ESTRO 2020 Abstract book

S1072 ESTRO 2020

Poster: RTT track: Patient preparation, positioning and immobilisation

1 Mount Vernon Cancer Centre, Radiotherapy, Northwood, United Kingdom ; 2 Sheffield Hallam University, Allied Health Department, Sheffield, United Kingdom Purpose or Objective With the recent developments in radiotherapy (RT) delivery, volumetric modulated radiotherapy (VMAT) can be used to reduce dose to organs at risk (OARs) when bladders are not filled. This study aims to examine the impact of bladder filling protocols on patients receiving radical RT for localised prostate cancer on post RT related toxicity and biochemical progression free survival (bPFS) at our institution. Material and Methods A retrospective review of treatment records of prostate patients who followed two different bladder-filling protocols was carried out. This included each patient’s bladder size on planning computed tomography (CT), daily online image guided radiotherapy (IGRT) setup data, treatment time and post treatment follow up (f/u) data. The f/u data included each patient’s bPFS and gastrointestinal (GI) and genitourinary (GU) toxicity scoring at 1, 6, 12, 24 and 36 months post RT using the CTCAE v4.0 protocol. bPFS and cumulative GI/GU toxicity rates were calculated using the Kaplan-Meier method and the resulting curves of patients using two different bladder filling protocols were compared using the Mantel-Cox log- rank test. The daily online IGRT setup data were defined as the vertical, longitudinal and lateral couch shifts required for each treatment fraction. The treatment time for each fraction was calculated as the time difference between the starts of first imaging field and the last treatment field. Mann Whitney U tests were used to investigate any significant differences in IGRT setup and treatment times between the two bladder filling protocols. All patients included in the study were planned and treated under the same departmental clinical protocol with VMAT and daily online IGRT corrections. Results 66 patients with localised prostate cancer were treated between March 2014 to August 2015 at our institution with a median follow up of 36 months. At 3 years bPFS in the empty bladder group was 100% and 97% in the full bladder group (p=0.32). As indicated in the figure, no greater than grade 3 GU and GI toxicities were found for both bladder filling groups over the 3 years post RT f/u period. There were no statistically significant differences in cumulative grade 1+ GU (p=0.37) and GI (p=0.85) toxicity rates between the two bladder filling protocols. As shown in the table, there were no statistically significant differences in couch shifts for the daily IGRT between the two groups. Despite the fact that the treatment times of the two groups are not significantly different, it was noted that all patients under the full bladder filling protocol were required to arrive at the department 30 minutes earlier than their scheduled appointment time for every fraction.

PO‐1828 Alignment of prostate cancer patients for radiotherapy on the MR‐linac H. Barnes 1 , J. Mohajer 1,2 , A. Dunlop 1,2 , G. Adair Smith 1 , T. Herbert 1 , R. Lawes 1 , A. Tree 1,2 , H. McNair 1 1 Royal Marsden Hospital, Radiotherapy, Sutton, United Kingdom ; 2 Institute of Cancer Research, Radiotherapy and Imaging, Sutton, United Kingdom Purpose or Objective The MR-Linac (Elekta AB, Stockholm, Sweden) provides a novel treatment modality that can adapt a patient’s treatment plan daily to account for positional errors and organ motion, but also creates new challenges for patient positioning as it is installed without calibrated lasers for alignment. An audit was undertaken to determine accuracy and duration of prostate patient set-up on the MR-Linac (MRL) using two methods for patient alignment, with the aim of establishing a current standard of practice on the MRL. Material and Methods Patients under the PRISM (Prostate Radiotherapy Integrated with Simultaneous MRI) trial (NCT03658525) were aligned for treatment using two different methods. The first (Set-up A) is vendor recommended and requires patients to be aligned right-left (RL) to the MRL sagittal indicator light (SIL) and superior-inferior (SI) in line with the couch index by eye. The second (set-up B), adapted from current protocol, requires the same RL positioning in addition to using MR safe rulers to align lateral tattoos anterior-posterior (AP) and SI to the couch index. Ten patients were included with set-up A and B used for 50% of fractions each. Population mean (M) and systematic (Σ) and random (σ) errors were calculated and compared using a paired t-test. Timing data were also analysed using a Wilcoxon signed rank to determine if there was a variance between set-up times. Table1.1 – Mean (M), standard deviation (SD), random (σ) and systematic (Σ) displacements. There was no significant difference between random and systematic error for the RL and AP directions. There was a significant difference in the SI direction M ( p <0.01) and Σ error ( p = 0.04). The population average set-up times of 4:02min (A) and 4:20min (B) were found not to differ significantly ( p =0.139). Conclusion The significant difference in the SI direction was possibly because of the large SD. As offsets are accounted for in online plan adaption on MRL, this SI displacement does not have a detrimental impact on delivered plans. The vendor recommended method (A) involves less manipulation of the patient and since there was no significant difference in the AP and RL directions, set-up A will be adopted as the standard for prostate and other pelvic patients on the MRL in future. Further investigations will be required to establish standards of practice for other body sites. PO‐1829 Can empty bladder filling protocol be implemented for localised prostate external beam radiotherapy? G. Chetiyawardana 1 , C. Holborn 2 , P. Hoskin 1 , Y. Tsang 1 Results