The histologic distinction between Type A and Type AB thymoma can be challenging at times.
Patient selection: thymoma
Common histologic features:
(1) presence of focal or diffuse bland-looking spindle epithelial cells (keratin positive)
(2) variable number of oval and polygonal tumor cells some of which may be CD20+
Features of Type AB thymoma - one or both of the following:
(1) any area showing a dense infiltrate of immature T-cell lymphocyte (type B-like)
(2) > 10% of tumor showing areas with moderate infiltration by immature T cells
where:
• The immature T-cells are TdT positive.
Features of Type A thymoma - both of the following:
(1) no lymphocyte dense areas
(2) <= 10% of tumor showing a moderate number of immature T cells
