Introduction: Fibroadenoma is the most common benign tumor found in young women. Transformation for Fibroadenoma to borderline phyllodes tumor is rare. Phyllodes tumor tends to recur locally due to the aggressive nature of the phyllodes tumor tissue, which tends to become malignant when recurrence occurs. Presentation of case: We report an 18-year-old woman with a lump in the right breast for one year, a mass in the
upper lateral quadrant measuring 20x15x10cm; the lump felt mobile, supple, cystic. 2​years earlier, lumpectomy
surgery with anatomical pathology of fibroadenoma mammae with 6​cm. A biopsy was performed, the result
was a borderline phyllodes tumor. Neoadjuvant chemotherapy was performed with tumor reduction >50%, followed by wide excision surgery with tumor-free margins. The first one-year evaluation showed no signs of recurrence, and no signs of distant metastases were found.
Conclusion: Our patient had a juvenile relapse FA which progressively changed to borderline tumor phyllodes borderline. We report this case to the need for continuous follow-up for fibroadenoma cases and the possibility of recurrence and becoming malignant.