[Mammary myofibroblastoma: a clinicopathologic analysis of nine cases].

Objective: To study the clinicopathologic characteristics and differential diagnosis of mammary myofibroblastoma. Methods: Nine cases of mammary myofibroblastoma diagnosed between 2006 and 2017 were collected from the Department of Pathology, Fudan University Shanghai Cancer Center. Clinical and histopathologic features of these nine cases were examined, immunohistochemical staining was performed, FISH analysis for the detection of FOXO1 gene was performed in one case, and follow-up data were also collected. Results: There were seven female and two male patients, with a mean age of 54 years, median age of 50 years (ranging from 40 to 83 years). Four lesions each were located in the left and right breast, and one was in the left subaxillary accessory breast tissue. Clinically, 8 patients presented with a breast mass, 3 of which accompanied with pain. All of the tumors were well-demarcated grossly with a mean diameter of 2.5 cm. Microscopically, there were no entrapped ductal or lobular structures within the tumor. Seven tumors were classic type, which were composed of bland-looking spindle neoplastic cells without mitoses, arranging in intersecting fascicles, and interrupted by thick hyalinized collagen bundles. One case was of epithelioid variant, demonstrating epithelioid neoplastic cells diffusely arranged or in cluster. The other one case was mixed spindle and epithelioid-cell type. Atypical tumor cells were observed in 3 cases. Immunohistochemically, tumor cells were diffusely positive for desmin (9/9) and CD34 (6/9), as well as ER (7/7), PR (6/6) and bcl-2 (3/3). SMA (4/7) and Calponin (1/2) were focally or partially positive in some cases. H-caldesmon (1/2) was weakly positive and epithelial markers were negative. Ki-67 proliferation index was low (<10%). There was no monoallelic loss of FOXO1/13q14 loci in the detected case according to FISH analysis. Follow-up data were available for all patients, and follow-up period ranged from 12 to 78 months. All patients remained well without recurrence. Conclusions: Mammary myofibroblastoma is a rare benign mesenchymal tumor. In some circumstances, it may exhibit confusing morphologies, including some variants. The epithelioid variant of mammary myofibroblastoma might mimic invasive lobular carcinoma, leading to the diagnostic dilemmas and even misdiagnosis, especially in core needle biopsy specimen or frozen sections. Familiarity with the characteristics of this tumor is of great importance for accurate diagnosis and proper treatment.