A 34 year old female presented with a palpable right breast mass
Contributed By: Kamaljeet Singh MD
Fibromatosis-like metaplastic carcinoma
Spindle cell type metaplastic carcinoma
The answer is Nodular Fasciitis.
The breast lesion shows typical histological features of nodular fasciitis. The mass is comprised of spindle cells without any entrapped lobules. The lesion is largely circumscribed. The spindle cells are arranged in fibrous and myxoid stroma, with scattered pseudocystic spaces. Extravasation of RBCs is present. The cells show spindle to oval and rarely epithelioid nuclei with pointed to rounded nuclear ends and fibrillary cytoplasmic contours. No hyperchromasia or pleomorphism is noted. Rare mitotic figures and scattered inflammatory cells are present.
On IHC the spindle cells show myofibroblastic/fibroblastic immunophenotype (desmin, SMA, and vimentin+). Keratins and p63 are negative. USP6 gene rearrangement, like soft tissue counterparts, has been demonstrated in nodular fasciitis involving breast.
Reference: WHO classification of Tumours Editorial Board. Breast tumours. Lyon (France): IARC; 2019 (pages 119-120).