Radiotherapy Targeting the Causative Lesion of Horner's Syndrome in Metastatic Breast Cancer: A Case Report and Literature Review.

Introduction: Horner's syndrome (HS) is a rare manifestation of metastatic breast cancer resulting from disruption of the sympathetic pathway. Only 6 cases have previously been reported in the literature, almost all of which were managed with systemic therapy. Detailed reports focusing on the role of local treatment, such as radiotherapy directed at 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 (MRI) 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 radiological tumor response on follow-up MRI, no improvement in HS was observed. Conclusion: This is the first reported case of radiotherapy directly targeting the lesion responsible for HS in metastatic breast cancer. No neurological improvement was observed despite radiological tumor response. This clinical course is consistent with previous reports involving systemic therapy and suggests that neurological recovery in breast cancer-associated HS may be limited, even when tumor response is achieved.