Primary Peritoneal High-Grade Serous Carcinoma Presenting With Diffuse Carcinomatosis and No Ovarian Mass: A Case of Exceptional Response.

Primary peritoneal serous carcinoma (PPSC) is a rare Müllerian malignancy that closely resembles ovarian carcinoma but is distinguished by minimal or absent ovarian involvement despite extensive peritoneal disease. Diagnosis becomes particularly challenging when no adnexal mass is identified, and in such cases, ascitic fluid cytology with Müllerian immunophenotyping may provide the only definitive diagnostic pathway. We report the case of a 63-year-old woman who presented with worsening abdominal pain, diffuse ascites, and imaging findings consistent with peritoneal carcinomatosis but without an ovarian or pelvic mass. Ascitic fluid cytology repeatedly demonstrated malignant epithelial cells expressing PAX8, CK7, and abnormal p53, confirming Müllerian origin and supporting a diagnosis of PPSC. Serial CA-125 levels rose markedly prior to treatment initiation, reaching 864.8 U/mL. The patient received neoadjuvant chemotherapy with carboplatin, paclitaxel, and bevacizumab, resulting in rapid symptomatic improvement, normalization of CA-125, and complete radiologic resolution of carcinomatosis. She subsequently underwent interval debulking surgery, during which no residual viable carcinoma was identified, consistent with a pathologic complete response. This case illustrates the diagnostic challenges of PPSC in the absence of an adnexal mass and demonstrates the value of ascitic cytology and Müllerian immunoprofiles when tissue biopsy is not feasible. It also highlights the prognostic significance of robust chemotherapy response, as pathologic complete remission remains uncommon in high-grade serous Müllerian carcinomas. Overall, this case underscores the importance of recognizing atypical presentations of PPSC and shows that exceptional therapeutic outcomes are achievable with appropriately selected neoadjuvant therapy.