A Comparative Dosimetric Baseline for Right-Sided Breast Cancer: Are We Ignoring the Lung in the Pursuit of Heart Sparing? A Multi-Dimensional Analysis of Laterality, Anatomical Asymmetry, and Organ-at-Risk Prioritization in Modern Radiotherapy

Purpose: The era of modern breast radiotherapy has been defined by "Cardiac Sparing," driven by the known cardiotoxicity of left-sided irradiation. This focus has led to the widespread adoption of Deep Inspiratory Breath Hold (DIBH) and intensity-modulated techniques for left-sided breast cancer (LSBC). However, a critical question remains: In the absence of a cardiac constraint, are Right-Sided Breast Cancer (RSBC) patients subjected to suboptimal lung dosimetry due to "benign planning neglect"? This study aims to establish a rigorous dosimetric benchmark for RSBC and compare it with LSBC to determine if lung sparing is being compromised when the heart is not at risk. Methods: We conducted a prospective comparative analysis of 150 patients (Right=75, Left=75) treated at a tertiary oncology center. To capture the nuance of anatomical asymmetry, the dataset was expanded to 100 high-dimensional variables, including cardiac contact distances, lung volume differentials, and tangential field geometry. All patients were treated with 40 Gy in 15 fractions. We utilized propensity score matching to control for PTV volume and BMI. The primary endpoints were Ipsilateral Mean Lung Dose (MLD) and V20; the secondary endpoint was the "Dosimetric Efficiency Ratio" (DER), defined as the ratio of PTV coverage to OAR integral dose. Results: Preliminary analysis reveals a paradox: Despite the lack of cardiac constraints, RSBC patients exhibited a mean MLD of 9.8 Gy compared to 8.4 Gy in the LSBC cohort (p < 0.05). This 1.4 Gy differential suggests that the rigorous optimization constraints applied to protect the heart in LSBC have a beneficial "bystander effect" on the left lung, which is absent in RSBC planning. Furthermore, DIBH usage was non-existent in the RSBC group, despite its potential to reduce lung density and dose. Conclusion: The pursuit of heart sparing in LSBC has inadvertently created a disparity in lung protection. Right-sided patients, considered "anatomically safe" from cardiac events, are receiving higher pulmonary doses. This study calls for a re-evaluation of planning priorities, advocating for the application of "Heart-Level Constraints" to the Right Lung to establish a true universal standard of care.