Table 1.

WHO classification for thymoma

2004 WHO classification	Description
A (spindle cell thymoma; medullary thymoma)	Bland spindle/oval epithelial tumour cells with few or no lymphocytes
AB (mixed thymoma)	Mixture of a lymphocyte-poor type A thymoma component and a more lymphocyte-rich type B-like component (smaller and paler than those of B1 or B2 thymomas). Lymphocytes are more numerous than in the type A component, but may be less numerous than in B1 thymomas
B1 (lymphocyte-rich thymoma; lymphocytic thymoma; organoid thymoma; predominantly cortical thymoma)	Epithelial cells with a histological appearance practically indistinguishable from the normal thymus, composed predominantly of areas resembling cortex with epithelial cells scattered in a prominent population of immature lymphocytes
B2 (cortical thymoma)	Tumour cells closely resembling the predominant epithelial cells of the normal thymic cortex. A background population of immature T cells is always present and usually outnumbers the neoplastic epithelial cells
B3 (epithelial thymoma; squamoid thymoma)	Medium–sized round or polygonal cells with slight atypia. The epithelial cells are mixed with a minor component of intraepithelial lymphocytes

From Travis et al. [24]