Mammary myofibroblastoma: a case report

Abstract

Background: Myofibroblastoma is a benign mesenchymal tumour which can be challenging to diagnose. It has a range of histological features that overlap with other benign or malignant neoplasms.1 Adjunct testing that demonstrates RB1 loss on immunohistochemistry and monoallelic loss of 13q14 region on FISH can seal the diagnosis of this lesion which is curative with local excision.2,3 Case summary: We report a case of a 45-year-old female who presented with a right breast nodule. A core biopsy showed a stromal spindle cell proliferation and the working diagnosis included myoid hamartoma, fibroepithelial lesion and a spindle cell carcinoma. The excision of the lesion revealed a vaguely circumscribed spindle cell proliferation with spindle cells arranged in fascicles interspersed by hyalinised collagen bundles. The tumour cells were positive on immunohistochemistry for SMA, Desmin and ER but negative for CKAE1/3, B-catenin, S100, CK broad spectrum and p63. It also showed a loss of RB1. The FISH analysis showed a loss of RB1 at 13q14 in 35.5% of the nuclei confirming the diagnosis of a benign myofibroblastoma. Conclusion: The constellation of histological features when supported with monoallelic loss of 13q14 region and RB1 loss on immunohistochemistry confirms the diagnosis of this benign entity preventing unnecessary further management.