Massive gastrointestinal stromal tumor with exophytic growth mimicking pelvic leiomyosarcoma: a case report and review of the literature.

Introduction: Gastrointestinal stromal tumors (GISTs) are mesenchymal neoplasms of the gastrointestinal (GI) tract. Large, exophytically growing GISTs from the small bowel can present as massive abdominopelvic masses, creating a diagnostic challenge by mimicking primary gynecologic malignancies such as uterine leiomyosarcoma. Case Presentation: A 55-year-old woman presented with a 20-cm pelvic mass, initially diagnosed on computed tomography as a uterine leiomyosarcoma. Exploratory laparotomy, however, revealed the mass originated from the small bowel. Following resection, histopathology confirmed a malignant GIST with significant mitotic activity. The diagnosis was secured by strong immunohistochemical positivity for CD117 and DOG1. The patient remained disease-free at her 6-month follow-up. Clinical Discussion: This case highlights a critical diagnostic pitfall where a GIST's exophytic growth pattern obscures its true origin on imaging. A high index of suspicion for GIST must be maintained in the differential for any large abdominopelvic mass, regardless of its apparent source. An accurate diagnosis requires integrating surgical findings with immunohistochemical profiling, which is paramount for guiding appropriate oncologic management. Conclusion: Massive small bowel GISTs can radiologically mimic primary pelvic malignancies. This case emphasizes including GIST in the differential diagnosis of large pelvic masses, even when imaging suggests a gynecologic origin. Definitive diagnosis relies on a multidisciplinary approach to ensure accurate classification and appropriate oncologic therapy.