Table 3 World Health Organization (WHO) histological classification of thymomas
From: Primary thymus tumors: retrospective case analysis at a reference center in Latin America, 2011–2019
Thymoma subtype | Obligatory criteria | Optional criteria |
|---|---|---|
Type A | Occurrence of bland, spindle shaped epithelial cells (at least focally); paucitya of 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 the 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 (< 3 contiguous epithelial cells) | Hassall’s corpuscles; perivascular spaces |
Type B2 | Increased numbers of single or c lustered 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 |
MNTb | Nodules of bland spindle or oval epithelial cells surrounded by an epithelial cell free lymphoid stroma | Lymphoid follicles; monoclonal B cells and/or plasma cells (rare) |
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 or epithelial cells; actin, keratin or EMA-positive spindle cells |
Rare othersc |