Is There a Spectrum of Cytologic Atypia in Type A Thymomas Analogous to That Seen in Type B Thymomas? A Pilot Study of 13 Cases

Thirteen cases of type A thymoma are reported showing the characteristic architectural attributes of this tumor type (World Health Organization), such as lobulation, perivascular spaces, gland-like formations, and cystic changes, but also displaying atypical features, as defined by increased mitotic activity, mild to moderate nuclear atypia, and/or scattered small foci of necrosis. The tumors were similar to type B3 thymomas in many respects, except for the fact that the tumor cells were spindled instead of round/polygonal. The existence of these cases suggests the desirability of an expansion of the type A thymoma category, analogous to that currently used for type B tumors. The various nomenclature options that could be used to name these tumors have been discussed.