A Case of Myofibroblastoma of the Breast Suspected to be Invasive Lobular Carcinoma on Needle Biopsy

Abstract

A 65-year-old man presented to our department having noticed a mass beneath his right nipple 2-3 years previously. A mass around 2 cm in size was present immediately below the right nipple, and since invasive carcinoma (invasive lobular carcinoma) was suspected on needle biopsy, surgery was performed. Histological testing showed invasive, irregular proliferation of somewhat atypical cells with little cytoplasm. Although the nuclei of individual cells were of different sizes, the nuclear chromatin was comparatively consistent. Immunostaining results were negative for epithelial and lymphocyte markers, but positive for the mesenchymal markers of α-smooth muscle actin, vimentin, desmin, and CD34, leading to a diagnosis of myofibroblastoma. Myofibroblastoma is an extremely rare tumor arising from mammary stromal fibroblasts or myofibroblasts that is reportedly difficult to diagnose. Immunostaining is essential for its diagnosis. The differential diagnosis includes invasive lobular carcinoma and malignant phyllodes tumor, and this disease should be borne in mind when making a pathological diagnosis.