Features of plexiform fibrohistiocytic tumor in skin punch biopsies: a retrospective study of 6 cases.

Plexiform fibrohistiocytic tumor (PFHT) is a mesenchymal neoplasm of intermediate malignant potential, which typically presents as a dermal or subcutaneous nodule, and is therefore often sampled by skin punch biopsy where diagnostic features may be subtle or absent. We retrospectively analyzed a series of 6 cases of PFHT to highlight for dermatopathologists the features of PFHTs useful to distinguish it from the other entities in the differential diagnosis. On the basis of the proportion of spindled fibroblastic cells to histiocytoid nodules in the biopsy specimen, we divided PFHT into 3 histologic variants: cellular, fibrous, and mixed. The biopsies also were compared with the final resection specimens, in an attempt to determine which histologic features in the original biopsies were most helpful in establishing a diagnosis. Clinical follow-up and immunohistochemistry were performed on all cases. The cellular and mixed variants were a lesser diagnostic challenge inasmuch as the distinctive features were more easily identifiable in small punch biopsy specimens. The fibrous variant proved more difficult to diagnose. Features most helpful in the diagnosis of PFHT were biphasic appearance with small, cellular, histiocytoid aggregates and accompanying plump spindled cells in the deep dermis and subcutis. Negative staining for CD34, NK1/C3, factor XIIIa, and beta-catenin by immunohistochemistry proved useful in excluding some of its mimics.