When fat goes bad

Excisional biopsy specimen from a 52 year old women with a 3.1 cm mass

low power magnification shows a fibroepthelial Lesion characterized by cellular stroma and distorted epithelial arrangements.  The lesion appears to have an irregular interface with adjacent adipose tissue.

Hypercellular stroma is condensed around the epithelial structures.  Variably sized adipocytes are intermixed with the cellular stroma.

Higher magnification shows hypercellular stroma with mild stromal atypia and mitotic activity.

High power shows variably sized fat cells, including numerous lipoblasts. 

Diagnosis:  Malignant phyllodes tumor with liposarcomatous elements

Discussion:  Features of malignant phyllodes tumor include stromal overgrowth (defined as a 4x field composed exclusively of stroma), stromal atypia, and mitotic activity.  The low power photomicrograph suggests an irregular interface with fat, but in fact the adipose tissue is actually part of the phyllodes tumor, and qualifies as stromal overgrowth.  Numerous lipoblasts are evident under high power.    Margin adequacy is the most important predictor of local recurrence.  Given the low grade nature of this phyllodes tumor, a good outcome is expected following adequate surgical excision.