Primary juxtacortical myoepithelioma/mixed tumor of the bone: a report of 3 cases with clinicopathologic, immunohistochemical, ultrastructural, and molecular characterization.

We describe the clinicopathological, immunohistochemical, and molecular features of 3 primary juxtacortical myoepithelioma/mixed tumor of bone. The patients were 2 males (13 and 23 years of age) and a 15-year-old female. The juxtacortical lesions were all located in the femur, and were surgically removed, 2 with wide margins and one with marginal margins. This latter tumor recurred locally 18 months later. The 3 patients were free of disease at 6 to 17 months follow-up. Histologically, all lesions showed a prominent multinodular architecture, and were formed by epithelioid and stellate elements, organized in solid sheets, or embedded in myxoid or chondroid matrix. Areas of osteoid formation were also observed. One tumor had the appearance of classical mixed tumor, showing aspects of duct formation and focal squamous differentiation. Immunohistochemically, all cases were positive for cytokeratins, epithelial membrane antigen, and S100 protein. The expression of other myoepithelial markers, including p63, glial fibrillary acid protein and calponin was more limited. No rearrangement of Ewing sarcoma region 1 (EWSR1) and fused in sarcoma (FUS) genes was observed by fluorescent in situ hybridization. To our knowledge, this is the first report of primary myoepitheliomas of bone arising at juxtacortical sites. These lesions must be distinguished from other benign and malignant bone and cartilage-forming surface tumors, including periosteal chondroma and chondrosarcoma, juxtacortical chondromyxoid fibroma, and periosteal and paraosteal osteosarcoma. The clinicoradiologic presentation and their histological and immunohistochemical features are distinctive enough to allow the separation from these entities.