Table 1. The World Health Organization histopathological classification of thymic epithelial tumours

Type	Pathological features	Invasiveness %	10-yr disease-free survival %
A	Spindle or polygonal cell lacking atypia	10–40	100
	Resembles medullary thymoma with paucity of immature thymocytes
AB	Mixed pattern of type A (lymphocyte-poor) and type B (lymphocyte-rich) thymoma	30–40	100
	Immature and mature lymphocytes
B1	Thymoma resembling the cortical thymus	45–50
B2	Subtypes B1, B2 and B3 differentiated by an increasing epithelial/lymphocyte ratio and the emergence of atypia	65–70	85
B3	Mature lymphocytes	85–90	35
Thymic carcinoma	Epithelial cell atypia	90–95	15
	Infiltration
	Absence of immature lymphocytes
	Frequent expression of CD5/KIT