Answer: Myofibroblastoma

Histology: The tumor is composed of plump spindle cells with somewhat purplish cytoplasm and nuclei with vesicular chromatin and punctate but prominent nucleoli. Mitoses are not evident. The stroma contains thickened collagen bundles, which appear to be condensed around blood vessels. Tumor cells label diffusely for CD34 and are negative for cytokeratin.

Discussion: Metaplastic carcinoma would be expected to show more cytologic atypia and would not label for CD34. Cytokeratin labeling could be patchy in metaplastic carcinomas, so a cytokeratin panel should be performed. Sarcomas should show greater mitotic activity and/or necrosis, neither of which is evident in the current lesion. A fibromatosis would feature more prominent thin-walled blood vessels and greater collagen deposition. Fibromatoses typically do not label for CD34, but in the breast do demonstrate nuclear beta-catenin labeling.

Myofibroblastoma was initially described in the male breast, and does appear to be more common in male breast than in female breast. This is a spindle cell neoplasm with ill-defined cytoplasm and somewhat tapered nuclei. An epithelioid variant may mimic invasive lobular carcinoma. The presence of dense, ropy collagen is a characteristic feature. Some of these lesions may label with desmin, in which case the lesion merges with the entity of myoid hamartoma. These lesions typically show no mitotic activity, and are uniformly benign clinically.