Peritoneal Sarcomatosis Secondary to a Gastrointestinal Stromal Tumor Treated With a Multimodal Approach: A Case Report.

We report the case of a 49-year-old woman diagnosed with peritoneal sarcomatosis secondary to an epithelioid gastrointestinal stromal tumor (GIST) (Ki-67 3%, CD117 and DOG1 positive), initially managed with empirical carboplatin-paclitaxel, which was discontinued after histological confirmation. Treatment with imatinib 400 mg/day was initiated, achieving sustained metabolic response and clinical stability. After 12 months of targeted therapy, the patient underwent cytoreductive surgery (CRS) with hyperthermic intraperitoneal chemotherapy (HIPEC) using mitomycin C. Intraoperative findings revealed extensive peritoneal disease with distal jejunal involvement, achieving CC-1 cytoreduction. The postoperative course was uneventful, and pathology confirmed a metastatic GIST with low mitotic activity. At one-year follow-up, the patient remains clinically stable, with preserved functional status and no evidence of radiological progression. This case illustrates the potential role of a multimodal strategy combining targeted therapy, CRS, and HIPEC in selected patients with peritoneal involvement from GISTs.