Primary poorly differentiated enteric-type adenocarcinoma of the thymus with characteristic intracytoplasmic vacuoles on cytology.

Primary thymic enteric-type adenocarcinomas are exceedingly rare. This rare case highlights the significance of intracytoplasmic vacuoles (ICVs) as a cytologic marker for primary enteric-type adenocarcinoma of the thymus. We present a case of a 62-year-old Japanese man who experienced progressive dyspnea for two months. Chest computed tomography (CT) revealed a 66 × 65 × 50-mm irregular mass in the right anterior mediastinum. Both CT-guided biopsy and needle-washing cytology were performed. Papanicolaou-stained specimens showed atypical cells with a high nuclear-cytoplasmic (N/C) ratio, bare nuclei on a hemorrhagic background, and small nucleoli with mild nuclear atypia, lacking glandular, luminal, or papillary structures. Giemsa-stained specimens revealed ICVs. Histological examination showed atypical cells arranged in solid nests. Immunohistochemical profiling showed CEA (+), CD5 (+), cytokeratin (CK) 7 (+), CK20 (+), adipophilin (+), CDX2 (-), TTF-1(-), p40 (-), and CD117 (KIT) (-). On the final diagnosis was poorly differentiated enteric-type adenocarcinoma, classified as cT3N2M0, cStage IVb. After neoadjuvant chemotherapy, the patient underwent surgical resection at Hamamatsu University Hospital in Japan. Postoperative findings revealed poorly differentiated adenocarcinoma with treatment-induced changes, measuring 60 × 50 × 40 mm, and classified as ypT3N0M0, with lymphovascular invasion. Despite adjuvant chemoradiotherapy, metastases developed 26 months postoperatively. The ICVs are immunoreactive for adipophilin and are therefore considered to represent lipid vacuoles. Identifying tumor cells with ICVs in cytology specimens and immunocytochemistry for CK7, CK20, CD5, p40, CDX2, MUC2, KIT, adipophilin, and CEA may be useful for diagnosis.