The World Health Organization (WHO) developed a classification for the histologic appearance of thymic epithelial tumors. This correlates with prognosis.
Histologic Features of Tumor Cells
Major Type
spindle or oval shaped epithelial cells with no or few non-neoplastic lymphocytes
A
dendritic or plump ("epithelioid")
B
mixture of both A and B features, with spindle or oval shaped epithelial cells and lymphocyte-rich areas
AB
carcinoma with obvious cytologic anaplasia
C
Type B is divided into subtypes B1, B2, and B3.
Features of Type B1:
(1) resembles normal thymus with cortical and medullary areas
Features of Type B2
(1) large numbers of lymphocytes
(2) scattered epithelial cells with vesicular nuclei and distinct nucleoli
(3) perivascular spaces are a common features, sometimes showing a palisading appearance
Features of Type B3:
(1) large numbers of epithelial cells, often sheetlike, with lymphocytes a minor component
(2) epithelial cells are round or polygonal in shape, with nuclei show no or minimal cytologic atypia
The classification correlates with (1) likelihood of invasion and (2) clinical aggressiveness, with risk C > B3 > B2 > B1 > AB > A.
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