Dermatomyofibroma: clinicopathologic and immunohistochemical analysis of 56 cases and reappraisal of a rare and distinct cutaneous neoplasm.

Dermatomyofibroma represents a rare and distinct benign cutaneous mesenchymal neoplasm of fibroblastic/myofibroblastic differentiation. A series of 56 cases of dermatomyofibroma has been analyzed to further characterize the clinicopathologic spectrum of this entity. Forty patients were female and 8 were male (gender was unknown in 8 cases). Patients' age ranged from 3 to 51 years (mean 30.8 years, median 30 years). Interestingly, 6 patients were younger than 16 years, and in this age group, 3 male and 3 female patients, respectively, were noted. The shoulder (13 cases) was the anatomic site most commonly affected, followed by the upper arm (7 cases), the neck (6 cases), the thigh (6 cases), the chest wall (4 cases), the back (3 cases), the axillary fold (2 cases), the abdominal wall (2 cases), and 1 case each was seen on the forearm, the buttock, and the popliteal fossa (exact anatomic location was unknown in 10 cases). One patient presented with 2 lesions arising simultaneously on both shoulders. Histologically, an ill-defined, plaque-like dermal neoplasm of varying cellularity was seen in all cases, composed of bland spindle-shaped tumor cells often oriented parallel to the overlying epidermis. An infiltration of superficial part of the subcutis was seen in 23 cases, and in 6 cases, deeper parts of the subcutis were involved by often perpendicular growing bands of neoplastic cells. Immunohistochemically, tumor cells in 11 of 48 cases tested stained positively for alpha-smooth muscle actin, and a focal expression of this marker was noted in 20 cases. In addition, a focal expression of CD34 was seen in 10 of 45 cases tested. Follow-up information was available in 38 cases (range from 3 to 156 months, median 34 months), and despite marginal or incomplete excision in 17 cases, none of the cases recurred. Dermatomyofibroma represents a benign fibroblastic/myofibroblastic dermal neoplasm.