Undifferentiated Pleomorphic Sarcoma Recurrence Observed two months Postoperatively in Case Report of Gastric Mass

Undifferentiated pleomorphic sarcoma (UPS) is a rare and aggressive malignant soft tissue sarcoma. While most commonly arising in the extremities and trunk, primary occurrence in the stomach is exceedingly rare. Due to its rarity and nonspecific presentation, preoperative diagnosis is challenging and often established postoperatively through detailed histopathological and molecular analysis. We report the case of a 71-year-old female presenting with a gastric mass and anorexia, who was preoperatively misdiagnosed with gastrointestinal stromal tumor(GIST) based on clinical and imaging characteristics. Due to the massive tumor burden and anatomical complexity, the patient underwent an R0 resection combined with splenectomy. The final diagnosis of primary gastric UPS was established through an exclusionary process involving comprehensive histopathological, immunohistochemical (including negativity for CD117, CD34, DOG-1), and molecular analyses to rule out other tumors with specific lines of differentiation. Despite adjuvant doxorubicin chemotherapy, follow-up imaging revealed tumor recurrence two months postoperatively. This case underscores the highly aggressive biological behavior and dismal prognosis associated with primary gastric UPS. The rapid recurrence following multimodal therapy highlights the critical importance of considering UPS in the differential diagnosis of gastric soft tissue masses and emphasizes the urgent need for more effective, comprehensive therapeutic strategies.