AN UNUSUAL SOFT-TISSUE TUMOR WITH FEATURES OF ANGIOMATOID MALIGNANT FIBROUS HISTIOCYTOMA COMPOSED OF BIMODAL CD34 AND FACTOR XIIIA POSITIVEDENDRITIC CELL SUBSETS - CD34 AND FACTOR XIIIA IN ANGIOMATOID MFH

CD34 and factor XIIIa (FXIIIa) antibodies delineate subsets of embryon ic dendritic stromal stem cells that persist in adult mesenchyme. CD34 stains interstitial and adventitial dendritic cells that may function as multipotential precursor cells. FXIIIa(+) dendrophages are tissue histiocytes active in tissue repair. Angiomatoid malignant fibrous his tiocytoma (AMFH) is an enigmatic fibrohistiocytic tumor with limited v ascular features. We examined an unusual soft tissue tumor from the na solabial subcutis of a 57 year old man that showed histologic features of AMFH. Most of the tumor cells expressed CD34. 10-30% of the tumor cells were FXIIIa(+) dendrophages. Sinusoidal areas were largely compo sed of FXIIIa(+) cells that also expressed HLA-DR and CD68 suggesting macrophage differentiation. CD31 and Factor VIII antigen highlighted c apillaries and single cells among the CD34(+) tumor cells. The Vessels had actin(+) myopericytes and there were single actin(+) tumor cells. Electron microscopy showed primitive dendritic cells and fewer histio cyte-like cells. The Ki 67 index was 15% including both FXIIIa(+) and CD34(+) cells. The patient is disease free three years after wide exci sion. We conclude that this AMFH-like neoplasm is a fibrohistiocytic t umor in which CD34(+) fibroblast-like precursors and FXIIIa(+) tissue dendrophages combine to build both sinusoidal tissue with endothelial and macrophage elements as well as capillary vascular tissue that is i nvested with myopericytes. Study of additional AMFH lesions from this standpoint is desirable.