Adult rhabdomyoma of the lip.

A 75-year-old man presented with a slowly enlarging crust on the left lower lip. Beneath the crust, there was a palpable nodule approximately 1 cm. A superficial punch biopsy demonstrated an actinic keratosis of the lip. A vermillonectomy with a wedge-shaped excision of the nodule was performed because invasive squamous cell carcinoma was suspected. Histologically, the tumor proved to be an adult rhabdomyoma with an overlying actinic keratosis. The tumor cells were positive for desmin and muscle-specific actin and negative for vimentin, alpha-smooth muscle actin (alpha-SMA), and S100 protein. Some areas of cross-striation were present. Only very rarely are dermatologists confronted with these slow-growing tumors with skeletal-muscle differentiation, occurring predominantly in the head and neck area of older adults. The histologic differential diagnosis includes granular cell tumor, hibernoma, oncocytoma and paraganglioma.