Abstract Background Conventional photon radiotherapy (RT) for breast cancer is associated with reduction in global longitudinal strain (GLS), an increase in both troponin and N-terminal pro-B type natriuretic peptide (NT-proBNP), and incident heart failure. The cardiac radiation exposure with proton-RT is reduced and, thus may be associated with less cardiotoxicity. Objectives To test the effect of proton-RT on GLS, troponin and NT-proBNP. Methods A prospective observational single center study of 69 women being treated with proton-RT for breast cancer. Serial measurements of GLS, high-sensitivity cardiac troponin-I (hs-cTnI), and NT-proBNP were performed at pre-specified intervals (pre proton-RT, 4 weeks after completion of proton-RT and again at 2 months post proton-RT). Results The mean age was 46±11 years, BMI was 25.6±5.2 kg/m2, 32% had hypertension and mean radiation dose to the heart and left ventricle (LV) were 0.44 Gy and 0.12 Gy respectively. There was no change in LV ejection fraction (pre proton-RT vs. 4-weeks post proton-RT vs. 2 months post proton-RT, 65±5 vs. 66±5 vs. 64±4%, p=0.15), global GLS (−21.7±2.7 vs. −22.7±2.3 vs. −22.8±2.1%, p=0.24) or segmental GLS from pre-to post proton-RT. Similarly, there was no change in hs-cTnI or NT-proBNP with proton-RT. However, post proton-RT, we found that patients with a history of hypertension had lower GLS when compared to women without hypertension (−21.3±3.5 vs. −24.0±2.4%, p=0.006). Conclusion Proton-RT did not impact LV function, or associate with an increase in biomarkers. These data support the potential cardiac benefits of proton-RT compared to conventional RT. Funding Acknowledgement Type of funding sources: None.