Mammary myofibroblastoma is a benign spindle cell tumor that can show variable morphologic patterns and lines of differentiation. Myofibroblastoma belongs to a family of CD34-positive tumors with ...similar morphology that show a deletion of 13q14, which includes RB1 and FOXO1A genes. A subset of these tumors demonstrates distinct smooth muscle differentiation which can be confused with other smooth muscle tumors of the breast, itself constituting a rarified morphological subgroup. We aimed to characterize 4 cases of the leiomyomatous variant of myofibroblastoma arising in the breast by clinicopathological, immunohistochemical, and molecular means. All 4 examples arose in women aged 41–62 years (median, 46.5 years). Tumors ranged in size from 1.7 to 2.5 cm (median, 2.2 cm). Morphologically, all tumors were characterized by bundles of smooth muscle cells with elongated cigar-shaped nuclei and eosinophilic cytoplasm. All four tumors showed diffuse positive staining with desmin, caldesmon, smooth muscle actin (SMA), estrogen receptor (ER), and Bcl-2. CD34 staining was diffusely positive in two cases, weak and patchy in one case, and was negative in one case. Two of four (50%) tumors showed deletion of RB1 by fluorescence in situ hybridization (FISH). Loss of Rb staining was seen in one tumor with RB1 deletion by FISH, while intact Rb staining was observed in one non-deleted case studied. In conclusion, this rare variant of myofibroblastoma is a distinct subgroup of tumors among an already uncommon category of (smooth muscle) breast tumors. Some reported examples of “parenchymal leiomyoma” may represent the leiomyomatous variant of myofibroblastoma.
