Thymic basaloid carcinoma is a rare subtype of thymic carcinoma, with < 40 cases published in the literature, mostly in the form of individual case reports.^1,2,3,4,5 The tumor is composed of compact lobules of neoplastic cells with peripheral palisading and a basophilic staining pattern due to a high nuclear-to-cytoplasmic ratio. Basaloid carcinoma shows a remarkable tendency to originate in multilocular thymic cysts.⁶

Most cases occur in the fifth decade of life with a male-to-female ratio of 5:1.⁷ The symptoms are nonspecific. Patients may show symptoms related to a mediastinal mass, for example, chest pain or dyspnea on exertion.⁸ In asymptomatic patients, the tumor may be detected as an incidental finding on radiographic imaging.⁸ No paraneoplastic autoimmune phenomena such as myasthenia gravis are observed.⁷

Grossly, thymic basaloid carcinomas are mostly well-circumscribed, gray to tan masses surrounded by a thin fibrous capsule with focal hemorrhage and cyst formation. In about 60% of reported cases, basaloid carcinomas were found as a mural nodule in a multilocular thymic cyst and/or showed cystic changes in the tumor.^7,8

Microscopically, thymic basaloid carcinoma is composed of monotonous, small- to medium-sized, columnar, round to oval, or vaguely spindled tumor cells with high nuclear-to-cytoplasmic ratios, hyperchromatic round to oval nuclei with inconspicuous nucleoli, scant amount of amphophilic cytoplasm, and indistinct cytoplasmic borders (Fig. 108.1).^7,8 The cells are haphazardly arranged in trabeculae, anastomosing cords, islands, and nests, and typically show prominent palisading at the periphery.^7,8 Occasionally, focal keratinization in the center of the cell nests is noted. Perivascular spaces can be prominent. Mitoses are frequent. There may be areas with numerous poorly formed glands lined by basaloid tumor cells containing pink, amorphous basement membrane-like material.^7,8 The multilocular thymic cyst frequently associated with thymic basaloid carcinoma is lined by benign-appearing squamous epithelium, which may imperceptibly blend with the basaloid tumor cells.⁷