[A Case of Spinal Cord Metastasis from Breast Cancer Successfully Treated with Trastuzumab Deruxtecan].

Intramedullary spinal cord metastases(ISCM)are very rare in patients with breast cancer and have a poor prognosis with no established treatment. We report a case of ISCM in a patient with HER2-positive breast cancer who was successfully treated with a novel anti-HER2 agent, trastuzumab deruxtecan(T-DXd, ENHERTU®).The patient was a 44- year-old woman who underwent surgery for right breast cancer. T-DXd was introduced as the fourth-line metastatic treatment for multiple metastases, including liver, bone, pituitary, brain, and spinal cord metastases. Hematologic and non- hematologic toxicities did not occur during the treatment with T-DXd. T-DXd could be administered continuously for 25 cycles, and symptoms such as numbness in the left lower limb were controlled without progression of the brain and spinal cord, although T-DXd-induced interstitial lung disease was a concern.ISCM is a rare metastatic lesion that is difficult to treat with chemotherapy due to the blood-brain barrier (BBB), and there is no established treatment for ISCM. T-DXd has shown promising results in previous clinical trials, including in patients with central nerve system (CNS) metastases, and is expected to be a good treatment option for CNS metastases in clinical practice.This successful case of T-DXd for ISCM suggests that T-DXd is an effective treatment option for patients with breast cancer and CNS metastases.