Lipomatous myofibroblastoma: a potential diagnostic pitfall in the spectrum of the spindle cell lesions of the breast

We report on two cases of myofibroblastoma (MFB) of the breast comprised pr edominantly of a mature fatty component, representing approximately three q uarters of the entire tumour area. Both tumours consisted of a well-circums cribed lipomatous tumour mass containing dispersed nodular or irregularly s haped spindled cellular areas. The fatty component was represented exclusiv ely by mature adipocytes, uniform in size and shape, lacking nuclear pleomo rphism. The cellular areas contained spindly to val cells with morphologica l and immunophenotypical features typical of MFB. The two components were s o intimately admixed that a finger-like infiltrating growth pattern was app arent. The cases reported here as "lipomatous MFB" aim to clarify further the morp hological spectrum of MFB of the breast. Lipomatous MFB may potentially mim ic other benign or aggressive tumor-like lesions or even bland-looking mali gnant spindle cell tumours such as fibromatosis, nodular fasciitis, spindle cell lipoma, spindle cell liposarcoma, spindle cell variant of metaplastic carcinoma, spindle cell malignant myoepithelioma, and low-grade fibrosarco ma/malignant fibrous histiocytoma. The histogenesis of the present bimorphi c mesenchymal tumours could be explained as the result of a dual, myofibrob lastic and lipomatous, differentiation from a common pluripotential mesench ymal precursor cell, probably represented by the vimentin(+)/CD34(+) fibrob last of the mammary stroma.