Abstract Book

S188

ESTRO 37

Results

Conclusion In interpreting the results, many factors were brought to light highlighting the complexity of this as a research area. Dissemination of this information will be of benefit to others interested in undertaking similar work as confounding factors unique to this patient group were identified which require careful consideration. It is our decision to focus any future research on other methods such as vestibular compensation to help dizziness and inbalance in this patient group PV-0368 Investigation of bladder volume and OAR protection in prostate radiotherapy L. Rice 1 , I. Maurenbrecher 1 , B. Earner 2 , M. Green 1 1 HCA International, Radiotherapy, London, United Kingdom 2 HCA International, Physics, London, United Kingdom Purpose or Objective Our pre-treatment clinical protocol for prostate radiotherapy strives to minimise inter-fraction targeting uncertainties by employing a pre-treatment enema and drinking protocol with ultrasound bladder volume (USBV) measurement. The aim is to achieve a reproducible rectal state (<4cm diameter) and bladder volume (150-250ml) to improve treatment quality. However, it is unclear whether these interventional measures achieve what they intend regarding organ-at-risk (OAR) protection. The important clinical questions are i) are OAR volume and OAR protection consistently achieved throughout a radiotherapy course? ii) do bladder volume variations relate to varied OAR dosimetry? iii) can USBV predict OAR protection in prostate radiotherapy? Material and Methods Data from prostate cancer patients undergoing radiotherapy were retrospectively analysied and underwent usual rectal evacuation and bladder filling protocols prior to planning CT and daily treatment appointments. OAR (bladder, rectum and bowel) were outlined on the planning CT and corresponding dose- volume-histogram (DVH) data and dose constraint information were acquired from the Eclipse™ planning system. Prior to daily treatment USBV was measured using a BladderScan® device. On the first three fractions and weekly thereafter CBCT images were acquired and transferred to the planning system to enable OAR contours and acquisition of corresponding DVH data. Internal bladder volume was calculated from planning CT images (CTBV) and CBCT (CBCTBV) after removing a 2mm margin equating to the bladder wall. A quality scoring system was developed for OAR DVH constraints: breach of departmental tolerances scored -3; worse than planned but better than departmental tolerances scored 0; and treatment DVH better or equal to planned DVH scored +2.

30 patients were studied giving a total of 259 complete inter-fraction datasets comprising comprehensive information on bladder filling, USBV, CTBV, CBCTBV, OAR contours and corresponding DVH data. Qualitative analysis showed considerable inter-fraction variation to CBCT bladder positioning and OAR DVH data (Fig 1A and B). Initial quantitative analysis has shown: 1. i) Pre-treatment USBV is confirmed as an acceptable surrogate for on-treatment CBCTBV measurement (Bland-Altman mean difference=0) and significant matching was observed between overall mean±sd CTBV, USBV and CBCTBV (RM ANOVA p<0.05). ii) Individual patients inter-fraction BSBV varied from -100% to +289% compared with CTBV and 22/30 patients breached 1-8 applied OAR dose constraints on 1-10 inter-fraction occasions. 2. iii) Bladder DVH constraint scores positively correlated with % change in CBCTBV (p<0.001), indicating on-treatment bladder volume reductions may increase bladder risk. Conclusion Results indicate that despite bladder and rectal volume regulation interventions, bladder volume and OAR dosimetry variations occur that may be inter-related. 3.