Phyllodes tumor with a benign heterologous osseous component: a diagnostic challenge.

Phyllodes tumors (PTs) are rare fibroepithelial neoplasms of the breast and constitute 0.3-1% of all primary breast tumors. They should be characterized in to a benign, borderline or malignant category based on a combination of histological features. PTs can show heterologous components, typically sarcomatous, including osteosarcomatous and chondrosarcomatous. Benign heterologous components are exceedingly rare with only one prior reported case in the English literature. This case emphasizes how difficult establishing a correct diagnosis in PTs with heterologous components can be, especially when the tumor consists mainly of a benign heterologous component. We report the case of a 65 year old woman with a recurrent breast tumor initially misdiagnosed as benign osseous metaplasia. The tumor re-occurred as a malignant PT dominated by benign osseous and chondroid metaplasia. Multiple metastases consisting of primarily mature bone and cartilage were seen in the lungs. On microscopic revision and considering the clinical course the primary breast tumor was re-classified as a borderline PT.

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© 2019 APMIS. Published by John Wiley & Sons Ltd.