Transformation of benign fibroadenoma to malignant phyllodes tumor.

The transformation of a benign fibroadenoma into a phyllodes tumor is uncommon and unpredictable. We report the case of a 40-year-old woman with a core biopsy proven fibroadenoma that underwent transformation into a malignant phyllodes tumor after 3 years of size stability. We present ultrasound and magnetic resonance images, as well as pathology slides from core biopsy and surgical excision, to illustrate this transformation. It has been suggested that phyllodes tumors may be misdiagnosed as fibroadenomas by core biopsy. However, in this case, pathology supports correct initial diagnosis of fibroadenoma and demonstrates a portion of the original fibroadenoma along the periphery of the malignant phyllodes tumor.