Primary peritoneal clear cell carcinoma with metastasis mimicking ovarian carcinoma: a case report and literature review.

BACKGROUND: Primary peritoneal clear cell carcinoma (PPCCC) is an exceptionally rare malignancy that closely resembles gynecologic epithelial cancers clinically and histologically. Its pathogenesis is poorly understood, with possible origins from Müllerian metaplasia or malignant transformation of endometriosis. CASE PRESENTATION: A 48-year-old woman with no history of endometriosis or hormone therapy presented with acute lower abdominal pain. Imaging revealed a pelvic mass and a lesion in the pancreatic tail. The patient underwent hysterectomy with bilateral salpingo-oophorectomy and distal pancreatectomy with splenectomy. Intraoperatively, the pelvic mass was located in the peritoneum, while the gynecologic organs appeared grossly normal. Histopathologic examination of the pelvic, pancreatic, and splenic lesions revealed identical features of clear cell carcinoma. Immunohistochemistry demonstrated positivity for PAX8, CK7, and HNF1β, with loss of MSH2 and MSH6. Next-generation sequencing revealed ARID1A loss and somatic PIK3CA mutations. No primary ovarian, endometrial, or renal tumor was detected, supporting a diagnosis of primary peritoneal clear cell carcinoma with metastases to the pancreas and spleen. CONCLUSIONS: This case highlights the diagnostic challenges of PPCCC and offers valuable insights into the clinical and pathological spectrum of this underrecognized malignancy.