Answer: Spindle cell hemangioendothelioma

Histology: On low power, this is a vascular lesion. There are large veins as well as thin-walled vessels, some of which contain thrombi. Some of the thin-walled vessels form cavernous spaces. There is also a proliferation of bland-appearing spindle cells with scattered, interspersed red blood cells. Epithelioid cells are also seen within the lesion. Some of these epithelioid cells contain vacuoles or intracytoplasmic lumina resembling fat cells.

Discussion: Spindle cell hemangioendotheliomas are non-metastasizing lesions that may recur locally and can result in extensive local disease. The tumor most commonly occurs in the subcutis or dermis of the distal extremities, especially the hand. Patients are usually young adults, however they may occur at any age. The presence of dilated thin-walled vessels containing organizing thrombi seen on low power is suggestive of a diagnosis of cavernous hemangioma, however a proliferation of spindle cells and epithelioid cells are seen between these cavernous spaces, ruling out that diagnosis. The proliferation of spindle cells is suggestive of Kaposi’s sarcoma. However, unlike Kaposi’s sarcoma, there is also a component of rounded or epithelioid endothelial cells, similar to those seen in an epithelioid hemangioendothelioma. In a spindle cell hemangioendothelioma, the epithelioid cells that contain vacuoles may occasionally cluster and resemble entrapped fat. Immunohistochemical stains for vascular markers such as CD31 and Factor VIII will stain the endothelial lining of the cavernous spaces and the epithelioid cells of the stroma. However, the spindle cell component is often negative for Factor VIII. These tumors are treated by conservative excision for limited disease. Approximately 60% will locally recur. However, distant metastases have not been documented in this disease.