GLYCOGEN-RICH CLEAR-CELL RHABDOMYOSARCOMA OF THE MEDIASTINUM - POTENTIAL DIAGNOSTIC PITFALL

A 63-year-old black man of Caribbean origin, seropositive for human T- cell lymphoma virus type I (HTLV-I), presented with a 4-week history o f progressive dyspnea, and was found to have a tumor of the anterior m ediastinum. Incisional biopsy revealed a malignant neoplasm with a sol id pattern of glycogen-rich clear cells. Diffuse expression of vimenti n was observed, whereas only rare cells were immunoreactive for muscle -specific actin and desmin. Ultrastructure revealed a large amount of monoparticulate glycogen in most cells and features of rhabdomyogenic differentiation in occasional cells. The autopsy revealed a 23 x 14-cm (1,345 g), soft and white mediastinal neoplasm bulging in the right t horax with right pleural metastases. HTLV-1 proviral genome was not de tected within tumor cells by polymerase chain reaction. This rhabdomyo sarcoma is best classified as the solid subtype of the alveolar varian t, with an unusually large amount of cytoplasmic glycogen. Clear cell rhabdomyosarcoma could be potentially confused with more commonly enco untered clear cell tumors, particularly in the mediastinum.