ESTRO 2020 Abstract Book

S1041 ESTRO 2020

RW3 and a pinpoint camera; a 3D measurement in 3D with the Octavius 4D phantom (PTW), which allows to perform gamma analysis comparing the calculated and measured doses; and finally portal dosimetry with the tool provided by Varian, which offers a gamma analysis of both measured and calculated fluences.

than 1Gy discrepancy. DMax figures were provided as a supplementary comparison Conclusion There are concerning factors when evaluating DVH data using larger voxels. This study demonstrated over 20% of the studied population have recorded higher dose received by an OAR by up 2Gy using a finer voxel resolution. This translates to a 1 in 5 chances of potential misinterpretation. A possible explanation to this observation could be related to the proximity of the OAR/s to the target volume where smaller voxels will be required to confidently evaluate dose within or near the high gradient zone. Clinicians should exercise higher degree of caution when evaluating DVH data in situation where the target is near any OAR. PO-1867 Dosimetric study of LINAC-based SRS/SRT of multiple brain lesions:multiple/single isocenter approach PO-1868 In silico hypofractionation feasibility study for lymph node oligometastases SBRT on the MRI-linac D. Winkel 1 , W.S.C. Eppinga 1 , M.P.W. Intven 1 , A.M. Werensteijn-Honingh 1 , J. Hes 1 , L.M.W. Snoeren 1 , G.H. Bol 1 , B.W. Raaymakers 1 , I.M. Jürgenliemk-Schulz 1 , P.S. Kroon 1 1 UMC Utrecht, Department of Radiotherapy, Utrecht, The Netherlands Purpose or Objective At our department, MR-guided SBRT using the 1.5T MR- linac system (MRL) (Unity, Elekta AB, Stockholm, Sweden) has been initiated for patients with lymph node oligometastases (LNO). Superior soft tissue contrast and the possibility for online plan adaptation on the MRL allows for hypofractionated treatment. The purpose of this study was to investigate the feasibility and compare plan quality of different hypofractionation schemes. Material and Methods Data was used from 12 patients with single LNO (10 pelvic, 2 para-aortic), which were all treated on the MRL with a prescribed dose of 5x7Gy to 95% of the PTV. Possibility for further hypofractionation was investigated based on the pre-treatment plan data. If all planning constraints (Table 1) were met for the 5x7Gy plan a 3x10Gy plan was created and if this plan met all constraints a 1x20Gy plan was generated (all BED 60Gy with α/β=10). All plans were made using IMRT with 7 or 9 beams, respectively. In case all three differently fractionated pre-treatment plans met all clinical dose criteria, the plans were adapted to the daily patient anatomy [1]. These were then used for quality and dose criteria based evaluation. The quality of the different plans was evaluated using four parameters: heterogeneity index HI (=PD* the actual prescription dose defined as D 95% of the PTV as a % of D max ), conformity index R 100% (=V PD* /V PTV ), R 50% (=V 0.5xPD* /V PTV ) and D 2cm (=D max at 2cm from PTV as % of PD*). These values were compared with benchmarks, of which R 50% and D 2cm depend on the size of the PTV [2]. Abstract withdrawn

Results Machine QA: The measurements made are well below the tolerances recommended by the TG-142 for SBRT treatments. Patient QA: The table shows the results of the QC of 100 treatment plans for SBRT with VMAT.

Conclusion Based on the results obtained, the tolerances of the QA for the treatment plans are established by applying a level of confidence of 95% and 99% to our data. Only in one of the 100 analyzed patients the results of the QA were not within the previous level of confidence and therefore a new treatment plan was made. In view of the results we are considering simplify ourQA procedure for future treatments of SBRT. PO-1866 Comparison of dose evaluation using difference voxel sizes in Stereotactic Radiotherapy Planning T.L. LAI 1 , E. Tsang 2 1 GensisCare Australia, WROC, Melbourne, Australia ; 2 GensisCare Australia, Epping ROC, Melbourne, Australia Purpose or Objective Genesis Care (GC) Australia has developed a national stereotactic team responsible for planning stereotactic brain radiotherapy using Monaco TPS for all Elekta based centres across Australia. All reporting for through Monaco is done using a 0.1cc volumetric matrix. GC Victoria provides stereotactic radiotherapy (SRT) across all sites using Pinnacle TPS and Varian Truebeam linear accelerators. Pinnacle TPS allow dose volume to be reported to as small as 0.01cc. Many international clinical trial report doses to 0.03cc whilst the AAPM Task Group 101 (2010) defined “point” as 0.035cc or less. This study aims to evaluate whether there are major differences in OARs dose reporting between voxel resolution sizes of 0.1cc Vs 0.03cc Material and Methods 22 SRT brain patients planned using Pinnacle TPS were reviewed retrospectively. A script was created to report OAR using dose to 0.1cc (D0.1cc), dose to 0.03cc (D0.03cc) and maximum point dose (DMax). DMax in this case is defined as the maximum dose reported on the dose volume histogram in Pinnacle. The OARs reported include the left and right optic nerves, optic chiasm, left and right lens and brainstem. Data are entered onto spreadsheet for comparison. Results Out of 22 samples, 5 (22.7%) showed 1 to 2Gy discrepancy for at least one OAR and 3 (13.6%) showed larger than 2Gy difference between values obtained using D0.1cc and D0.03cc voxels. The reminder of the cohort showed less