Case Report: Intramedullary spinal cord metastasis in HER2-low breast cancer: near-complete response to multimodal management and trastuzumab deruxtecan

Background Intramedullary spinal cord metastasis (ISCM) is a rare neurological complication of breast cancer. While trastuzumab deruxtecan (T-DXd) demonstrates robust central nervous system efficacy in HER2-low metastatic disease, real-world evidence regarding its specific activity against ISCM remains exceedingly limited. Case presentation A patient initially diagnosed in 2021 with hormone receptor-positive (HR+), human epidermal growth factor receptor 2 (HER2)-low (1+) breast cancer with sternal metastasis presented in July 2025 with severe right upper extremity weakness (Medical Research Council [MRC] grade 3/5). Cervical magnetic resonance imaging (MRI) revealed a C2 intramedullary lesion, prompting immediate surgical resection and laminoplasty. Pathological evaluation of the spinal lesion confirmed breast cancer-derived ISCM, revealing a HR-negative (ER 0%, PR 0%), HER2-low (1+) profile with a high Ki-67 index (70%). Two months post-surgery, follow-up spine MRI detected a 9-mm C2 nodule, designated as radiologically suspected local recurrence or residual disease. Guided by the identified HER2-low target, the patient received T-DXd therapy concurrently with intensity-modulated radiation therapy to the C1-C4 cervical spine (30 Gy in 10 fractions). Follow-up imaging demonstrated a near-complete radiological resolution of the spinal nodule, accompanied by substantial neurological improvement (motor strength recovering to grade 4/5). As of September 2026, the patient reached 13 months of postoperative follow-up with sustained disease control. Conclusion This case underscores the critical importance of metastatic re-biopsy in capturing spatial heterogeneity and identifying actionable targets. Furthermore, it provides compelling real-world evidence supporting the intraspinal therapeutic activity of T-DXd within a multimodal treatment strategy for refractory HER2-low ISCM.