Optimizing SRS for Brain Metastases: Understanding the Volume-Gradient Index Relationship

Background/Objectives: The goal is to provide a straightforward framework for generating SRS-SRT plans that reliably meet high-quality dosimetric standards. That posed some questions such as which plan quality metrics should be utilized to evaluate a plan, what influences plan quality metrics the most, and finally, how to best optimize plan geometry. Our work has primarily concentrated on the second question, guided by our clinical experience. Methods: A dataset for statistical analysis was compiled by retrospectively reviewing 200 individual SRS-SRT target volumes from two centers. From the gathered data, several Linear Regression models were generated to assess the variability of plan quality metrics using statistical analysis. The most important regressor in the models were revealed to be target volume (TV), followed by a flag type variable that indicates whether the plan used to treat the referenced target contained multiple targets (MT) or not. Results: Every doubling of TV lowers Gradient Index (GI) sharply (-0.55 to -0.17) while Gradient Measure (GM) increases moderately (+0.024 cm to +0.07 cm). The model explains 85% of the variation in GI (R² = 0.85) and 84% of GM. Conclusions: In small lesions, GI seems to be a more sensitive evaluation metric for sub-CC SRS targets, compared to GM. Dose per fraction appeared to have had no significant effect. Treating multiple targets in the same plan appears to add an average of +0.19 to GI, independent of volume, while for GM by +0.027 cm.