Diagnostic challenge of Mixed Endometrial Adenocarcinoma with a Small Cell Neuroendocrine Carcinoma A Case Report

Small cell neuroendocrine carcinoma (SCNEC) of the endometrium is a rare, high-grade neuroendocrine neoplasm with aggressive biological behavior. It commonly coexists with endometrioid carcinoma and is often misdiagnosed as undifferentiated carcinoma, carcinosarcoma, primitive neuroectodermal tumor (PNET), or carcinoid tumor due to similar morphological features. We report the case of a 52-year-old female initially diagnosed with a primitive neuroectodermal tumor not otherwise specified (PNET-NOS) associated with endometrioid carcinoma. The patient underwent surgical resection followed by adjuvant chemotherapy; however, the disease progressed during treatment. A second pathological review was requested, leading to the diagnosis of mixed endometrial adenocarcinoma with a small cell neuroendocrine carcinoma component, FIGO stage IIIC2. This final diagnosis was based on the distinct pathological and immunohistochemical findings, along with the absence of EWSR1 gene rearrangement on fluorescence in situ hybridization (FISH) analysis, effectively excluding PNET. The patient is currently receiving second-line treatment for progressive disease.