Mammary Myofibroblastoma: A mimicker of benign and low-grade malignant tumors with multipotent mesenchymal differentiation

Mammary myofibroblastoma (MFB) is a rare benign spindle cell neoplasm of the breast that can exhibit both epithelial and mesenchymal elements. The histomorphologic features of MFB mimic other potentially more aggressive neoplasms of the breast that may require more involved clinical management beyond a conservative local excision. We present the case of a 71-year-old male with a mammary MFB who underwent core needle biopsy (CNB) of a well-defined, bland spindle cell neoplasm with a subsequent local excision. The case highlights triage of MFB among a broad diagnostic differential at biopsy, particularly given its multipotent differentiation potential and highly variable presentation from the standpoint of both radiology and histopathology. We also discuss the role of RB1 deficiency in distinguishing MFB from other tumor types and non-neoplastic/reactive proliferations.