Rhabdomyoma of the head and neck: morphology and differential diagnosis.

Rhabdomyomas of the head and neck are exceptionally rare benign mesenchymal tumors. Although histology is very characteristic, several differential diagnoses have to be considered. We investigated five patients with extracardiac rhabdomyoma of the head and neck (median age 65.9 years), four of them presenting with adult rhabdomyoma (AR) and one with fetal rhabdomyoma (FR). We analyzed the histological findings, with special regard to separation from hibernoma (two patients) and granular cell tumor (GCT; six patients, median age 31 years). Both FR and AR showed polygonal eosinophilic cells with peripherally or centrally localized nuclei and cross striations, while in hibernoma, multivacuolated cells with centrally localized nuclei were detected. In GCT, polygonal eosinophilic cells with granular periodic-acid-Schiff-positive cytoplasm were found; in one case, atypical GCT with increased pleomorphism and mitotic rate was observed. Pseudoepitheliomatous hyperplasia occurred both in FR and GCT. Immunohistochemically, rhabdomyomas were strongly positive for myogenic markers (desmin, actin, and myoglobin) but negative for S-100, while hibernoma and GCT strongly expressed S-100. Concerning the differential diagnosis of rhabdomyoma, GCT has to be especially considered since this tumor can undergo malignant transformation.