(Saturday, 4/4/2020) 4:30 PM - 5:30 PM [Mountain Time (GMT-6)]
Measurements of the small fields used in SRS/SBRT, IMRT/VMAT, TomoTherapy, CyberKnife and Gammaknife pose significant dosimetric challenges mainly due to non-equilibrium conditions. Charge-particle equilibrium is a phenomenon associated with the range of secondary particles and hence dependent on the beam energy, particle spectrum and the composition of the medium. Source size, even when relatively small, depends on the accelerator design with any obstructions by the collimation system impacting the beam fluence, output factors, and profile shape.
This session will educate attendees on how to address these challenges with a thorough commissioning strategy which includes detector selection, measurement media, energy considerations, and accuracy requirements. Characteristics to be considered when selecting appropriate detectors will also be described including any perturbations to the measurement field. It is crucial that multiple detectors be used and that the use of any detectors which have significant volume averaging be avoided. From the perspective of the IAEA-TRS-483 and TG-155 reports, this presentation will address the myths associated with small field dosimetry, including the definition of small fields and how to identify suitable detectors and associated correction factors. Finally, treatment planning considerations when performing beam modeling and the value of an independent end-to-end test will be discussed in support of patient safety.
Objectives
1. History and physics of small fields will be discussed
2. How to achieve +/-3% dosimetry in small fields
3. Take away message for clinical physicist for implementation of small fields in SRS/SBRT
Funding Support, Disclosures, and Conflict of Interest: J. Moran: disclosures - Blue Cross Blue Shield of Michigan and Varian Medical Systems.
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