Room: Exhibit Hall
Purpose: This study is to evaluate the dosimetric impact of inter-fraction variations in bladder and rectal volumes based on daily CBCT for VMAT SBRT Prostate patients prescribed to 40Gy in 5 fractions.
Methods: A total of 50 CBCT images approved by physicians for treatment of 10 consecutive prostate SBRT patients were used to measure bladder and rectal volumes for comparison to planning volumes. For simulation and each treatment, all patients were instructed to drink 16oz of water 45 minutes prior and administer an enema 3 hours prior. All VMAT (Eclipse AAA 13.6.23) plans met institutional dose constraints for rectum (D1cc<38.5Gy, V24<25%, V10<52%, mean<13Gy) and bladder (V36<10%, V20<50%). Daily delivered doses were evaluated using the DVH of the CBCT contoured structures on the original plan blended with daily CBCTs.
Results: Overall, the mean CBCT bladder volume was 17% [-78%, 163%] lower and the mean rectal volume was 13% [-13%, 79%] higher than at the time of simulation. Despite variability, bladder and rectal dose constraints were met over the entire course of treatment. However, there were individual fractions that exceeded dose tolerances. On these fractions, the CBCT bladder volume was >50% smaller and the rectal volume was >25% larger than the simulation volumes. Considerable overfilling of the bladder, which consistently occurred upon rejection of the initial CBCT for bladder underfilling and required the patient to drink more, does not seem to pose any negative dosimetric impact to the patient, since the inferior bladder wall and prostate volumes are consistent with planning upon CBCT alignment.
Conclusion: Despite significant variations in bladder and rectal fillings over the entire course of treatment, all 10 patients met MSK dose constraints. To consistently meet institutional constraints and effectively evaluate CBCT bladder and rectal volumes, a volume variance threshold still needs to be determined.