This goal of this thesis was to contribute to the current body of evidence of anti-cancer therapy related cardiotoxicity, with particular focus on trastuzumab-induced cardiotoxicity during HER2-positive metastatic breast cancer (MBC). We found that serial cardiac monitoring is warranted in patients with HER2-positive MBC and certain risk factors. A novel imaging method is speckle tracking 2D-echocardiography (2DE-ST). In this thesis we found that GLS measurement with 2DE-ST could predict cardiotoxicity earlier and with greater accuracy. Further research is warranted on the role of NT-proBNP to identify cardiac dysfunction. Regarding the trastuzumab treatment, discontinuation after complete radiological remission should be carefully considered. In addition, a baseline LVEF