ESTRO 38 Abstract book

S1204 ESTRO 38

Electronic Poster: RTT track: Treatment planning and dose calculation / QC and QA

1 Clatterbridge Cancer Centre, Radiotherapy, Bebington- Wirral, United Kingdom; 2 The University of Liverpool, School of Health Sciences, Liverpool, United Kingdom Purpose or Objective The urinary bladder is subject to variation in shape and size which, to some extent, be controlled using an empty bladder protocol for planning and treatment. Bladder can still be influenced by adjacent organs, notably the rectum. Micro-enemas are used with a view to controlling the rectal volume. An earlier study, showed improve consistency in rectal diameter and content between planning and treatment with the use of a micro-enema. The aim of this study is to identify any significance in the relationship between rectal consistency and bladder deformation with a view to reconsidering appropriate treatment margins. Utilising on treatment imaging technology for soft tissue visualisation enabled the variation in bladder and rectal volumes to be quantified. Material and Methods A control group consisted of patients asked to empty their bladder immediately before planning and treatment. No guidance was given on rectal state. An intervention group consisted of patients asked to use a micro-enema 20 mins prior to planning and each treatment and empty their bladder immediately before both. Treatment CBCT images were analysed on-line using an automatic match algorithm to provide a rigid bone registration and a 3mm Action level and a CTV coverage check. Retrospective analysis was carried out on 97 CBCT scans from 15 patients, control group (n = 41) and intervention group (n= 56). Volume and positional organ variations were determined from the rigid bone registration of CT and CBCT. Bladder and rectum volumes were manually contoured on planning CT and each CBCT using Varian Eclipse™ contouring module. Centre of mass shifts for bladder and rectum were determined in the X,Y and Z axis and the magnitude of Centre of Mass shift calculated for the planning scan and all fractions. The Dice similarity coefficient was taken from the Ecliplse planning system.

EP-2181 Use of treatment log-files for QA of cranial radiosurgery adaptive plans S. Moragues Femenia 1 , J.F. Calvo-Ortega 1 , M. Hermida- López 2 , J. Casals 1 1 Hospital Quirónsalud Barcelona, Radiotherapy, Barcelona, Spain; 2 Hospital Universitario Vall d'Hebron, Servicio de Física y Protección Radiológica, Barcelona, Spain Purpose or Objective To analyze the accuracy of delivered adaptive stereotactic radiosurgery (SRS) plans. Material and Methods Adaptive cranial SRS is performed in our department [Med Dosim. 2013 Autumn;38(3):291-7] as a strategy to compensate for the patient's roll and pitch rotational errors not supported by the 4D couch of the Varian 2100 CD linac used for treatment delivery. Our current adaptive SRS policy requires adapting the original treatment plan (Plan_ref) when pitch and/or roll errors detected by CBCT imaging are ≥ 0.5 degrees. The beam fluences of the Plan_ref are adapted to the actual patient's position such that an adaptive plan (Plan_adapt) is created while patient is lying on the linac couch. Obviously, it is not possible to perform a fully patient-specific QA. In order to check the Plan_adapt delivery, an off-line re-calculation of the Plan_adapt is done using the Varian Dynalog files recorded by the linac during the delivery [J Appl Clin Med Phys. 2014 Mar 6;15(2):4665], such that a reconstructed plan is generated (Plan_adapt_Dyn). Dose calculations were done with the Analytical Anisotropic Algorithm (AAA) of the Eclipse v.13.6 treatment planning system using a 1 mm calculation grid size. The accuracy of this algorithm for SRS was previously investigated by our team [Med Dosim. 2014 Summer;39(2):129-33]. Plan_adapt plans were computed with AAA modelled with the beam data (6 MV) of our linac, while the Plan_adapt_Dyn plans were calculated with the AAA modelled in an independent way with the Golden Beam Data (6 MV) supplied by Varian for our linac model.In order to assess the accuracy of the delivered dose, Plan_adapt and Plan_adapt_Dyn plans were compared for 20 multitarget SRS cases using the 3D gamma tool of the Computational Environment for Radiotherapy Research (CERR) software. It was compiled from the source code available at https://github.com/cerr/CERR on July 8, 2018. 3D gamma calculations were focused on the planning target volumes (PTVs) and the OARs (brainstem, optic nerves and optic chiasma). Gamma passing rates were computed for several dose difference criteria (1%, 2% and 3% of the reference maximum dose), 1 mm distance-to-agreement-distance, and threshold of 10% of the reference maximum dose. Results Sixty PTVs were analyzed: average gamma passing rates were 87.4% (95% CI [81.9, 92.9]), 97.7% (95% CI [95.1, 100.0]) and 99.6% (95% CI [98.9, 100.0]) for 1%/1 mm, 2%/1 mm and 3%/1 mm criteria, resp. Minimum passing rate of 100% was reached for all 20 OARs of each type regardless of the gamma criteria used. Conclusion The proposed log-file-based method is a useful tool to assess the accuracy of the dose delivery when adaptive SRS is performed. A high agreement within 3% and 1 mm between the designed adaptive plans (Plan_adapt) and the corresponding delivered ones (Plan_adapt_Dyn) was found for all cases. EP-2182 Skin dose calculation in breast cancer when the air surrounding the patient contour is considered

Results

Conclusion The introduction of a micro-enema shows significant worsening of bladder and rectum stability relevant to the control group. The significant result for correlation between rectum changes in the Y and Z directions and bladder centre of mass shift in the same direction suggest that the use of micro-enema may, in some cases, destabilise the rectum. It is acknowledged that the small study size may influence the results. Bladder radiotherapy may benefit from daily imaging with appropriate justification and optimization of imaging dose. The majority of target volumes in the pelvis are moving to daily online imaging acknowledging the physiological movements of organs.