Myofibromatosis in adults (adult counterpart of infantile myofibromatosis).

Five solitary, benign, soft-tissue tumors histologically resembling infantile myofibromatosis but which occurred in adults were found among more than 5,000 benign soft-tissue tumors from a tumor registry. The tumors clinically presented as superficial, painless, and slowly enlarging nodules, usually of more than 10 years' duration, that occurred in the upper (two cases) and lower (two cases) extremities or the buccal mucosa (one case). They developed in the dermis and subcutis as well-circumscribed nodules with an average diameter of 1.2 cm. They were composed of discrete and confluent fibrous tissue with a mixture of bundles of smooth muscle-like cells and a hemangiopericytoma-like area of immature mesenchymal cells. Immunostaining for actin and intermediate filaments revealed the myofibroblastic nature of the tumor cells. The tumors were surgically excised, and there has been no recurrence. Clinicians and pathologists should note that the lesion of infantile myofibromatosis can and does occur in adults.