Table 2.

Histological classification for thymoma according to the World Health Organization (WHO) [13].

Thymoma Subtypes	Obligatory Criteria	Optional Criteria
Type A	Occurence of bland, spindle shaped epithelial cells; paucity or absence of immature T cells	Polygonal epithelial cells CD20+ Epithelial cells
Atypical type A variant	Criteria of type A, in addition comedo-type tumor necrosis; increased mitotic count, nuclear crowding	Polygonal epithelial cells CD20+ Epithelial cells
Type AB	Occurrence of bland, spindle shaped epithelial cells; abundance of immature T cells	Polygonal epithelial cells CD20+ Epithelial cells
Type B1	Thymus-like architecture and cytology; abundance of immature T cells, areas of medullary differentiation; paucity of polygonal or dendritic epithelia cells without clustering	Hassall’s corpuscles; perivascular spaces
Type B2	Increased numbers of single or clustered polygonal or dendritic epithelial cells intermingled with abundant immature T cells	Medullary islands; Hassall’s corpuscles; perivascular spaces
Type B3	Sheets of polygonal slightly to moderately atypical epithelial cells; absent or rare intercellular bridges; paucity or absence of intermingled T cells	Hassall’s corpuscles; perivascular spaces
MNT (micronodular thymoma with lymphoid stroma)	Nodules of bland spindle or oval epithelial cells surrounded by an epithelial cell-free lymphoid strome	Lymphoid follicles; monoclonal B cells and/or plasma cells
Metaplastic thymoma	Biphasic tumor composed of solid areas of epithelial cells in a background of bland-looking spindle cells; absence of immature T cells	Pleomorphism of epithelial cells; actin, keratin, or EMA-positive spindle cells
Rare others (microscopic thymoma, sclerosing thymoma, lipofibroadenoma)