Correlations between baseline anatomy, respiratory motion, and cardiac dose in DIBH breast radiotherapy

Purpose: Deep inspiration breath-hold (DIBH) is widely used to reduce cardiac dose in breast radiotherapy; however, its dosimetric benefit has not been well characterized at the individual patient level. This study aimed to evaluate the relationships between anatomical parameters, respiratory motion, and cardiac dose, and to identify predictors of treatment benefit. Methods: A retrospective analysis was conducted on 23 patients who underwent CT simulation under free-breathing (FB) and DIBH conditions. Anatomical parameters derived from FB CT included cardiac contact distance (CCD), heart–chest distance (HCD), and cardiac–lung (CL) ratio. Respiratory parameters (respiratory gap and gating window width) were recorded. Dosimetric parameters for the heart and left anterior descending artery (LAD) were analyzed. Spearman’s correlation analysis was performed. Results: HCD showed the strongest inverse correlation with LAD Dmean (r=-0.686, p<0.001). The CL ratio was positively correlated with LAD Dmean in both left–right (r=0.697, p<0.001) and anterior–posterior directions (r=0.502, p=0.015). CCD showed limited association, significant only for LAD Dmax (r=0.471, p=0.023). Respiratory parameters had minimal impact; only respiratory gap correlated with LAD Dmax under FB (r=-0.436, p=0.037). CL ratio (L–R) was moderately correlated with respiratory gap (r=-0.475, p=0.022). Conclusion: Baseline anatomical parameters, particularly HCD and CL ratio, are the primary predictors of cardiac dose in breast radiotherapy using DIBH. Respiratory motion has limited direct impact on dosimetric outcomes. These findings highlight the dominant role of anatomical geometry and support the potential use of pre-treatment anatomical screening to optimize patient selection for DIBH.