Congenital Primary Cutaneous Rhabdomyosarcoma of the Perineum with Mixed Histopathological Features and Unusual Expression of Smooth Muscle Actin: Report of a Case

Rhabdomyosarcoma (RMS) affecting the soft tissue of the perianal and perineal regions in children is uncommon and account for 2% of all rhabdomyosarcomas. Primary cutaneous presentation in this anatomical location is extremely rare. We report a 9-month-old female infant with a congenital primary cutaneous RMS presenting as a polypoidal exophytic mass in the perineum mimicking giant condyloma acuminatum (Buschke and Löwenstein tumor). Histopathological examination revealed a predominantly alveolar RMS with areas showing embryonal and pleomorphic features. In addition to expressing desmin and myogenin, the tumor cells also expressed smooth muscle actin.