Minimum Bladder Reference Contour to Guide Bladder Filling for Prostate Radiotherapy

Abstract

The purpose of this study was to evaluate if a minimum bladder reference contour could help reduce treatment delays and imaging needed for pre-treatment setup in patients undergoing prostate stereotactic body radiation therapy (SBRT).20 patients undergoing 5-fraction SBRT to the prostate and seminal vesicles were included in this pilot study. Patients were simulated with a full bladder (1 cup, 45min prior). The simulated bladder was contoured and then a script automatically generated four possible "min reference contours as a contraction by 1, 2, 3, and 4 cm from the superior bladder border. The smallest reference contour that still met our bladder dose constraints was selected as the min_bladder to be used to guide pre-treatment setup. Radiation therapists and treating physicians were instructed to consider bladder filling sufficient if the volume on the cone-beam CT (CBCT) upon visual inspection appeared larger than the min_bladder reference even if smaller than the simulated bladder. Approved CBCTs were used to evaluate bladder and bowel dosimetry. Treatment time (first kV image to beam-off) and number of CBCTs were compared to 66 consecutive patients treated prior to introducing the min-bladder reference contour.The average simulated bladder, min_bladder, and treated bladder sizes were 398 ± 139cc, 157 ± 54cc and 300 ± 150cc, respectively. There was 1 of 100 total fractions where bladder constraints were exceeded, however the approved CBCT bladder volume was less than the min_bladder reference and the total cumulative bladder dose for this patient was within constraints. Average cumulative max bowel dose was 14.5 Gy in 5 fractions with a maximum cumulative max bowel dose 32.5 Gy in 5 fractions. Compared to our historical average treatment time of 31.5 ± 20 min/fraction, the pilot study average was 26 ± 15 min/fraction (P = 0.003). Average number of CBCTs/fraction also decreased from 1.38 ± 0.69 to 1.24 ± 0.60 (P = 0.048). Based on our annual SBRT treatment volume, we estimate that use of this reference contour will save > 150 hours and > 260 CBCTs per year in our department.Our results suggest that the reference contour accurately predicted the minimum bladder volume needed to avoid exceeding bladder constraints, did not inadvertently overexpose bowel, and significantly reduced treatment time and number of pre-treatment CBCTs. A similar reference contour is now being developed for other prostate definitive and post-op regimens at our institution.