Nodular fasciitis. Its morphologic spectrum and immunohistochemical profile.

Although nodular fasciitis (NF) is a well recognized pseudosarcomatous proliferation, it continues to cause diagnostic problems. We reviewed the clinical, histologic and immunohistochemical features of 53 lesions from 30 male and 23 female patients, ages 8-76 years, that involved the upper and lower extremities, trunk, and head and neck. Sizes ranged from 0.6 to 6.5 cm. The morphologic spectrum was broad, including the classic pattern of delicate fibroblasts suspended in a myxoid matrix, granulation tissue-like areas, solid and whorled myofibroblastic proliferations with multinucleated cells, mucoid cysts, and so-called "ancient" forms with dense, refractile strands of keloid-like collagen. Nodular fasciitis was correctly diagnosed in 23 cases (43%); a sarcoma was diagnosed in 11 (21%). A characteristic immunohistochemical profile emerged wherein 49 of 53 cases stained for smooth-muscle and muscle-specific actins, vimentin, and KP1 (a histiocyte marker), indicating dual myofibroblastic and histiocytic differentiation. None of the lesions expressed keratin, S-100 protein, or desmin. Knowledge of the immunohistochemical profile of nodular fasciitis and its overlap with certain sarcomas can decrease the likelihood of misdiagnosis.