Closed biopsy shows vascular fibroblastic tissue with foci of woven bone formation. The fibroblasts display mild to moderate degree of cellular pleomorphism.
Open biopsy shows a lesion which infiltrates the skeletal muscle. The central portion is composed of loosely textured vascular fibroblastic tissue. The fibroblasts display a mild degree of cellular pleomorphism and contain occasional mitotic figures. Bordering these areas is a zone with ill-defined bony trabeculae. Toward the periphery trabeculae mature and form woven bone.