Abstract
INTRODUCTION: Phyllodes tumor of the breast is a rare biphasic neoplasm, accounting for less than 1% of all breast tumors. Coexistence of phyllodes tumor and breast cancer in distinct breasts is extremely rare. CASE PRESENTATION: A 47-year-old Japanese woman presented with bilateral breast lumps. A HER2-positive, unresectable invasive carcinoma in the right breast and fibroadenoma in the left were diagnosed via core needle biopsy. During chemotherapy with anti-HER2 therapy, the breast cancer shrank quickly, while the left breast lump suddenly enlarged. Under a diagnosis of malignant neoplasm of the breast, left mastectomy was performed. Malignant phyllodes tumor was diagnosed by postoperative histological examination and recurred in multiple areas as early as 2 months after surgery. DISCUSSION: Only 10 cases of coexisting phyllodes tumor and breast cancer in distinct breasts have been reported in the English literature. Phyllodes tumor associated with breast cancer in distinct breasts tends to be malignant. This is the first case of phyllodes tumor rapidly enlarging during anti-HER2 chemotherapy for locally advanced HER2-positive breast cancer. CONCLUSION: Even during effective treatment of advanced or recurrent breast cancer, attention should also be paid to the contralateral breast for the possible association of a second malignancy such as phyllodes tumor.