Table 4. Major and minor histological feature of type B1 versus B2 thymomas.

Features	Type B1 thymoma	Type B2 thymoma
Major criteria
Thymus-like pattern throughout	Consistently present	Rarely present
Medullary islands (+/− Hassal’s corpuscles)	Consistently present	Occasionally presenta
Confluence of epithelial cells in cortical areasb	No (like in the NT)	Yes
Absence of type A areas (even if <10%)	Yes	Yes
Minor criteria
Small lobular growth pattern	Rare	Common
Large lobular growth pattern	Common	Rare
Perivascular spaces	Commonly present	Commonly present
Keratin+c network like in NT	Yes	Denser than in NT

^a, These features are, therefore, minor criteria of type B2 thymomas; ^b, defined as at least three contiguous epithelial cells; ^c, on immunostaining. NT, normal thymus.