A case report of a 52-year-old woman with HER2-positive breast cancer and multi-organ metastases including brachial plexus neuroinvasion

Abstract

HER2-positive breast cancer is an aggressive subtype, accounting for 15–20% of all cases, with a high tendency to metastasize to the lungs, liver, bones, and brain. Although targeted therapies have significantly improved survival, some patients still experience disease progression. Peripheral nervous system involvement, such as brachial plexus invasion, is rare but usually indicates advanced disease with poor prognosis. A 52-year-old woman was diagnosed with non-luminal HER2-positive breast cancer in 2022. She received multiple lines of therapy, including epirubicin–cyclophosphamide–trastuzumab, vinorelbine–trastuzumab, and capecitabine–lapatinib. Following further progression, she underwent six cycles of trastuzumab deruxtecan (T-DXd) with initial improvement. In April 2025, progressive weakness and paresthesia developed in her left arm. MRI revealed an infraclavicular mass invading the brachial plexus, along with lung, liver, bone, and brain metastases. Electromyography confirmed severe C5–T1 polyradiculopathy. Pathology reconfirmed HER2 positivity, while PET/CT demonstrated further progression. Palliative radiotherapy was administered, and ado-trastuzumab emtansine (T-DM1) was planned as fifth-line therapy, but the patient deteriorated and passed away. This case highlights the aggressive nature of HER2-positive breast cancer, with extensive metastases despite sequential targeted therapy. Although clinical trial data, such as DESTINY-Breast03 clinical trial data showed T-DXd to be superior to T-DM1, rapid progression acould still occur, reflecting the heterogeneity of treatment response. Brachial plexus invasion is an uncommon manifestation, typically resulting from direct extension or metastasis, and often leads to disabling neurological symptoms. Management in such cases is largely palliative, focusing on symptom relief and quality of life. Metastatic HER2-positive breast cancer with multi-organ involvement and brachial plexus neuroinvasion represents a rare yet challenging clinical scenario, emphasizing the importance of multidisciplinary care and careful sequencing of HER2-targeted therapies.