Session Item

Adaptive radiotherapy (ART) using a plan of the day (PoD) approach is a modern technique accounting for variability in bladder volume. Inspired by participation in the RAIDER trial (CRUK/14/016), it was deemed necessary to investigate if a simple estimation of the delivered dose to the rectum and bowel could be calculated based on the plans selected daily on treatment. This offers an opportunity to learn from every patient and tailor our care plan and treatment support for each patient when the dose to the rectum or bowel is higher, and there is a risk of increased toxicity. The objective of this preliminary study was to develop a simple estimation of radiation dose delivered to rectum and bowel after a course of bladder ART.

7 retrospective patient verification CBCT data sets were evaluated. Bladder translational displacements including a measurement in the Superior-inferior direction at 45˚were measured (Fig 1.).

The percentage of plans selected per treatment course, and mean D50% dose constraint for rectum and bowel; small, medium, large plans were collated. The Common Terminology Criteria for Adverse Events (CTAEv.4) was used to collect patient reported acute and late (up to 24 months) rectum and bowel toxicity.

The mean dose to rectum and bowel was calculated by multiplying the plan selected by the mean D50%.

Example calculation; mean dose to D50% rectum = (small PoD% * small mean D50%) + (medium PoD% * medium mean D50%) + (large PoD% * large mean D50%)

The estimated mean bowel dose calculated was then validated for agreement against a composite small bowel dose calculated in Pinnacle TPS for each patient dataset.

Bladder displacements per population (n=7) were notable in the SI and SI 45˚directions with a systematic error of 0.98cm and 0.99cm and a random error of 0.74cm and 1.17cm respectively. The percentage of plans selected per population was 35% small, 38% medium and 27% large. The mean dose per population was 15.94 (range 7-26) Gy for bowel and 19.98 (range 14-36) Gy for rectum. The agreement with the TPS was 98%. The maximum CTAE for bowel and rectum acute and late toxicities were ≤2.

This preliminary study suggests that the estimation of bowel and rectum mean D50% dose from PoD selection can be easily calculated and validated against the TPS. Future work should include the exploration of the relationship between the CTAE toxifies in a larger dataset, apply this learning to other volumes or dose constraints could be more favorable to individualising the patients care pathway.