摘要Background: Breast cancer is one of the most common malignant tumors among women worldwide. Chemotherapeutic and targeted agents, as important adjuvant therapy for breast cancer, can also cause cardiotoxicity, leading to cardiac dysfunction. It is essential to recognize cardiotoxicity early, cease drug exposure when appropriate, and initiate heart failure therapy. Currently, echocardiography is routinely used to monitor cardiac function during treatment. However, normal left ventricular ejection fraction (LVEF) measured by echocardiography cannot exclude cardiotoxicity. Therefore, more sensitive cardiac monitoring tools are needed. Optical pumped magnetometer-magnetocardiography (OPM-MCG) has been proved to be a noninvasive and effective means to detect and monitor myocardial injury.Case description: In this case, we presented a patient diagnosed with early breast cancer with human epidermal growth factor receptor 2 (HER2) overexpression, following adjuvant therapy with paclitaxel liposomes, trastuzumab, and pertuzumab. Heart failure with reduced ejection fraction (HFrEF) occurred after five cycles of anti-HER2 therapy, which improved with chronic heart failure (CHF) treatment. The MCG scan of this patient was significantly abnormal when she developed symptomatic HFrEF, which improved gradually during CHF treatment.Conclusion: The patient's heart failure was most likely caused by HER2-targeted agents, which was reversible and could be improved with the administration of angiotensin receptor neprilysin inhibitor (ARNi) and sodium-glucose cotransporter-2 inhibitor (SGLT2i). In the future, OPM-MCG may act as a safe, accurate, and efficient evaluation tool for cardiotoxicity monitoring to detect early myocardial injury in cancer patients.