A predictive model for Gamma Knife intermediate dose spill: R50%Analytic-GK.

Abstract

Minimizing intermediate dose spill in stereotactic radiosurgery (SRS) for brain treatment is crucial. Intermediate dose spill correlates with the exposure of normal brain tissue to high doses, which increases the risk of radionecrosis. R50%, defined as the volume of the 50% of prescription isodose cloud/planning target volume, is one metric for intermediate dose spill. A predictive model for R50% in linear accelerator VMAT-delivered SRS has been developed Desai etal. (2020) and is called R50%Analytic. This study extends the R50%Analytic model to Gamma Knife (GK) delivered SRS, resulting in the R50%Analytic-GK model. Phantom calculations were performed on 11 spherical target volumes ranging from 0.001 to 44cm3 to develop the R50%Analytic-GK model. R50%Analytic-GK was tested against clinical data from 18 brain metastasis cases with one to 11 targets treated on GK Icon and planned in GammaPlan with lightning dose optimizer. Thirty-five targets with volumes between 0.011 and 27.4cm3 were analyzed by extracting the R50% achieved clinically (R50%Clinical) for comparison to the predicted intermediate dose spill from R50%Analytic-GK. The predicted R50%Analytic-GK values generally represent a lower bound for the R50%Clinical values as the model would predict. The Difference, R50%Clinical - R50%Analytic-GK, has a median value of 0.92, which quantifies the lower bound nature of R50%Analytic-GK. The model reflected the character of intermediate dose spill for the clinical cases. A few outliers were likely due to specific planning complexities. The R50%Analytic-GK model for intermediate dose spill successfully extends the theoretical framework of R50%Analytic to GK-delivered SRS. It provides a method to predict the intermediate dose spill for GK Icon treatments. This model can aid in assessing SRS treatment plans by providing a benchmark for the intermediate dose spill for comparison.