2278 - A Novel Radiotherapy Immobilization Device Enabling Pre-Positioned Hippocampal Avoidance for Hippocampal Sparing During Whole-Brain Radiotherapy

Purpose/Objective(s): To evaluate a novel, low-cost radiotherapy immobilization device designed to enable pre-positioned hippocampal avoidance during CT simulation.

Materials/Methods: Patients were pre-positioned to achieve the hippocampal avoidance by simulating the diagnostic MRI posture, with the temporal frontal line aligned perpendicular to the simulator couch. Used the novel immobilization device . Hippocampal volumes on diagnostic MRI were compared with those obtained under conventional positioning (control group) and under pre-positioned hippocampal avoidance (experimental group). The ratio of hippocampal slice number to whole-brain slice number was also compared between the two positioning approaches. The whole-brain planning target volume (PTV) was prescribed 30 Gy in 10 fractions, and hippocampal doses were compared between the two plans. Hippocampal dose constraints were defined , and target dose coverage was evaluated and compared between the two plans.

Results: Hippocampal volume under pre-positioned hippocampal avoidance was closer to that measured on diagnostic MRI. Hippocampal volume under conventional positioning was significantly larger than that under hippocampal avoidance (p=0.027). The ratio of hippocampal slice number to whole-brain slice number was significantly lower under hippocampal avoidance compared with conventional positioning (p=0.003). When equivalent target dose coverage was achieved, the median hippocampal Dmax, Dmean, and D₄₀ in the experimental group were 5.64, 3.07, and 2.99, respectively, compared with 16.6, 3.97, and 3.94 in the control group. Significant differences were observed in median hippocampal D40, D50, and Dmin between the two groups (p<0.01). When equivalent hippocampal dose constraints were applied, D95% of the PTV, homogeneity index (HI), and conformity index (CI) were significantly higher under hippocampal avoidance than under conventional positioning (p<0.01).

Conclusion: Pre-positioned hippocampal avoidance during CT simulation for prophylactic whole-brain radiotherapy results in hippocampal volume closer to that observed on diagnostic MRI, a significantly reduced proportion of hippocampal slices within the whole-brain CTV, and a significant reduction in hippocampal dose, thereby achieving effective hippocampal sparing. By using the novel radiotherapy immobilization device to obtain CT simulation and MRI images with identical hippocampal avoidance positioning and consistent scanning direction, target delineation accuracy is improved and radiation exposure to organs at risk is reduced. This low-cost immobilization device combined with pre-positioned hippocampal avoidance enables effective hippocampal sparing and accurate delineation, while optimizing the dosimetric protection in HA-WBRT. This technique provides a cost-effective and accessible approach for hippocampal sparing, and may facilitate the broader clinical implementation of HA-WBRT across healthcare institutions.