Post‐Irradiation Vascular Proliferations: Potential Mimic of Kaposi’s Sarcoma

A variety of benign and malignant cutaneous vascular lesions occur in irradiated skin. We report two cases of vascular lesions that developed on the breast within a year after radiation therapy for breast carcinoma. Both cases mimicked Kaposi’s sarcoma (KS) histologically. We compare these with one case of post‐irradiation angiosarcoma and 3 cases of acquired progressive lymphangioma (APL) unrelated to radiation therapy. The post‐irradiation vascular lesions showed slit‐like vessels dissecting through superficial dermal collagen, leading to an initial diagnosis of KS by one contributing pathologist. Immunohistochemically, the vessels in both cases expressed CD31 and CD34, stained only partially for actin, and had a low Ki67 proliferation index. Neither case expressed HHV8. The post‐irradiation angiosarcoma occurred three years after radiation, and also exhibited a focal slit‐like growth pattern resembling KS. However, there was marked cytologic atypia, a high proliferation rate, and negative staining for HHV8. The APL were characterized by more ectactic vascular spaces and deeper involvement of the dermis than was present in the post‐irradiation lesions. These cases also failed to stain for HHV8. Recognition of the phenomenon of post‐irradiation vascular proliferations is important, especially because such lesions may resemble KS, yet invariably follow a benign clinical course.