Clear cell rhabdomyosarcoma.

A clear cell rhabdomyosarcoma was studied by light microscopy, histochemistry, immunohistochemistry, and electron microscopy. It was a large, painful left parapharyngeal mass in a 10-year-old boy with intracranial extension and cervical metastatic enlarged lymph nodes. Tumor tissue was macroscopically grayish. At microscopic examination, the architecture was diffuse and focally alveolar. Tumor cells were of three types. Most cells were large, round or polygonal, with abundant clear vacuolated cytoplasm. Fibrils were sometimes found to be present around the nucleus. Nuclei often had irregular outlines and multiple nucleoli. Mitotic activity was high. Some round or elongated cells had eosinophilic fibrillar cytoplasm and were found to have a few double striations. A few cells were round and medium sized with a high nucleocytoplasmic ratio. Periodic acid-Schiff stain demonstrated huge amounts of intracytoplasmic glycogen in clear cells. Tumor cells showed positive immunostaining for muscle markers (desmin, muscle actins, dystrophin). Electron microscopy showed large lakes of glycogen, lipid droplets, and striated muscle features.