Myofibroblastoma of the breast: report of a case with radiological, cytological and histological findings

Background. Myofibroblastoma of the breast is an uncommon benign mesenchymal neoplasm that occurs predominantly in elderly men. Methods. A 64-years-old woman presented a slowly growing lump in her left breast. A mammogram revealed a 15 mm radiopaque lesion with regular margins and no microcalcifications and ultrasonography showed well-circumscribed, oval, hypoechoic mass measuring 28 mm, in the upper outer quadrant of the left breast. A US-Guided Fine Needle Aspiration Cytology (US-FNAC) was performed, showing tumour cells arranged in cohesive groups or in fascicular clusters and, in the background, naked nuclei. The cells were spindle shaped with a scanty amount of cytoplasm, oval nuclei with finely granular chromatin and occasional inconspicuous nucleoli. Fragments of stromal collagen were observed. The lesion was diagnosed as a mesenchymal proliferation without atypia and surgical biopsy was recommended. The patient underwent a surgical excision biopsy and histologic examination showed a 2.8 x 2.1 cm nodular mass, well- circumscribed although not truly capsulated, firm, rubbery, with a whirling appearance and a greyish external surface on cut sections. The slides showed a well-defined hyalinised tumour with a spindle cell proliferation and abundant dense fibrous collagen; the lesion was circumscribed by a thin “pseudocapsule” of compress glandular stroma. The spindle cells were arranged in short and haphazardly intersecting fascicles and clusters and were mixed with broad bands of brightly eosinophilic hyalinised collagen. Individual cells were bland-looking and showed a single ovoid nucleus with, occasionally, one or two nucleoli. Mitotic figures were uncommon and cytological atypia was not observed. Mast cells were conspicuous. The spindle cells stain positively for vimentin, CD34, desmin, H-caldesmon, CD99, bcl2. Results. The morphologic features, in conjunction with the immunophenotype, were consistent with a myofibroblastoma of the breast (classic type).