Cardiac Substructure Dose Reduction and Toxicity Risk Assessment: IMPT Versus IMRT for Breast Cancer

Intensity-modulated proton therapy (IMPT) significantly reduces mean heart dose (MHD), but data on cardiac substructure dose and toxicity compared to intensity-modulated radiation therapy (IMRT) are limited. This study evaluated dose reduction in cardiac substructures between IMPT and IMRT and assessed cardiac toxicity risks using 2 normal tissue complication probability models.

A retrospective analysis was conducted on 30 breast cancer patients from a randomized trial with the highest MHD receiving IMRT. IMPT plans were created for a prescribed dose of 4005 cGy(RBE) in 15 fractions. Normal tissue complication probability models were used to compare individual acute coronary events (ACEs) risk between IMPT and IMRT.

Intensity-modulated proton therapy reduced cardiac substructure doses by 63.34% to 100%, with greater absolute reductions in left-sided and relative reductions in right-sided patients. For left-sided internal mammary node irradiation (IMNI), IMPT achieved an 82.25% reduction for left anterior descending coronary artery (P = .009), 79.45% for RV (P < .001), and over 90% for other substructures. Right-sided patients had near-zero mean doses in most substructures. The Darby model indicates IMPT reduces individual ACE risk by 1.58% to 5.16% for left-sided IMNI (P = .001) and 0.59% to 1.05% for right-sided IMNI (P = .063). The Bogaard model shows a 0.19% to 2.75% reduction in individual 9-year ACE risk-based MHD for left-sided IMNI (P = .0015). Risk reduction variations are influenced by dose reduction and other risk factors.

Intensity-modulated proton therapy provides excellent cardiac protection, particularly for left-sided IMNI and high-risk patients.