Primary thymic mucoepidermoid carcinoma with positive MAML2 gene rearrangement: a case report

Background: Mucoepidermoid carcinoma (MEC) is a malignant tumor primarily composed of epidermoid, mucous, and intermediate cells. While it predominantly occurs in the salivary glands, MEC can also develop in the lungs, esophagus, breast, and thymus. Thymic MEC is rare, accounting for 2–3% of primary thymic malignancies. The primary clinical characteristic is incidental discovery in asymptomatic individuals. Due to the rarity of thymic MEC, much of the literature consists of case reports, and no standardized treatment protocol exists. Surgical resection is the primary treatment for MEC. Case Description: This report presents a case of thymic MEC identified incidentally during a routine physical examination in an asymptomatic patient. Chest computed tomography (CT) revealed a well-defined, rounded cystic-solid mass measuring approximately 61 mm × 32 mm in the right anterior mediastinum, with mild enhancement. Following comprehensive assessment to rule out surgical contraindications, the mass was resected. Postoperative pathology indicated a low-grade primary MEC, positive for epithelial membrane antigen (EMA), cytokeratin (CK)19, p63, CK7, cluster of differentiation (CD)5, and mucicarmine/Alcian blue (AB)-periodic acid-Schiff (PAS) staining. Ki-67 was ~5%, and fluorescence in situ hybridization (FISH) analysis confirmed mastermind-like transcriptional coactivator 2 (MAML2) gene rearrangement. The patient received no additional treatment. Conclusions: This report details a rare case of thymic MEC. The diagnosis required pathology and molecular testing. MAML2 rearrangement suggests a better prognosis; further research is needed to elucidate its pathogenesis and improve its treatment.