Type B3 thymoma is an organotypic thymic epithelial tumor predominantly composed of medium-sized round or polygonal cells with slight atypia. The epithelial cells are mixed with a minor component of intraepithelial lymphocytes, resulting in a sheet-like growth of epithelial cells. The postulated cell of origin is a thymic epithelial cell capable of differentiating toward a less differentiated cortical-type epithelial cell than in B2 thymoma.¹

Type B3 thymomas account for ˜7% to 31% of all thymomas, the range reflecting imprecision in distinction from other B thymomas or thymic carcinoma. Patients’ ages range from 14 to 78 years, with a mean of 45 to 50 years. There is no consistent gender predominance.^{2,3,4,5,6,7,8,9}

The most frequent manifestations are symptoms of myasthenia gravis in 27% to 77% of cases.^{2,3,4,5,6,7,8,9} Rare complications are hypogammaglobulinemia (Good syndrome) and pure red cell anaplasia.^6,7,10 Patients with type B3 thymoma may experience symptoms caused by compression of the surrounding organs by an expansive mass, including but not limited to superior vena cava syndrome, dysphagia, cough, or chest pain. Some patients with type B3 thymoma have no symptoms at all, and the tumor can be detected for an unrelated problem as an enlarged mediastinal area or mass by x-ray, CT, or MR imaging.

Grossly, type B3 thymomas are usually not encapsulated but show a vaguely infiltrative border with extension into mediastinal fat or adjacent organs (Fig. 102.1).¹ The cut surface is typically firm and exhibits gray to white nodules separated by white fibrous septa.¹ Soft yellow or red foci, cyst formation, or hard calcified regions indicative of regressive changes may be observed.

Histologically, B3 thymoma demonstrates a lobular growth pattern with irregular septa, somewhat resembling the lobular architecture of the normal thymic cortex. The neoplastic cell component consists of sheets of medium-sized polygonal epithelial cells with eosinophilic to clear cytoplasm admixed with a minor component of lymphocytes (Figs. 102.2 and 102.3). In some type B3 thymomas, immature T lymphocytes might be absent. Foci of medullary differentiation are absent. The nuclei are round or elongated, often folded or grooved,
and characteristically smaller with less prominent nucleoli than in B2 thymoma (Figs. 102.2 and 102.3). The neoplastic epithelial cells might form palisades around perivascular spaces and along septa. Small foci of keratinization mimicking Hassall corpuscles may be present. In a minority of cases, tumor cells with round nuclei and prominent nucleoli resembling those of type B2 thymomas may be observed. Clear cell changes, spindle cell formation, and occasionally giant cells may also occur. Mitoses are rare.