Phyllodes tumors are rare breast neoplasms that share many clinical characteristics with fibroadenomas, their benign counterpart. Despite their shared features, few reports have been made about the potential mechanisms by which a fibroadenoma can convert into a phyllodes tumor. This case report describes a large phyllodes tumor presenting in a 50-year-old female patient with a history of an excised biopsy-proven fibroadenoma eight years prior. Here, we exhibit our imaging findings and discuss plausible mechanisms for the conversion of a fibroadenoma into a phyllodes tumor, including genomic alterations.