Mammary-type myofibroblastoma of soft tissue: a tumor closely related to spindle cell lipoma.

Mammary myofibroblastoma is a benign breast tumor, with a reported predilection for older men. It is composed of fascicles of spindle cells having features of myofibroblasts, with intervening hyalinized collagenous stroma and a variably prominent component of adipose tissue. The spindle cells characteristically express both CD34 and desmin. Herein, we report the clinicopathologic features of nine tumors that were morphologically and immunohistochemically identical to myofibroblastoma of breast; however, they arose in subcutaneous soft tissue at extramammary sites. The study group comprised seven men and two women with an age range of 35-67 years (median 53 years). Lesions presented as either a slowly growing painless mass or were incidental findings at the time of surgery. The site distribution was as follows: inguinal/groin area (five cases) and one case each in posterior vaginal wall, buttock, anterior abdominal wall, and mid-back. Tumor size ranged from 2 to 13 cm (median 6 cm), and all lesions were well circumscribed. Eight tumors had a component of adipose tissue (ranging from 10% to 60%), within which some variation in adipocyte size was often seen. One case showed epithelioid cytomorphology and three cases showed rare atypical or multinucleated cells. Focal myxoid stromal change was seen in four cases. Tumor cells were positive for desmin (9 of 9 cases), CD34 (8 of 9 cases), and occasionally positive for smooth muscle actin (3 of 9 cases). Lesions were marginally excised with no recurrences to date, although follow-up is very limited. Lesions with morphologic and immunophenotypic features similar to myofibroblastoma of breast can arise at extramammary sites, with an apparent predilection for the inguinal area of older men. Both mammary and extramammary lesions show morphologic overlap with spindle cell lipoma and are likely closely related.