIPEC) plus intravenous cisplatin resulted in a 5-year disease-free survival.Clinical discussionThe specimens removed at CRS showed marked progression within large masses. Smaller masses resected with CRS showed fibrosis and calcification. The response to Nivolumab was heterogeneous with smaller masses with good blood supply adequately treated but larger masses markedly progressed.ConclusionsA combination of partial response to immunotherapy with a complete CRS plus HIPEC and NIPEC can result in a long-term favorable outcome.