Plexiform fibrohistiocytic tumour: clinicopathological, immunohistochemical and ultrastructural analysis in favour of a myofibroblastic lesion.

Plexiform fibrohistiocytic tumour is a recently described, seemingly benign neoplasm of superficial soft tissue which is poorly recognized and the differentiation pattern of which remains obscure. Fourteen new cases are presented here. These presented predominantly in the upper limb of infants and children, although the age-range was wide. A morphological spectrum depending on the relative proportions of the spindle cellular and nodular histiocyte-like components was evident. Immunohistochemical analysis revealed positivity of tumour cells in both components for smooth muscle actin, suggestive of myofibroblastic differentiation, as was borne out ultrastructurally in two cases. In addition, a minority of the histiocyte-like cells were also CD68 positive but negative for leucocyte common antigen, HLA-DR, Mac387 and lysozyme. In view of the ultrastructural and other immunohistochemical results, this is regarded as further evidence that the CD68 epitope recognized by KP-1 is not confined to cells of monocyte/macrophage or myeloid lineage. Plexiform fibrohistiocytic tumour appears to be a clinicopathologically distinctive myofibroblastic neoplasm which may warrant reclassification in due course.