Program Information
A Simple Model of Dose Falloff for Highly Convergent SBRT Beams
P Hoban1*, Y Niu1 , C Yu1,2 , (1) Xcision Medical Systems, Columbia, MD, (2) Univ Maryland School of Medicine, Baltimore, MD
Presentations
SU-E-T-74 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: The purpose of this study was to create a simple model of dose falloff outside the target for a SBRT system employing a highly convergent beam geometry. With an estimate of the rate of reduction in dose away from a target of a given size, dose to tissues at a given distance from the target can be estimated. Our hypothesis is that dose at a given distance away from the edge of a target can be approximated using an inverse square model.
Methods: With the GammaPod™ system used for this study, thirty-six rotating Co-60 beams with circular cross section converge at an isocenter and then diverge distal to the isocenter. Dose falloff beyond the target is mainly due to reduction in beam overlap, which in turn depends on projected area of all beams and projected beam area swept out during rotation. Beams also decrease in intensity beyond the isocenter. The GammaPod™ TPS calculates dose via superposition of Monte Carlo-generated kernels, each corresponding to the convergence of beams at an isocenter within a stereotactic breast cup.
Results: The factors above yield a simple inverse square-based model that has been compared with TPS predictions and measurements for a variety of targets. Good agreement is seen considering the simple nature of the model. Fitting of the treatment planning and measured dose falloff data with a power law expression yields a negative exponent close to 2, confirming the inverse square nature of dose falloff.
Conclusion: It is possible to apply a simple inverse square dose falloff model for SBRT or SRS systems such as the GammaPod which employ a highly-convergent beam geometry. This can be useful in estimating the “worst case scenario” for dose to critical organs at a known distance from the edge of the target volume.
Funding Support, Disclosures, and Conflict of Interest: Authors are employees of Xcision Medical Systems
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