PET/CT-Detected myofibroblastoma of the breast with bizarre cells: A potential diagnostic pitfall of malignancy.

Myofibroblastoma (MFB) is a rare benign mesenchymal tumor usually occurring in the breast parenchyma. This tumor can present as a palpable nodule or can be incidentally detected as a nonpalpable mass on routine screening mammogram. We first report a rare case of histologically proven MFB of the breast revealed by fluoro-deoxyglucose uptake on PET-CT examination in a patient with a lung nodule. Tumor exhibited an unusual morphology, being predominantly composed of polygonal, epithelioid, and deciduoid-like cells set in a myxoid stroma. The most striking feature was the multifocal presence of atypical/bizarre, mono/bi-nucleated cells that, in addition to diffuse myxoid stromal changes, were a concern of malignancy, especially on core biopsy. The final diagnosis of MFB was achieved on surgically resected specimen and, similarly to other benign soft tissue tumors (especially leiomyoma and schwannoma/neurofibroma), the term "bizarre cell MFB of the breast" is proposed to emphasize the degenerative/reactive nature of the atypia.