Metastatic breast cancer with leptomeningeal carcinomatosis treated with trastuzumab deruxtecan – a case report

Breast cancer (BC) is the most common malignancy in women. Both histology and immunochemistry are core to treatment choice and the knowledge of these fields is expanding. Metastatic breast cancer (MBC) has significantly lower overall survival (OS) than early stages, and leptomeningeal carcinomatosis (LMC) and brain metastasis (BM) have particularly worse prognosis. We describe a case of LMC BC treated with trastuzumab-deruxtecan (T-DXd). Thirty-seven years old woman, diagnosed with a HER2-positive invasive lobular carcinoma (cT2N0M0). After initial treatment with neoadjuvant chemotherapy plus anti-HER2 double blockade, surgery and postoperative radiotherapy, the patient relapsed with <i>de novo</i> LMC and BM, two years after initial diagnosis. Systemic therapy with first line off-label T-DXd was initiated, resulting in disease response, neurological recovery and improved quality of life (QoL). Disease complexity and expansion of new therapeutic options in BC has made multidisciplinary team discussion mandatory. In the HER2-positive MBC setting, antibody-drug conjugates (ADC), such as trastuzumab-emtansine and T-DXd, have shown important disease outcomes and QoL improvement. Equally important, these treatments are well tolerated and have manageable adverse events, making them safe and effective drugs. This solid evidence is seemingly leading into a new and groundbreaking BC treatment era.