Ossifying fibromyxoid tumour of soft parts: immunohistochemical and ultrastructural analysis.

Ossifying fibromyxoid tumour of soft parts is a recently described benign neoplasm which usually presents in soft tissue. The histogenesis or pattern of differentiation is a source of controversy. Thirteen cases are reported herein. All arose in adults, principally on the upper trunk or head and neck region. None recurred. All but one tumour showed a shell of lamellar bone, laid down by reactive osteoblasts, at the tumour-host interface. The characteristic laciform, glomoid and fascicular patterns, usually in combination, necessitate differential diagnosis from chondroid, smooth muscle and neural tumours especially. Immunohistochemistry revealed positivity for S-100 protein in 10/12, desmin in 7/10 and smooth muscle actin in 4/8. Ultrastructural examination of four cases revealed prominent intermediate filaments, without myofilamentous organization, and a discontinuous external lamina. Immunoelectronmicroscopy localized desmin positivity to the filamentous meshwork. Differentiation therefore appears to take the form of an incomplete neural and smooth muscle phenotype, without evidence of complete maturation.