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 seen. The stroma contains thickened collagen bundles.

Discussion: The spindle cell proliferation raises the possibility of a sarcoma, however sarcomas show greater mitotic activity and/or necrosis, which are not seen in the current lesion. Metaplastic carcinoma would show more cytologic atypia. Immunostains for cytokeratin and CD 34 would be helpful, as metaplastic carcinomas would show patchy cytokeratin staining and would be negative for CD34. A fibromatosis would feature more prominent thin-walled blood vessels and greater collagen deposition. In contrast to myofibroblatoma, fibromatosis typically does not label for CD34.

Myofibroblastoma was first described in the male breast, and is more common in males than females. This spindle cell neoplasm with ill-defined cytoplasm and somewhat tapered nuclei shows prominent dense, ropy collagen. The tumor labels with CD34 and Some lesions may label with desmin. These lesions typically show no mitotic activity, and are clinically benign. It has been proposed in the literature that this lesion is related other benign spindle cell lesions in the breast that arise from the mammary stromal cell.

Reference(s):
Magro G, Bisceglia M, Michal M, Eusebi V. Spindle cell lipoma-like tumor, solitary fibrous tumor and myofibroblastoma of the breast: a clinico-pathological analysis of 13 cases in favor of a unifying histogenetic concept. Virchows Arch. 2002 Mar;440(3):249-60

Pauwels P, Sciot R, Croiset F, Rutten H, Van den Berghe H, Dal Cin P. Myofibroblastoma of the breast: genetic link with spindle cell lipoma. J Pathol. 2000 Jul;191(3):282-5.