Thymic enteric type adenocarcinoma: A case report with cytological features.

Primary thymic adenocarcinoma is an extremely rare tumor, and thymic enteric type adenocarcinoma has recently been proposed as a distinct pathological entity. Herein, we report the first cytological description of thymic enteric type adenocarcinoma. A 29-year-old Japanese female without a significant past medical history was found to have an abnormal chest shadow. Chest computed tomography demonstrated a well-circumscribed tumor in the anterior mediastinum, and thymectomy was performed. The Papanicolaou staining of the touch smear of the resected tumor demonstrated tightly cohesive epithelial cell clusters in a necrotic background. These cells were cuboidal to columnar in shape and had large round to oval nuclei with conspicuous nucleoli. Some of these neoplastic cells had intracytoplasmic mucin. Immunocytochemically, the neoplastic cells were positive for cytokeratin 20 and CDX-2. Histopathological study revealed tubular and papillotubular neoplastic growth composed of cuboidal to columnar neoplastic cells that contained large round to oval nuclei. Some of the neoplastic cells had intracytoplasmic mucin. Immunohistochemical study confirmed the expression of cytokeratin 20 and CDX-2. The final diagnosis of thymic enteric type adenocarcinoma was made. The cytological and immunocytochemical features of this case led to a diagnosis of enteric type adenocarcinoma. However, these features alone cannot be differentiated from a metastatic adenocarcinoma arising from the gastrointestinal tract. Cytological examination of a fine-needle aspiration of the mediastinal tumor has been reported to be useful in making a diagnosis. Therefore, an awareness of this new pathological entity is important for differentiating a thymic tumor from a metastatic carcinoma in the thymus.