Giant Malignant Phyllodes Tumor with Secondary Thyroid Involvement

<b>Background/Objectives</b>: Phyllodes tumors are rare fibroepithelial neoplasms of the breast, accounting for less than 1% of all breast tumors and approximately 2-3% of breast fibroepithelial tumors. They can be benign, borderline, or malignant, depending on cellular atypia and stromal invasion. Although most display local behavior, malignant forms can metastasize hematogenously, most frequently to the lungs, bones, and liver, with lymph node metastases being exceptional. <b>Case Presentation:</b> We analyzed the case of a 47-year-old woman with a phyllodes breast tumor weighing approximately 5 kg. The tumor evolved slowly over five years, followed by accelerated growth, cutaneous necrosis, superinfection, and severe anemia. Total mastectomy was performed, and histopathological examination confirmed the diagnosis of a malignant phyllodes tumor. Five months after surgery and adjuvant radiotherapy, the patient developed laterocervical metastases with thyroid invasion and life-threatening tracheal compression, an extremely rare presentation for this type of tumor in adults. <b>Discussion/Conclusions:</b> This case illustrates the aggressive and unpredictable behavior of malignant phyllodes tumors and underscores the necessity of careful oncological monitoring and a multidisciplinary approach, given their potential for dissemination to unusual locations.