Abstract
RATIONALE: Well-differentiated papillary mesothelial tumor (WDPMT) is an uncommon, slow-growing neoplasm that is often an incidental finding in the peritoneum of women of reproductive age. It can be misdiagnosed as malignant mesothelioma on routine histology, potentially leading to unnecessary aggressive therapy. The role of modern molecular techniques in resolving this diagnostic dilemma and guiding conservative management warrants emphasis. PATIENT CONCERNS: A 32-year-old woman presented with vague abdominal discomfort. Imaging revealed bilateral ovarian cysts and significant pelvic free fluid. DIAGNOSES: Laparoscopy identified multiple small peritoneal nodules. Initial frozen-section histology suggested malignant mesothelioma. Definitive diagnosis of WDPMT was established through comprehensive immunohistochemistry (retained BAP1 expression) and next-generation sequencing, which revealed a low tumor mutational burden and a pathogenic GPR124 mutation. This genetic profile distinct from malignant mesothelioma. INTERVENTIONS: After definitive diagnosis and thorough counseling, the patient opted against cytoreductive surgery or systemic therapy. A strategy of active surveillance with serial imaging and tumor marker assessment was implemented. OUTCOMES: The patient remained asymptomatic with no evidence of disease recurrence or progression 24 months after diagnosis. LESSONS: This case highlights that a definitive distinction between WDPMT and malignant mesothelioma is paramount, as it dictates a radically different management strategy. Integration of immunohistochemistry (particularly BAP1) and molecular profiling is essential for accurate diagnosis and can prevent overtreatment. For appropriately selected patients with WDPMT, conservative management with active surveillance represents a safe and effective approach, underscoring the value of precision medicine in guiding patient-centric care.