[HER2-Positive Breast Cancer Brain Metastasis Recurred after Whole-Brain Radiotherapy and Successfully Treated with Trastuzumab Deruxtecan].

A 50-year-old female patient noticed a mass and pain in her left breast, and needle biopsy revealed mucinous carcinoma that was negative for hormone receptors and positive for HER2/neu. After further examination, she was diagnosed with left breast cancer, cT3N2aM1 (bone), cStage Ⅳ, and was scheduled to start trastuzumab + pertuzumab + docetaxel (HPD) and denosumab. However, she was urgently admitted to the hospital due to pain caused by bone metastasis, and radiation therapy was performed. After discharge from the hospital, she was started on HPD and zoledronic acid treatment. After 6 doses of HPD, because of Grade 2 lower leg edema, treatment was continued with trastuzumab + pertuzumab (HP) and zoledronic acid only. Eighteen months after the first presentation, she complained of nausea and headache and was diagnosed with breast cancer brain metastasis. Therefore, whole brain irradiation was performed. HP was continued after radiation therapy, but headaches reappeared 33 months after the first presentation, accompanied by recurrence of brain metastasis. Trastuzumab deruxtecan was initiated, and the lesions had almost disappeared on the MRI scan 3 months later. The treatment is ongoing without any major adverse events.