Diagnosis: A. Fibromatosis.

Histology:
The stellate tumor is composed of bland spindle cells with no nuclear atypia nor mitotic activity. The capillaries within the lesion appear to “stand out” at low power because the lesional cells are hypochromatic relative to the endothelial nuclei, and there is perivascular clearing leading to a small gap between the blood vessels and the lesional spindle cells. There are scattered lymphoid aggregates within and adjacent to the tumor. Malignant epithelial elements or leaf-like architecture is not identified. The lesional spindle cells are negative for cytokeratin labeling and display nuclear beta-catenin labeling.

Discussion:
This lesion is a fibromatosis, or extra-abdominal desmoid tumor, involving the breast. Breast fibromatoses are histologically and immunphenotypically identical to fibromatoses elsewhere in the body. The leading differential diagnoses are spindle cell metaplastic carcinoma (low-grade fibromatosis-like spindle cell carcinoma), scar, and nodular fasciitis. Spindle cell metaplastic carcinoma will be immunoreactive for cytokeratins, although not necessarily for every cytokeratin antibody, whereas fibromatosis is negative for cytokeratin. Nuclear-beta catenin labeling is seen in approximately 75% of fibromatoses and labeling supports the histologic impression of fibromatosis…however! Nuclear beta-catenin labeling can be seen in a minority of both malignant phyllodes tumors and spindle cell metaplastic carcinomas, such that cytokeratin must always be performed on a potential fibromatosis in the breast in order to exclude spindle cell carcinoma.

References:
Charu V, Cimino-Mathews A. Spindle cell lesions of the breast. Am J Surg Pathol: Reviews & Reports. 2017 Mar-Apr;22(2):116-124