Primary thymic mucinous adenocarcinoma: a case report

Abstract

Primary thymic mucinous adenocarcinoma is an extremely rare aggressive subtype of thymic carcinoma. With a review of literatures, only nine cases have been reported up to present. A 36-year-old woman was admitted for further evaluation and treatment of a mediastinal mass. The patient had no medical history of cancer. The clinicoradiological examination disclosed no tumor elsewhere. After the surgical excision of mediastinal mass, it was grossly a round semi-solid mass with mucin-filled cystic areas. Microscopically solid areas showed cords, small nests and dilated glands infiltrating the fibrotic parenchyma, while the cystic areas were lined by mucinous epithelium with tumor cells floating in extracellular-mucin pools. Some cystic walls underwent malignant transformation of the benign thymic epithelium. Immunohistochemically, the tumor cells were positive for cytokeratin (CK) 7, CK20, CD5, and CDX-2, and negative for thyroid transcription factor-1. In conclusion, the mucinous thymic adenocarcinoma should be recognized as a separate histopathological entity and considered in the differential diagnosis of mediastinal carcinomas.

Keywords: Adenocarcinoma, mucinous; Immunohistochemistry; Mediastinal cyst; Thymus gland.

Fig. 1

Radiologic and gross findings. (A) The chest computed tomography (CT) shows a 5.6×4.4 cm sized, multiloculated cystic mass (arrow) with heterogenous enhancing septa (37→57 hounsfield unit) in the right anterior mediastinum. (B) On positron emission tomography-CT scans, the solid part of the mass shows nodular F-18 fluorodeoxyglucose (FDG) uptake. There is no remarkable FDG uptake in the entire body. (C) A round mass with a smooth, lobulated external surface. (D) The cut surface reveals yellow-white solid mass with multiple cysts of variable sizes, filled with mucinous fluid.

Fig. 2

Microscopic findings. (A) The low microscopic field shows the residual thymic tissue in lymphocyte-rich cortex (right lower) and multifocal cyst formation. (B) The cyst is lined by malignant mucious epithelium undergoing the transition from the benign flattened thymic epithelium of the cyst wall. (C) The malignant mucinous glands infiltrates in the fibrotic parenchyma of tumor. (D) Island of atypical cells floats in pools of extracellular mucin.

Fig. 3

Immunohistochemical stains. (A) The immunhistochemical stains reveal strong positivity for cytokeratin (CK) 7 and (B) CK20 and focal positivity for (C) CDX-2. (D) The thymic tissue and epithelium lining the thymic (benign) cyst shows a positive reaction for CD5.