Diagnosis: D. Malignant phyllodes tumor

Histology: The resection specimen contains a fibroepithelial lesion that displays prominent leaf-like
architecture, marked stromal cell cytologic atypia, and a brisk mitotic rate (>10 mitoses per 10 high power fields) including numerous atypical mitotic figures. There is no stromal overgrowth in this section, nor elsewhere in the specimen. The tumor displays predominantly circumscribed borders in this section, but in other areas has a nodular infiltrative growth. The degree of stromal cellularity is relatively uniform on this particular section, but in other sections displays variation in the stromal cellularity with regions of marked hypercellularity. Despite the lack of stromal overgrowth, the constellation of features is in keeping with a malignant phyllodes tumor.

Discussion: Fibroepithelial lesions of the breast consist of fibroadenomas and phyllodes tumors. Fibroadenomas are vastly more common and are entirely benign. Phyllodes tumors are fibroepithelial lesions of the breast that display stromal hypercellularity and a prominent leaf-like architecture, often with cystic areas. In fact, their original name was “cystosarcoma phyllodes,” referring to the gross appearance: cystic (“cysto-“) and fleshy (“-sarcoma”), with projections resembling leaf fronds (“phyllodes” comes from the Greek “phyllon”, or leaf). Phyllodes tumors are subclassified into benign, borderline and malignant on the basis of a constellation of histologic features. When evaluating a fibroepithelial lesion, it’s helpful to first scan all slides of the lesions at low power to get an overall sense of the lesion, and then to evaluate these histologic features from low power to high power. In general, the severity of the atypia increases across the diagnostic categories of benign, borderline and malignant. The histologic features include: the degree of circumscription (circumscribed vs. infiltrative), the degree of stromal cellularity (mild to marked hypercellularity), any variation in stromal cellularity and the presence of stromal overgrowth (defined as one 4x lower power field entirely comprised of stroma), the degree of stromal atypia (mild to marked), the mitotic rate, and the presence of malignant heterologous elements (limited to malignant phyllodes). The constellation of features here is most in keeping with a malignant phyllodes tumor.

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