Microcytic variant of thymoma: histological and immunohistochemical findings in two cases.

Two cases of microcytic variant of thymoma are presented. Both tumours were well encapsulated with a yellow-whitish colour and soft consistency. Microscopically, they consisted of round cells, having ample vacuolated cytoplasm. Fat droplets were not detected in one case where fat staining was performed. Immunohistochemically, the tumour cells were strongly positive for AE1/AE3, MB1, MB2, and LN1 and faintly positive for epithelial membrane antigen. They lacked any other leucocyte antigens. Leu 7 showed a positive immunoreaction in a ring-like or homogeneous pattern, compatible with the cytoplasmic vacuoles or cytoplasm. Ultrastructurally, the vacuoles resembled cystically dilated rough-surfaced endoplasmic reticulum. Desmosome-like structures (case 1) and intermediate junctions (case 2) were identified between adjacent cells. These findings indicate that the present tumours belong to a category of microcystic thymoma. The vacuoles were attributed to excess accumulation of Leu-7-positive material, probably in the cystically dilated endoplasmic reticulum.