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Evaluation of Dose Spillage From the Gamma Knife Perfexion Vs Volumetric Modulated Arc Radiosurgery When Treating Multiple Metastases in a Single Fraction

D McDonald*, J Peng, N Koch, M Ashenafi, D Jacqmin, J Jenrette, I Takacs, K Vanek, Medical University of South Carolina, Charleston, SC

TH-A-137-5 Thursday 8:00AM - 9:55AM Room: 137

Purpose: To explore differences in the patterns of dose spillage to normal brain in stereotactic radiosurgery when using the Gamma Knife Perfexion (GKP) versus single-isocenter volumetric modulated arc radiosurgery (VMAS) to treat multiple metastases.

Methods: Radiosurgery plans were generated on five T1-weighted MRI data sets for delivery with a Gamma Knife Perfexion (Elekta GammaPlan V10.1) and a Varian accelerator using VMAS (Philips Pinnacle V9.2). Each data set contained between three and five brain metastases. The percent of the target volume receiving the prescription dose (V100%) was >99% for each target. No heterogeneity corrections were used with either planning system. Following planning, the images, contours, and dose distributions were exported to an analysis program (VelocityAI). On each data set, the normal brain volumes receiving 20, 30, 40, 50, 60, 70, 80, and 90% of the prescription dose were determined and tabulated. Normal brain was defined as brain minus any target volumes. A percent change from GKP to VMAS was calculated for each dose level. Isodose lines and DVHs were generated for each modality and overlaid.

Results: Dose spillage to normal brain was consistently higher for single-isocenter VMAS compared to GKP, even when the volume of brain receiving the prescription dose remained similar or decreased. This increase in dose spillage could be seen for all dose levels (V90%-V20%), and was more significant for lower dose levels. Levels V20-40% showed an average increase of +653% compared to GKP. Levels V70-90% showed an average increase of +355%. The average increase across all levels was +546%.

Conclusion: Dose spillage to normal brain is significantly increased when treating multiple metastases with single-isocenter VMAS compared to multi-shot treatment with GKP. The clinical implications of this increased spillage should be examined further, particularly in situations where patients may receive multiple SRS treatments or radiosurgery combined with whole-brain radiotherapy.

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