Lipomatous myofibroblastoma of the breast: case report with diagnostic and histogenetic considerations.

We report rare case of myofibroblastoma (MFB) of the breast comprised predominantly of a mature fatty component, representing approximately 70% of the entire tumour area. This tumour, designated "lipomatous MFB", should be interpreted as the morphological result of an unbalanced bidirectional differentiation of the precursor mammary stromal cell, with the adipocytic component overwhelming the fibroblastic/myofibroblastic one. Lipomatous MFB is a rare variant of mammary MFB, which can mimic malignancy because of the close juxtaposition of fibroblasts/myofibroblasts with mature adipocytes, resulting in a finger-like infiltrative growth pattern of the former towards the latter. Histogenetic considerations and differential diagnostic problems with other bland-looking spindle cell tumours containing infiltrating fat are provided.