The impact of vertebral body irradiation techniques on dose distribution to cardiac substructure during craniospinal irradiation for pediatric medulloblastoma: A detailed dosimetric study.

72 Background: With improved long-term survival in pediatric medulloblastoma, radiotherapy-related cardiac toxicity—particularly delayed injury to critical functional cardiac substructures—has become an increasingly important concern in cardio-oncology. In craniospinal irradiation (CSI), whole vertebral body irradiation (WVB) is commonly employed to mitigate vertebral growth deformities; however, its characteristic dose distribution may differentially affect specific cardiac substructures, effects that may not be adequately captured by conventional whole-heart dosimetry. Methods: Within the context of photon radiotherapy, this study provides the first systematic comparison of vertebral body sparing (VBS) versus WVB techniques in pediatric medulloblastoma CSI with respect to dose distribution across 16 key cardiac substructures. These substructures were grouped into four anatomical/functional categories: cardiac chambers, coronary arteries, great vessels, and cardiac valves. A paired dosimetric analysis was performed in 22 children treated with helical tomotherapy, for whom both VBS and WVB CSI plans were generated. Dose–volume metrics—including mean dose (Dₘean), maximum dose (Dₘax), and volumetric parameters (e. g. , VₓGy) —were extracted and statistically compared. Results: WVB exhibited a distinct substructure-specific dose pattern. Although it modestly reduced the mean whole-heart dose, it significantly increased the left anterior descending artery (LAD) Dₘean relative to VBS (18. 2 ± 3. 1 Gy vs. 15. 8 ± 2. 7 Gy, p<0. 01), accompanied by an approximately 15% increase in LAD V₂0Gy. The right ventricle also received a significantly higher radiation burden with WVB. In contrast, VBS resulted in higher doses to the left ventricular myocardium and the mitral valve region. Atrial substructures showed heterogeneous dose-impact patterns between the two techniques, indicating nonuniform exposure across atrial regions. Conclusions: This study demonstrates that vertebral body irradiation strategies during pediatric medulloblastoma CSI lead to markedly different dose distributions across cardiac substructures. While WVB helps preserve spinal development, it may increase radiation exposure to specific coronary segments—particularly the LAD—and to the right ventricle. Conversely, VBS may preferentially increase dose to the left ventricular myocardium and the mitral valve region. These findings underscore that whole-heart dose alone is insufficient for plan optimization in pediatric CSI and support incorporating substructure-level dosimetric evaluation to enable earlier cardiovascular risk stratification and individualized cardioprotection against late effects such as ischemic heart disease, cardiomyopathy, and valvular dysfunction.