Room: Room 209
Purpose: Gas release rectal balloons are used routinely for mitigating intra-fraction motion in our clinic. The hydrogel spacers were recently introduced to improve rectum sparing and were not used in combination with rectal balloon. This study is to evaluate and compare the intra-fraction prostate motion by analyzing pre-treatment setup and post-treatment radiographs from prostate patients treated with proton using the hydrogel spacers or rectal balloons.
Methods: Total 200 prostate patients were selected for the study, with 100 using hydrogel spacers and 100 using rectal balloons. Pre-treatment setup radiographs and post-treatment radiographs were obtained to evaluate the setup residual errors and prostate intra-fraction motion errors for both cohorts of patients. In-house 2D-3D image registration software was used for the study and the implanted fiducial markers in the prostate were used for target registration between DRR and radiographs. The systematic and random components of the residual errors and intra-fraction motion errors were calculated and compared.
Results: For the intra-fraction motion errors of hydrogel spacer patients, the setup residual systematic errors are 0.02, 0.06 and 0.05cm in lateral (LR), longitudinal (SI) and Vertical (AP) directions. The residual random errors are 0.03, 0.11 and 0.10cm in LR, SI and AP directions, respectively. For rectal balloon patients, the setup residual systematic errors are 0.02, 0.04 and 0.04cm in LR, SI and AP directions, respectively. The residual random errors are 0.08, 0.15 and 0.15cm in LR, SI and AP directions, respectively. The setup residual systematic and random errors are less than 0.05cm for both hydrogel and balloon patients. The calculated population based PTV margins are similar between the two patient populations.
Conclusion: The error analyses of the patient initial setup residual errors and intra-fraction motion showed that similar characteristics between the patients using rectal balloon and hydrogel spacer.