Proton beam therapy has recently been proposed as a noninvasive approach for treating ventricular tachycardia (VT), where target regions are identified in the myocardium and treated using external beam therapy. Effective treatment requires that lesions develop at target sites of myocardial tissue in order to stop arrhythmic pathways. Precise characterization of the dose required for lesion creation is required for determining appropriate dose levels in future clinical treatment of VT patients. In this work, we use a deformable registration algorithm to align proton beam delivery isodose lines planned from baseline computed-tomography scans to follow-up delayed contrast-enhanced magnetic resonance imaging scans in three swine studies. The relationship between myocardial lesion formation and delivered dose from external proton beam ablation therapy is then quantitatively assessed. The current study demonstrates that myocardial tissue receiving a dose of 20Gy or higher tends to develop into lesion, while tissue exposed to less than 10Gy of dose tends to remain healthy. Overall, this study quantifies the relationship between external proton beam therapy dose and myocardial lesion formation which is important for determining dose levels in future clinical treatment of VT patients.