ESTRO 37 Abstract book

S185

ESTRO 37

2 Bristol Haematology and Oncology Centre, Radiotherapy, Bristol, United Kingdom

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.

Purpose or Objective Dizziness is a major factor affecting quality of life in patients with vestibular schwannoma (VS) post stereotactic radiosurgery (SRS). The aim of this study was to investigate if there is any indication that dizziness relates to labyrinth dosimetry in our patients treated with gamma knife SRS in order to guide the direction of local research into reducing the impact of dizziness symptoms in this patient group. Material and Methods The patient group consisted of consecutive patients treated at the Bristol Gamma Knife Centre October 2013- March 2016 with SRS for VS who had completed a Dizziness Handicap Inventory (DHI) at a minimum of treatment day and 3 months post SRS. Labyrinth and labyrinth vestibule were retrospectively contoured on all treatment plans and length of vestibular nerve treated with treatment dose measured. Dosimetric data collected from this was related to change in DHI scores utilizing Pearson correlation for statistical analysis. Results 114 patients were treated with SRS for VS in the period of whom 86 patients had intact labyrinth and DHI at 0 and 3 months, 69 at 1 year and 38 at 2 years. The VS treated was mean 1.8cc [0.07-8.9], received a mean 12.3Gy to 50.1% isodose SRS with the Perfexion/Icon Gamma Knife. Coverage mean 0.99, gradient index mean 2.83 and Paddick Conformity Index mean 0.82. The mean vestibule dose was 5Gy [1.3-8.7Gy] and maximum to 1mm 3 mean 7.3Gy [1.5-16.1Gy]; labyrinth mean 3.3Gy [1-6.2Gy] and maximum to 1mm 3 mean 5.9Gy [1.8-9.5Gy]; and length of nerve receiving treatment dose mean 18.5mm (6.9- 28.2mm). There was no statistically significant correlation between any of the dose measurement and changes in DHI at 3, 12 or 24 months post SRS.

In addition, DHI score changes for all patients were mapped across the 4 measurement periods. Variability in results was considerable, and showed no discernable trend.