The Correlation Between Target Volume (PTV) and Mean Heart Dose in Breast Cancer Patients: Developing a Predictive Model for Dosimetric Outcomes

Purpose: Modern radiotherapy techniques strive to maximize planning target volume (PTV) coverage while minimizing dose to Organs at Risk (OARs). In breast cancer, the heart is a critical OAR, where the Mean Heart Dose (MHD) is linearly associated with late cardiac events. This study investigates the correlation between PTV size and MHD to determine if target volume can serve as a reliable predictor for dosimetric outcomes in the era of IMRT and Deep Inspiration Breath Hold (DIBH). Methods: A retrospective analysis was conducted on 40 breast cancer patients (24 Right, 16 Left) treated with 3D-CRT, IGRT, or IMRT. PTV (cc) and MHD (cGy) were extracted from Dose-Volume Histograms (DVH). Pearson correlation coefficients and linear regression models were utilized to evaluate the predictive power of PTV volume on cardiac exposure, stratified by laterality and technique. Results: The mean PTV was 4031.7 cc (Range: 3899–4839 cc). The mean MHD for the entire cohort was 279.7 cGy (Left: 328.7 cGy; Right: 247.1 cGy). A weak negative correlation (r = -0.33) was observed between PTV and MHD in left-sided patients, described by the regression model MHD = 1318.4 - 0.24* PTV. This inverse relationship indicates that larger volumes were preferentially treated with advanced sparing techniques, effectively neutralizing the expected volume-dose penalty. Conclusion: PTV volume alone is a poor positive predictor of cardiac dose in a modern clinic. The implementation of DIBH and IMRT successfully decouples the relationship between target size and cardiac exposure, ensuring that patients with large treatment volumes do not necessarily incur higher cardiotoxicity risks.