BACKGROUND: Trastuzumab-related cardiotoxicity is a common adverse effect of HER2-positive breast cancer treatment, especially when combined with anthracyclines. However, to date, no definitive prognostic markers have been found to predict trastuzumab-related cardiotoxicity. METHODS: Patients diagnosed with HER2-positive breast cancer, scheduled to receive anthracyclines followed by 12 months of trastuzumab or with pertuzumab, were prospectively followed up for 27 months. Measurements of left ventricular ejection fraction LVEF, high-sesitivity troponin I hs-Tn I, and a full cardiac examination were performed at baseline, after anthracycline treatment, and after four cycles of anti-HER2 agents. Subsequently, LVEF measurement and full cardiac examination were conducted every 3 months until the end of the follow-up. Cardiotoxicity was defined as an absolute decrease in LVEF of ≥15%, or a drop in LVEF of ≥10% from the baseline to <
50%. RESULTS: Among 78 patients, cardiotoxicity occurred in 13 (16.7%). A higher risk of cardiotoxicity was linked to hs-Tn I measured after four cycles of anti-HER2 agents ( CONCLUSION: Hs-Tn I measured after four cycles of trastuzumab in HER2-positive breast cancer patients could be an important predictor of cardiotoxicity induced by chemotherapy followed by anti-HER2 agents, particularly in the first year post-treatment, with a different cutoff value than that used in other cardiac conditions.