Diagnosis: B. Malignant phyllodes tumor

Histology:
The tumor is composed of markedly atypical epithelioid spindle cells with regions of fascicular architecture, areas of stromal myxoid change, and variable cellularity, including abrupt transition from hypercellular to hypocellular areas. There is brisk mitotic activity. Malignant epithelial areas are not identified. At one end of the section, hyalinized leaf-like projections are identified; they are ischemic and devoid of epithelial lining.

Discussion:
This lesion is a malignant phyllodes tumor of the breast, characterized here by extensive stromal overgrowth (defined as one 4X low power field entirely composed of stroma), markedly atypical stromal cells with brisk mitotic activity, and focal leaf-like architecture. Phyllodes tumors are rare fibroepithelial lesions of the breast and are subdivided into benign, borderline, and malignant classification. The diagnosis of malignant phyllodes tumor can be challenging due to the often extensive stromal overgrowth and paucity or absence of the benign epithelial component. The presence of a leaf-like architecture and/or an associated lower grade fibroepithelial lesion makes it possible to diagnose a malignant phyllodes tumor on the H&E alone. The differential diagnosis includes metaplastic carcinoma, metaplastic carcinoma, and metaplastic carcinoma, followed by primary or metastatic sarcoma to the breast. The most useful diagnostic tool is to take additional tissue sections to assess for tumoral heterogeneity and the possibility of a focal leaf-like component. A subset of malignant phyllodes tumors are immunoreactive for CD34, which is negative in metaplastic carcinomas and can be a useful diagnostic tool if positive.

Reference:
Tan BY, Acs G, Apple SK, et al. Phyllodes tumours of the breast: a consensus review. Histopathology. 2016 Jan;68(1):5-21.