Dermal and subcutaneous variants of plexiform fibrohistiocytic tumor.

We report five cases of plexiform fibrohistiocytic tumors, three classic subcutaneous lesions and two dermal ones. Both variants had similar profiles and were clinically indistinguishable. The lesions affected the trunk more than the upper extremities and were found in children and young adults (18.4 +/- 12.8 years). They showed a marked female predominance (4:1, including both dermal variants). Clinically, they were skin-colored, hard nodules of 1-2-cm diameter that resulted in such differential diagnoses as fibroma, histiocytoma, pilomatricoma, or cyst. Interestingly, one subcutaneous case with a painful "worms in the sack" presentation was thought to represent a plexiform neurofibroma. Histology revealed well-circumscribed dermal or subcutaneous plexiform lesions with a characteristic biphasic appearance. Most of the tumor bulk consisted of spindle-shaped to stellate myofibroblasts with a variable admixture of collagen or loosening of stroma. In the center of the plexiform strands and nodules, a few osteoclast-like giant cells as well as epithelioid mononuclear cells (< 10%) were found. Myofibroblasts were positive with HHF35 and for smooth muscle actin in three of five cases. Osteoclast-like giant cells were positive with KP1 in all five cases. Both types of cells stained with the macrophage marker Ki-M1p. A broad panel of other markers was negative. This series expands the spectrum of plexiform fibrohistiocytic tumor, but it also broadens the differential diagnosis of (dermal) plexiform lesions, which at present includes spindle cell nevi, schwannomas, neurofibromas, granular cell tumors, nerve sheath myxomas (neurothekeomas), spindle cell lipomas, and tufted angiomas.