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We present a new case of retiform hemangioendothelioma (RH), an entity first described by Calonje et al. in 1994. The tumor was intradermal and located on the toe of an 11-year-old boy. Histologically, in addition to the distinctive retiform pattern of proliferating vessels in RH there are intraluminal papillae with hyaline cores similar to those seen in malignant endovascular papillary angioendothelioma (Dabska's tumor), but usually they are infrequent, focal and poorly developed. In our case, these papillary structures were well formed and distributed in a diffuse way. They were most conspicuous in superficial areas where the blood vessels were dilated. In deep areas, where the pattern of neoplastic vessels was retiform, the papillae filled their lumina totally, resembling solid cords. Our case shares the clinical and morphologic features of both retiform hemangioendothelioma and Dabska's tumor, supporting a relationship between these two kinds of neoplasms. The benign behavior of this case, with no recurrence or metastases over a 4-year follow-up, corresponds to the low malignancy of this kind of vascular neoplasm.

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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