Small-cell neuroendocrine carcinoma in directly sampled endometrial cytology: A monocentric retrospective study of six cases.

Small-cell neuroendocrine carcinoma (NEC) of the endometrium is extremely rare, and demonstrates an aggressive clinical course. Since reports describing its cytological features are scarce, we aimed to retrospectively analyze these features. Patients with a histopathological diagnosis of NEC who underwent preoperative cytological examination, were enrolled in this study. The cytological features including the background, arrangement, and shape of the neoplastic cells, and the nuclear and cytoplasmic features were reviewed; six patients were enrolled. The conventionally stained, directly sampled cytological specimens showed small neoplastic cell clusters in all cases, as well as isolated neoplastic cells and large clusters in 3 and 2 cases, respectively, in inflammatory or necrotic backgrounds. These neoplastic cells had a high nuclear/cytoplasmic ratio, and round to oval nuclei with powdery chromatin, inconspicuous nucleoli, and scant cytoplasm. Nuclear molding was a characteristic finding. An adenocarcinoma component was also present in 3 cases. Initial cytodiagnosis revealed small-cell NEC and adenocarcinoma or suspected adenocarcinoma in 1 and 4 cases, respectively. The one remaining case was found to be negative and was considered as degenerated endometrial stromal cells. Primarily owing to overlooking this component, the initial cytodiagnostic accuracy of small-cell NEC was low, particularly in cases with coexisting adenocarcinoma. However, the cytological features of this tumor were characteristic. Therefore, although extremely rare, careful observation is essential for an early and accurate diagnosis and to prevent overlooking the small-cell NEC component.