Case Report of Mammary-Type Myofibroblastoma with Sarcomatous Features

Abstract

Mammary-type Myofibroblastoma (MFB) is a rare benign mesenchymal neoplasm that most commonly occurs in the breast and along the embryonic milk line. Histologically, it is characterised by spindle-shaped cells with myofibroblastic differentiation and typically demonstrates an indolent clinical course. Although several histological variants have been described, tumours exhibiting atypical or sarcomatous-like features are exceptionally uncommon. We report the case of a 78-year-old woman who presented with a slowly enlarging right breast mass. Core needle biopsies performed one year apart demonstrated the characteristic morphology and immunophenotype of mammary-type myofibroblastoma. Two years later, following interval enlargement of the lesion, lumpectomy revealed a biphasic tumour composed of a conventional mammary-type myofibroblastoma and a sharply demarcated atypical component exhibiting increased cellularity, marked nuclear pleomorphism, hyperchromasia, multinucleation, and increased mitotic activity. Immunohistochemical analysis showed diffuse expression of desmin, Estrogen Receptor (ER), Androgen Receptor (AR), and Smooth Muscle Actin (SMA) in both components. In contrast, the atypical component demonstrated reduced CD34 expression, loss of progesterone receptor (PR) expression, increased Ki-67 proliferative activity, and diffuse p53 positivity. These immunohistochemical alterations were confined to the atypical area and may suggest biological progression; however, molecular genetic studies would be required to establish a clonal relationship between the conventional and atypical components. This case expands the limited literature on mammary-type myofibroblastoma with sarcomatous-like features and emphasises the importance of recognising this rare morphological variant to avoid diagnostic pitfalls and unnecessary overtreatment.