Malignant Phyllodes Tumors of the Breast With Rhabdomyosarcomatous Differentiation: A Case Report and Literature Review.

Phyllodes tumor (PT) is a rare fibroepithelial breast neoplasm that is typically graded histopathologically as benign, borderline, and malignant. Malignant PTs (MPTs) exhibit marked stromal cellularity, atypia, overgrowth, increased mitotic activity, and the propensity to metastasize. MPTs represent 10%-15% of all PT cases and often have a notably aggressive disease course. Infrequently, these tumors contain heterologous histological elements, including liposarcoma and fibrosarcoma, among others. Rhabdomyosarcomatous differentiation is an exceptionally rare example of such variation. This report documents the clinical presentation and disease course of a 62-year-old woman diagnosed with MPT with rhabdomyosarcomatous differentiation, just the seventh such confirmed case in the English literature. The patient experienced an arduous disease course, developing metastases to her lungs and axial skeleton just months after her initial diagnosis. Palliative radiation and chemotherapy were initiated, but the patient unfortunately succumbed to her disease just 10 months after the initial diagnosis. This case adds to the scarce literature surrounding the rare development of a heterologous rhabdomyosarcomatous element in an MPT, as well as the decision-making process surrounding the use of radiation to treat such lesions. The details discussed in this paper may inform future approaches for patients diagnosed with this disease.