We report 5 cases of the fibrosarcomatous variant of dermatofibrosarcoma protuberans, 4 of which presented a morphologic change of intraneoplastic blood vessels not previously recognized. This change consisted of focal proliferation of smooth muscle cells, resulting in hypertrophy, generally eccentric, of vascular walls with reduction and collapsing of vascular lumina. In 3 cases the proliferation was so intense it formed leiomyomatous nodules and bundles. This proliferation may originate in the smooth muscle cells of the vessel walls either by means of a hyperplastic mechanism or in the pericytes via a line of differentiation leading to mature smooth muscle cells. In either case, we believe that it concerns a reactive process of the vessel walls very probably induced by adjacent neoplastic cells. The cases recently reported by Calonje and Fletcher as "myoid differentiation" of neoplastic cells in dermatofibrosarcoma protuberans (DFSP) may well be an expression of the same phenomenon, and therefore the presence of leiomyomatous areas in this tumor should not be used to support the theory of a fibroblastic/myofibroblastic line of differentiation for DFSP.

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