Intravascular angiomatosis: development and distinction from angiosarcoma.

Histologic features similar to those present in angiosarcomas were found in organizing thrombi. Within the thrombi, freely anastomosing small channels, often lined by one or more layers of prominent, occasionally atypical, endothelial cells were noted. Papillary-like projections of organizing thrombus material lined by similar cells heightened the similarity of the process to angiosarcomas. The papillary-like structures appeared to develop due to a combination of endothelialization of thrombus fragments and of ingrowth of interlacing vessels. The peculiar process of thrombus organization, which has been called "intravascular angiomatosis," may be mistaken for true angiosarcoma and thus lead to unnecessary irradiation or radical surgery. The pseudoangiosarcoma differed from angiosarcoma in its confinement entirely within large vascular lumens and in its lack of mitoses, necrosis, and true solid cellular areas devoid of vascular differentiation.