Supplementary Material for: Radiotherapy Targeting the Causative Lesion of Horner’s Syndrome in Metastatic Breast Cancer: A Case Report and Literature Review

Background: Horner’s syndrome (HS) is a rare manifestation of metastatic breast cancer resulting from disruption of the sympathetic pathway. Only six cases have been previously reported in the literature, almost all of which were managed with systemic therapy. Detailed reports focusing on the efficacy of local treatment, such as radiotherapy, specifically targeting the causative lesion of HS in breast cancer are lacking. Case presentation: We report the case of a 60-year-old Japanese woman with HER2-positive, hormone receptor–positive invasive ductal carcinoma who developed left-sided HS due to lymph node metastasis. Five months after stereotactic body radiotherapy for cervical spine metastasis, she presented with left ptosis, miosis, and anhidrosis. Magnetic resonance imaging revealed a cervicothoracic lymph node metastasis adjacent to the first thoracic vertebra, compressing the cervicothoracic sympathetic trunk. Palliative radiotherapy was delivered to the enlarged lymph node. Despite slight tumor shrinkage on follow-up MRI, no improvement in Horner’s syndrome was observed. Conclusion: This is the first reported case of radiotherapy directly targeting the lesion responsible for HS in metastatic breast cancer. No improvement in Horner’s syndrome was observed despite slight radiological tumor shrinkage. These findings are consistent with previous reports involving systemic therapy and collectively suggest that neurological recovery in HS is unlikely, regardless of whether radiotherapy or systemic treatment is used.