ESTRO 2024 - Abstract Book

S4047

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

1611

Digital Poster

Effect of bladder volumes changes on bowel doses during prostate with pelvic lymph-node radiotherapy

Suzanne Wong 1 , Helen Mayles 1 , Isabel Syndikus 2

1 Clatterbridge Cancer Centre, Physics, Liverpool, United Kingdom. 2 Clatterbridge Cancer Centre, Clinical Oncology, Liverpool, United Kingdom

Purpose/Objective:

The aim of this study is to measure the changes in Bladder Volume (B_Vol) during treatment, for patients receiving IMRT to the prostate and pelvic nodes, within the PIVOTALboost trial [1], and quantify the effect that these changes have on the bowel dose and dose constraint values.

Material/Methods:

A preliminary retrospective study has been performed, looking at the CBCT data for 5 patients in Arm D of the PIVOTALBoost trial (prostate boost and pelvic nodes treatment) [1]. The bladder and bowel were outlined on the CBCT’s for fractions 1, 2, 3, 8, 13, 18. These structures were transferred to the original structure set, using the online match from treatment. The B_Vol for each CBCT was measured and the difference in volume from the Planning CT (P-CT) calculated. The height from the top of the femur to the dome of the bladder (Bladder Height (B_Ht)) was also measured for each CBCT and the difference in height from the P-CT found [2]. The field of view of the CBCTs is limited compared with the P-CT. Image artifacts further reduce the range over which bowel can be outlined. To account for this, bowel was outlined to the same superior level on all CBCTs and a new plan bowel structure created, with the outline limited to the same level. The doses to the bowel structures were then calculated from the original plan, giving an estimate of the dose received at that fraction, including the Maximum dose to bowel. The values of the following PIVOTAL Boost dose constraints were found from the DVHs: V36Gy, V40Gy, V44Gy, V48Gy (V represents absolute volume in cc). The differences in dose constraints values between the plan and each CBCT were then calculated.

Results:

The results show that there is very good correlation between the B_Vol and the B_Ht (0.85≥ r ≤1) (Figure 1). B_Ht can be measured easily and quickly on CBCTs making it a useful surrogate for B_Vol, which requires outlining and measurement offline.