Atypical vascular proliferation in benign soft tissue tumors

Atypical endothelial proliferation and continuing growth of small vessels were found to be the main features of two benign soft tissue tumors. Lack of history of injury and insidious onset suggested neoplasia, but the morphology was that of a reactive rather than neoplastic process. Ultrastructural study of the relationship of the endothelial cells to one another and to perithelial cells was helpful in making this distinction. The process appeared to have begun in an organizing thrombus or hematoma, in one case in relation to an occluded artery, and in the other in an abnormally vascular area of skeletal muscle. It is known that endothelial hyperplasia can occur in reparative reaction of this type. The prominent muscular development of small vessels in the cases reported here suggests that abnormal flow or intraluminal pressure could have been a stimulant to atypical growth.