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

Purpose: 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 et al. (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.

Methods: Phantom calculations were performed on 11 spherical target volumes ranging from 0.001  to 44 cm³ 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 27.4 cm³ were analyzed by extracting the R50% achieved clinically (R50%_Clinical) for comparison to the predicted intermediate dose spill from R50%_Analytic-GK.

Results: 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.

Conclusion: 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.