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Fibroadenomas are common benign lesions that are typically managed conservatively. However, transformation into malignant phyllodes tumor is a rare but critical progression that demands a shift from surveillance to surgical intervention. We present the case of a patient who underwent multiple core-needle biopsies over several years, all confirming a fibroadenoma, yet the mass demonstrated progressive growth, ultimately exceeding 7 cm. Ultimately, surgical excision revealed a malignant phyllodes tumor characterized by stromal overgrowth, cellular atypia, increased mitotic activity, and infiltrative margins. This case highlights the diagnostic limitations of core-needle biopsy in fibroepithelial lesions. Although the vast majority of fibroadenomas remain benign, a minority may undergo malignant transformation. Physicians must maintain a high degree of clinical suspicion and closely monitor longstanding lesions for any signs of growth or change. Early distinction between fibroadenomas and phyllodes tumors is essential, as management strategies differ substantially.