Mammary-type Myofibroblastoma: Clinicopathologic Characterization in a Series of 143 Cases

Mammary-type myofibroblastoma (MTMF) is a benign mesenchymal neoplasm initially described to occur in the breast. MTMF is typically CD34 and desmin positive and genetically has rearrangement or deletion of 13q14, resulting in loss of Rb expression by immunohistochemistry (IHC). Although the wider anatomic distribution of MTMF is increasingly recognized, no large series with clinicopathologic information has been reported to date. Archival cases were retrieved, and the diagnosis of MTMF was confirmed. Hematoxylin and eosin-stained slides and IHC slides were reviewed when available (CD34, Desmin, Rb, SMA, S100, EMA, MDM2, CDK4). The patient age, sex, tumor anatomic location and size, preceding symptoms, and margin status were recorded when possible. Clinical follow-up data were requested for tumor recurrence, metastasis, and patient status at last follow-up. A total of 143 cases of MTMF comprised this study, affecting 94 (66%) male and 49 (34%) female individuals. Mean tumor size was 6.6 cm (range, 1 to 22 cm). Anatomic locations included: inguinal/groin region (65; 45%), breast (15; 10%), chest wall/axilla (7; 5%), trunk (17; 12%), lower (18; 13%) and upper (2; 1%) extremities, or intra-abdominal/retroperitoneal (14; 10%). MTMFs were characterized by spindle cells with relatively short, stubby nuclei and a variable adipocytic component. Hyalinization and myxoid stroma were common. Less common morphologic features included nuclear atypia, epithelioid tumor cell morphology, and neurilemmoma-type nuclear palisading. CD34 and desmin were positive in 89% and 91%, respectively, and were both negative in 3%. Rb expression was lost in 92% (57/62). No cases with follow-up data available had tumor recurrence, although 1 case was reportedly a recurrence itself. In summary, MTMF appears more common at extramammary sites than in the breast and can cause diagnostic difficulty when atypia or epithelioid morphology is present or when located in an unusual anatomic location. MTMF is frequently positive for CD34 and desmin by IHC; however, rare cases are negative for both. There is no evidence of any significant recurrence risk for MTMF, even in the presence of positive resection margins. The degree of morphologic overlap between spindle cell lipoma, cellular angiofibroma, and MTMF, in combination with shared genetics and slightly overlapping anatomic distribution, raises the question of whether or not these tumors are truly distinct entities or instead represent points along a single spectrum of genetically related tumors.