Giant malignant phyllodes tumor: a case report.

We present a case of a 57-year-old woman with a giant malignant phyllodes tumor (PT) in her right breast, with maximum diameter of 20 cm. The core-needle and excisional biopsy specimens were diagnosed as suspicious for low-grade myofibroblastic sarcoma (LGMS). The subsequent total mastectomy with partial resection of the pectoral muscles showed predominance of stromal hypercellularity without an epithelial component. However, we diagnosed this as a malignant PT because focal areas showed a leaf-like pattern. In the case of large malignant PTs that exhibit stromal predominance, it can be difficult to distinguish between a pure sarcoma and malignant PT. It is important to thoroughly examine multiple sections from the view point of residual epithelial structure in morphological diagnosis.