Background. Left ventricular ejection fraction (LVEF) is used routinely to monitor cardiac dysfunction associated with breast cancer treatment. In this study the prevalence of early left ventricular diastolic dysfunction (LVDD) and its relationship to the dose‐volume of the heart irradiated were evaluated in HER2‐positive breast cancer patients undergoing concurrent trastuzumab and adjuvant radiotherapy (RT). Materials and Methods. Data from 40 breast cancer patients treated with concurrent trastuzumab and left‐sided adjuvant RT between September 2011 and October 2012 were collected prospectively. For comparison, 32 patients treated with concurrent trastuzumab and right‐sided adjuvant RT and 71 patients treated with left‐sided RT alone were collected retrospectively. Echocardiography was obtained before RT, immediately following RT, and 3 and 6 months after RT. Doses to the heart and left ventricle (LV) were quantified. Results. Prior to RT with concurrent trastuzumab, 11 of 29 (left) and 8 of 25 (right) patients with normal baseline left ventricular diastolic function (LVDF) developed LVDD. In patients receiving left‐sided RT alone, 12 of 61 patients with normal baseline LVDF developed LVDD. Dmean, D15–D40, D60–D70, and V3–V10 of the LV were significantly higher in patients who developed LVDD after concurrent trastuzumab and left‐sided RT. In contrast, only two patients developed grade 1 LVEF decrease after both concurrent treatment and left‐sided RT alone. Conclusion. Changes in LVDF compared with LVEF are more sensitive for early detection of cardiotoxicity. The dose‐volume of the heart contributes significantly to the risk of LVDD in patients with left‐sided breast cancer treated concurrently with trastuzumab. Implications for Practice: Abnormalities in diastolic function are more sensitive than changes in the left ventricular ejection fraction for detecting acute cardiotoxicity and are related to the dose‐volume of the heart irradiated in patients with left‐sided breast cancer receiving radiotherapy concurrently with trastuzumab. This result highlights the importance of decreasing the dose‐volume of heart irradiated as a protective strategy in the treatment setting of concurrent trastuzumab and radiotherapy. Diastolic dysfunction may serve as a more sensitive tool for the early detection of cardiac damage and should be incorporated as a routine parameter in the functional monitoring of cardiotoxicity. 摘要 背景. 左心室射血分数（LVEF）常规用于监测与乳腺癌治疗相关的心脏功能障碍。本研究在接受曲妥珠单抗联合同期辅助放疗（RT）的HER2阳性乳腺癌患者中，对左心室舒张功能障碍（LVDD）的患病率及其与心脏照射剂量‐体积的关系进行了评价。 材料和方法. 我们前瞻性地在2011年9月至2012年10月期间收集了40例接受曲妥珠单抗联合同期左侧辅助放疗的患者数据。另外，我们还回顾性地收集了32例接受曲妥珠单抗联合同期右侧辅助放疗患者数据，以及71例仅接受左侧放疗的患者数据用于对比。在放疗前、放疗后即刻，以及放疗后3个月和6个月行超声心动图检查。确定心脏和左心室（LV）接受的照射剂量。 结果. 在给予曲妥珠单抗联合同期放疗前，基线左心室舒张功能（LVDF）正常的患者中，分别有11/29例（左侧）和8/25例（右侧）在治疗后发生了LVDD。在仅接受左侧放疗的患者中，12/61例基线LVDF正常的患者发生了LVDD。曲妥珠单抗联合同期左侧放疗后发生LVDD的患者左心室Dmean、D15‐D40、D60‐D70和V3‐V10均显著较高。相反，在接受同期治疗和仅左侧放疗的患者中，仅2例患者发生了1级LVEF下降。 结论. 通过比较LVEF检测到的LVDF改变在早期发现心脏毒性方面更为敏感。对于接受同期放疗联合曲妥珠单抗治疗的左侧乳腺癌患者而言，心脏剂量‐体积为LVDD的显著风险因素。The Oncologist 2015;20:605–614 对临床实践的意义： 在接受放疗联合同期曲妥珠单抗治疗的左侧乳腺癌患者中，舒张功能异常在发现急性心脏毒性方面比左心室射血分数改变更为敏感，且与心脏剂量‐体积相关。这项结果强调了在接受曲妥珠单抗联合同期放疗患者中，采用减少心脏照射剂量‐体积作为保护策略的重要性。对于心脏损伤，舒张功能障碍可以作为更为敏感的早期检测工具，并且应该纳入常规心脏毒性功能监测参数之中。 Abnormalities in diastolic function are more sensitive than changes in the left ventricular ejection fraction for detecting acute cardiotoxicity and are related to the dose‐volume of the heart irradiated in patients with left‐sided breast cancer receiving radiotherapy with trastuzumab, highlighting the importance of decreasing the dose‐volume as a protective strategy. Therefore, diastolic dysfunction should be incorporated as a routine parameter in the monitoring of cardiotoxicity.