Case Report: Coexistence of a giant borderline phyllodes tumor of the breast and a contralateral fibroadenoma: a challenging case with clinical manifestations and PET-CT imaging mimicking advanced cancer.

Background: Phyllodes Tumor (PT) is a rare fibroepithelial tumor, which is classified into three categories: benign, borderline, and malignant. The three types show a gradually increasing trend in terms of local recurrence and distant metastasis. Giant PT usually refers to a mass with a diameter larger than 10 cm, accounting for approximately 20% of PT cases. Borderline PT (BPT) has overlapping clinical manifestations with both benign fibroadenomas and malignant breast tumors. Especially when it presents with rapid growth and ulceration, it is easily misdiagnosed as advanced breast cancer. Case presentation: This paper reports a case of a 44-year-old female patient who was admitted to the hospital with the chief complaint of "discovering a painless mass in the left breast several months ago and presenting due to rapid enlargement accompanied by skin ulceration in the past month". Physical examination revealed a huge mass (approximately 24×17×15 cm) in the left breast, with diffuse skin edema, redness, increased skin temperature, and multiple skin ulcers.A small nodule can be palpated in the right breast. Preoperative auxiliary examinations: Breast ultrasound indicated a nodule in the right breast (BI-RADS category 4a) and disordered glandular structure in the left breast (BI-RADS category 0) accompanied by axillary lymph node enlargement; PET-CT showed significantly increased metabolism in the left breast mass (SUVmax 11.5) and ipsilateral axillary lymph nodes (SUVmax 9.0), highly suggestive of malignancy with axillary lymph node metastasis. Core needle biopsy (CNB) of the left breast mass suggested a fibroepithelial tumor, favoring PT, but the grade could not be determined. After discussion by the multidisciplinary team (MDT), the patient underwent total mastectomy of the left breast + axillary lymph node dissection(ALND) of the left side + resection of the right breast mass. Postoperative paraffin pathology and extensive immunohistochemistry confirmed BPT of the left breast (mitotic figures approximately 7/10 HPF, negative surgical margins), and all 21 dissected lymph nodes on the ipsilateral side showed reactive hyperplasia; the nodule in the right breast was a FA. Conclusion: This case highlights the diagnostic dilemma of giant BPT, which clinically and radiologically mimics locally advanced or metastatic breast cancer. Preoperative CNB has limitations in PT grading, and the final diagnosis depends on histopathological examination of completely resected specimens. For tumors with necrosis, ulceration, and infection, the high metabolic findings on PET-CT should be interpreted with caution, as they may be caused by secondary inflammatory responses rather than specifically indicating highly invasive cancer or metastasis, thus avoiding overstaging and overtreatment before surgery. The core of treatment is wide surgical resection with negative margins, and a prudent approach should be taken towards axillary management, with routine axillary dissection not recommended. The multidisciplinary collaborative model is crucial for integrating conflicting information and achieving individualized and precise decision-making. In the future, it is necessary to explore a dynamic risk assessment system integrating clinical, pathological, and molecular features to improve the individualized management level of PT.