Table 3.

Diagnostic criteria of thymomas in the 2015 World Health Organization classification of thymic tumors

Thymoma subtype	Obligatory criteria	Optional criteria
Type A	Occurrence of bland, spindle-shaped epithelial cells (at least focally); paucity1 or absence of immature (TdT+) T cells throughout the tumor	Polygonal epithelial cells; CD20+ epithelial cells
Atypical type A variant	Criteria of type A thymoma; in addition, comedo-type tumor necrosis; increased mitotic count (> 4/2 mm2); nuclear crowding	Polygonal epithelial cells; CD20+ epithelial cells
Type AB	Occurrence of bland, spindle-shaped epithelial cells (at least focally); abundance of immature (TdT+) T cells focally or throughout tumor	Polygonal epithelial cells; CD20+ epithelial cells
Type B1	Thymus-like architecture and cytology: Abundance of immature T cells, areas of medullary differentiation (medullary islands); paucity of polygonal or dendritic epithelia cells without clustering (i.e., < 3 contiguous epithelial cells)	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 TdT+ T cells	Hassall’s corpuscles; perivascular spaces
MNT	Nodules of bland spindle or oval epithelial cells surrounded by an epithelial cell-free lymphoid stroma lymphoid stroma	Lymphoid follicles; monoclonal B cells and/or plasma cells (rare)
Metaplasticthymoma	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 others2

The atypical type A thymoma variant is the only new addition to the “historic” list of thymoma subtypes.

¹Paucity versus abundance: Any area of crowded immature T cells or moderate numbers of immature T cells in more than 10% of the investigated tumor are indicative of “abundance”.

²Microscopic thymoma, sclerosing thymoma, lipofibroadenoma.

MNT: Micronodular thymoma with lymphoid stroma.