Purpose: In theory, image-guided online adaptive therapy (IGART) is a good choice for treating the prostate and the pelvic nodes by allowing the treatment plan to be adapted to account for daily changes in the OARs (bladder, rectum and bowel). However, present forms of the IGART process, including patient setup, imaging, contouring, reoptimizing, QA and re-imaging, have been shown to require 40-60minutes and bladder filling during this time could invalidate the potential dosimetric benefits offered by plan adaptation. The goal of this work is to evaluate the bladder-filling during IGART and its spatial effect on adjacent OARs.
Methods: A single patient underwent twenty-five-CBCTs as part of 5-fraction-SBRT to the prostate and pelvic nodes. The first three-fractions of treatment were delivered without adaptation, and the last two were adapted based on daily CBCT. Additional CBCTs were acquired prior to each VMAT beam. The bladder and rectum were manually contoured on each CBCT. The relationship among the bladder volumes, the CBCT scan time, and the rectum anterior wall positions were studied.
Results: Two out of three non-adapted fractions were delivered within a reasonable period (<15minutes). However, the patient’s bladder filled quickly in all fractions (see supporting Fig.3), for example, from 200cc to 760cc in 50minutes. The patient had to discharge urine in 1 extended adaptive fraction (~70minutes). The position of the anterior rectal wall corresponded well with the bladder volume (Fig.4). The rectum anterior wall was pushed 1.9cm further posterior when the bladder was filled from 500cc to 900cc.
Conclusion: In this study, bladder-filling was seen to affect the positioning of the rectum in a clinically meaningful way. For IGART to be effective, it is paramount to maximally reduce the total plan adaptation time, e.g. <20minutes, and carefully manage patients’ liquid intake prior to treatment fractions to minimize intra-fraction bladder filling effects.
Not Applicable / None Entered.