ESTRO 2025 - Abstract Book

S4310

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

177

Digital Poster Adaptive bladder new proposed lateral margins. Roeum Butt 1 , Najib Abubakr 1 , Daniel Megias 2 1 Radiotherapy, Mount Vernon Cancer Centre, London, United Kingdom. 2 Radiotherapy#, Mount Vernon Cancer Centre, London, United Kingdom Purpose/Objective: This observational study aimed to see if the small plan utilisation within the adaptive bladder radiotherapy patient population can be improved. As anecdotal data suggested this was low due to the lateral margin expansion. Material/Methods: A retrospective review assessed plan utilisation for bladder patients treated with an adaptive approach between Jan 2023 and March 2024. The plan selected daily per patient was logged on an Excel spreadsheet. The true bladder maximum lateral extension beyond the CTV on CBCT was collected per fraction per patient. The systematic error was collected based on online match position, this was then used within the Van Herk margin recipe to calculate the increase in margin potentially required based on systematic shifts. A box and whisker diagram was created to assess if any overlap was present between the maximum lateral extension of the bladder and plan selection.

Results: An overview of total plan utilisation can be found in table 1. Table 1: Shows plan utilisation Times S plan used Times I plan Used

Times L plan used

Total reviewed

686

129

339

218

Percentage utilisation

Percentage utilisation

Percentage utilisation

18.80%

49.40%

31.80%

Table 2 shows systematic error as well as standard deviation. This was then used to calculate the Van Herk value suggesting a potential margin increase of 0.25cm. Table 2: Shows systematic error values Margin in cm Per pt breakdown Standard deviation VHF vrt= 0.193904

VHF lng=

Mean Vrt

-0.0337

0.397365

0.508141

Mean Lng

0.106256

0.346431

VHF lat=

0.259874

Mean Lat

-0.02427

0.457939

Table 3 shows the new proposed margin for the 3 different expansions.