Consistency of Organ Geometries during Prostate Radiotherapy with Two Different Bladder and Bowel Regimens

The majority of Ontario cancer centres incorporate bladder and bowel preparation protocols for the treatment of prostate cancer with radical radiotherapy. Differing methods are used to achieve a full bladder and empty rectum for planning and treatment. We compared the effects of two different bladder and bowel preparation regimens on bladder, rectum, and prostate +/- seminal vesicle geometries through a course of radiotherapy. An optimal preparation would achieve reliable spatial arrangements and a high therapeutic ratio.

This prospective longitudinal study involved 59 prostate cancer patients treated with radical radiotherapy, of which half followed cohort 1 (laxative cohort) and the other cohort 2 (consistent timing cohort) bladder and bowel preparation regimen. Participants were asked to maintain an empty rectum for both planning and daily treatment appointments in cohort 1 through a fleet enema the morning of the planning appointment, and intake milk of magnesium during daily treatments. No specific bowel preparation was provided to cohort 2 patients. Instead, their appointment times were aligned with their natural bowel habits. This information was collected through a prescreening tool before treatment booking. All cohort 1 and 2 participants were asked to drink 250 mL of water 1 hour before planning and daily treatment appointments. Cohort 2 participants who identified no pre-existing urinary conditions were also asked to drink 2 L of water within 24 hours before the planning session and to continue this during treatment trajectory unless unable to do so because of treatment-induced bladder toxicities later in the treatment. A total of 1,335 structures (bladder, rectum +/- gas, and prostate +/- seminal vesicles) were contoured on the cone beam computerized tomography scans by three radiotherapists. A stringent quality assurance process was performed to assure quality and consistency of contours. Organ volumes were measured and evaluated for consistency over time from planning to completion of radiotherapy. Data analysis included the Fischer exact test and mixed effect modelling for total and subvolumes for bladder, rectum, rectal gas, and prostate +/- seminal vesicles.

Baseline total volumes for bladder ranged from 132 mL to 501 mL with means of 325 mL and 315 mL in cohorts 1 and 2, respectively. Bladder volume declined 3.6 mL per fraction and 2.4 mL per fraction in cohorts 1 and 2, respectively. The volume of the bladder structure inside the planning target volume (PTV) on simulation showed no difference by cohort (P = .095) but there was an effect of time (linear P < .0005). Baseline total volumes for rectum ranged from 19.2 mL to 106.3 mL with means of 52.0 mL and 54.7 mL in cohorts 1 and 2, respectively. The volume of the rectum inside the PTV on simulation showed no difference by cohort (P = .12) or time (P = .30) during the treatment course. Volume of gas in the rectum did not vary by cohort (P = .6) or time (P = .08). Baseline total volumes for the clinical prostate +/- seminal vesicles target ranged from 37.1 mL to 167.5 mL with means of 76.2 mL and 66.0 mL in cohorts 1 and 2, respectively. The clinical target decreased by 3% in total volume during the course of radiotherapy in both cohorts, with similar rates of the target falling outside the planned PTV structure.

No significant difference was found between cohorts for rectal volume, gas volume, target coverage, and rectal and bladder volumes in the PTV. Hence, patients should be offered a choice between cohort 1 and 2 bowel preparation regimens to allow for patient preference customization. Cohort 2 bladder preparation regimen was shown to be superior for consistency with slightly larger volume over time.