Undifferentiated component of dedifferentiated endometrial carcinoma presenting as a "small round cell" malignancy in a mediastinal lymph node, mimicking small cell carcinoma.

Undifferentiated/dedifferentiated endometrial carcinoma is an uncommon malignant neoplasm of the endometrium that can present as a diagnostic challenge, especially in a metastatic setting. We present a case of a 70-year-old woman with a prior endometrial biopsy diagnosed as endometrioid carcinoma, FIGO Grade 2. Chest computerized tomography showed moderate to severe centrilobular emphysema with a 3 mm nodule in the right upper lobe and posterior mediastinal lymphadenopathy. Fine needle aspiration smears of the mediastinal lymph node showed predominantly single and loosely cohesive tumor cells with scant basophilic cytoplasm, prominent nuclear streaking, and molding. Inconspicuous nucleoli and mitotic figures were present. Immunohistochemical (IHC) stains showed the tumor cells were positive for CD56 and synaptophysin but negative for AE1/AE3, CAM5.2, CK7, CK20, TTF-1, INSM1, chromogranin, CD99, HMB45, SOX10, EBV-LMP1, and desmin. Flow cytometry was negative for lymphoma. Based on the overall cytologic findings and significant smoking history, a small cell carcinoma could not be excluded. Similar morphologic findings were identified on the corresponding lymph node biopsy. Because of the history of endometrial carcinoma, additional IHC stains (PAX 8, ER, and EMA) were done but were negative. However, the mismatch repair proteins revealed loss of MLH1 and PMS2 with retained MSH2 and MSH6 nuclear expression. Hence, a metastatic undifferentiated component of a dedifferentiated carcinoma from the patients' endometrial primary was favored and subsequently confirmed on the hysterectomy specimen.