Evaluation of spatial and dosimetric accuracy of adaptive radiotherapy on Ethos using polymer gel dosimetry

Adaptive radiotherapy is an emerging technology that requires the development of more advanced quality assurance techniques. It is hypothesized that deformable polymer gels can provide volumetric dosimetric information for the assessment of spatial and dosimetric accuracy of adaptive radiotherapy on Ethos. This hypothesis will be investigated by (1) using stationary polymer gel dosimeters (PGDs) to benchmark HyperSight cone-beam CT (CBCT) imaging on Ethos relative to imaging done on a TrueBeam CBCT using signal-to-noise ratio (SNR) and gamma analysis and (2) establishing the feasibility of using deformable gel dosimeters to evaluate dose deformation on Ethos using gamma pass rate (GPR). It was found that Ethos’s SNR was up to 40% lower than TrueBeam, using stationary PGDs. The gamma analysis showed that with a criteria of 3%/3mm, Ethos had a GPR of 83.1%, compared to a GPR of 94.1% on TrueBeam. To evaluate the second objective, a deformation phantom was designed and fabricated. When the 3-field unadapted plan was used on the undeformed gel, the GPR was found to be 88.1% with a 3%/3mm criteria. When the gel was both deformed and undeformed in a different experiment, the GPR of the undeformed gel was 85.3% and the GPR of the deformed gel was 85.4%. Finally, an adaptive radiotherapy plan was made and delivered to a deformed gel. When the gel was compared to the adapted treatment plan, the GPR was 85.8% for the deformed gel. This adapted GPR shows that the adapted plan was successfully delivered and subsequently measured using PGD. When the gel was compared to the scheduled unadapted treatment plan, the GPR was 78.0% for the undeformed gel. The lower GPR with the scheduled treatment plan is due to the dosimeter not returning to its original shape when uncompressed. This work showed that PGD is possible on Ethos. Although the SNRs and GPRs are lower using HyperSight, PGD is still feasible for Ethos adaptive radiotherapy quality assurance. Further work on refining imaging methodology and deformation techniques is necessary to improve the accuracy of deformable PGD on the Ethos platform.