Myogenic stromal tumor of the male breast (so-called myofibroblastoma).

A case of a distinctive stromal tumor of the male breast, recently designated as myofibroblastoma, is reported. A fascicular clustering of spindle cells with intervening hyaline bands was the dominant pattern, but areas of myxoid transformation were present. Ultrastructure revealed a large predominance of fibroblastlike cells, often containing dispersed or aggregated intermediate and/or thin filaments. These cells depicted numerous thin processes encircling collagenous bundles. Primitive mesenchymal and typical smooth muscle cells were present, but myofibroblasts were not identified. Vimentin was diffusely expressed, whereas desmin and muscle-specific actin were detected in, respectively, 50% to 60% and 15% to 25% of the cell population. The designation of myogenic stromal tumor is, therefore, considered more appropriate.