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thymoma type B3

Wednesday 29 May 2013


Type B3 thymoma is a thymic epithelial tumor composed predominantly of sheets of moderately atypical polygonal epithelial cells intermingled with a few non-neoplastic immature T-cells.

Type B3 thymoma accounts for between 10-25% of all thymomas and are frequently associated with myasthenia gravis.

At low power, type B3 thymomas have a lobular architecture with coarse or delicate fibrous bands dividing the tumor into irregular lobules as seen here.

Due to predominance of neoplastic epithelial cells and paucity of lymphocytes, type B3 thymoma has a pink appearance at low magnification, unlike type B1 or type B2 thymomas which appear basophilic.

The tumor cells are arranged in clusters or solid sheets separated into lobules by coarse or delicate fibrous bands.

Areas of hemorrhage, necrosis, and cystic change are commonly seen.


- Type B3 thymoma at Webpathology


Type B3 thymoma is composed of medium-sized round or polygonal cells arranged in clusters and sheets separated by thick fibrous septa. The tumor cells may exhibit mild cytologic atypia. They are admixed with a small component of lymphocytes.

Mild cytologic atypia and scant numbers of intraepithelial lymphocytes can be better appreciated in a high power view of a type B3 thymoma.

The tumor cells are polygonal with distinct cell borders and clear or eosinophilic cytoplasm.

Type B3 thymomas may show spindling of tumor cells.

The tumor can be widely invasive and infiltrated into the mediastinal fat – a feature commonly observed in type B3 thymomas. The paucity of lymphocytes is a diagnostic feature.

The tumor cells are arranged in solid sheets and show rare lymphocytes.

An extensive squamoid differentiation. Such thymomas are often referred to as squamoid thymomas. Type B3 thymoma can show a squamoid morule surrounded by polygonal tumor cells and scant lymphocytes.

Steroid treatment in thymoma patients presenting with myasthenia gravis may produce a sponge-like appearance with formation of microcysts containing foamy histiocytes.


Type B3 thymoma are usually invasive at presentation, often unresectable, and show recurrences and metastases in 15% and 20% of cases respectively.

See also

- thymoma / thymomas