Room: AAPM ePoster Library
Purpose: To assess the accuracy of treatment planning system (TPS)-estimated water equivalent thickness (WET) and stopping power ratio (SPR) of selected immobilization devices compared to measurements using a multi-layer ionization chamber (MLIC).
Methods: A selection of clinical immobilization devices were scanned using four clinical CT acquisition protocols. The WET and physical thickness of these devices were measured in the clinical TPS (Eclipse, Varian), and the material-to-water SPR was determined. The WET of each device was measured using a MLIC (Giraffe, IBA Dosimetry). Based on the measured results, a preliminary dosimetric analysis was performed to determine the effect of overriding the SPR of one of the immobilization devices to the measured value on the quality of 3 clinical treatment plans.
Results: The TPS underestimated the SPR of the immobilization devices we investigated by 12.1% to 35.1%, with the exception of a dental putty. However, due to the relatively low SPR values of these devices, the WET differences were <2mm for all the devices we investigated. The largest WET difference was 1.9mm for the body portion of the kVue Bos couch insert. The Coefficient of Variation of WET obtained from different CT protocols was minimal, ranging from 0.0% to 3.9%. The dosimetric analysis showed minimal changes in GTV V100% (<0.4%), CTV D95% (<0.2%) and relevant DVH metrics for organs at risk (<1.0%).
Conclusion: The TPS tended to underestimate the WET and SPR of commonly used immobilization devices, and the TPS estimation is independent of CT acquisition protocol. However, such underestimation results in minimal change in plan quality for the clinical proton plans investigated. Further studies are needed to investigate the dosimetric impact for additional tumor sites and immobilization devices.