ESTRO 36 Abstract Book

S419 ESTRO 36 2017 _______________________________________________________________________________________________

Conclusion We developed a fast end-to-end test for stereotactic radiation therapy with the ArcCHECK phantom which minimizes user influences for high reproducibility and was easily included into clinical routine. It compares the dose distribution on a helical diode array and a cumulative central dose with the doses from the treatment planning system. By additionally comparing each of the over 1300 diode values to a corresponding average dose derived from previous measurements, the method simultaneously serves as a constancy test of all involved components and is able to reliably detect a vast variety of even very small errors. PO-0795 Comparison of Service graph log and Dynamic linac log of Elekta Linacs for patient QA. M. Kowatsch 1 , M. Meinschad 1 , G. Leitold 1 , P. Szeverinski 1 , T. Künzler 1 1 LKH Feldkirch, Institut of Medical Physics, Feldkirch, Austria Purpose or Objective The complexity of intensity modulated radiation therapies (IMAT, IMRT) requires patient specific pretreatment verification of calculated dose distributions which is time consuming. Elekta linacs provide 2 different log files. One is the Service graph (SG) with a resolution of 4 Hz and is directly accessible through the service mode on the linac. The second one is the Dynamic linac log (DLL) with a resolution of 25 Hz. The aim of this study is to compare both types of log files for dose recalculation with Monte Carlo and beam statistics for an Elekta Synergy linac with Agility MLC (Elekta, Crawley). Material and Methods To compare the log files 2 head & neck, a mamma left side, an abdomen with simultaneous integrated boost, a thoracic spine with 3 dose levels and 1 brain case were chosen. Different parameters like leaf travel (LT), the sum of travel of all leaves between the open jaws, leaf speed (LS), leaf position (LP) and modulation complexity score (MCS) (Masi, Med. Phys. 40, 071718, 2013) were compared between the SG and the DLL. The DICOM RT file was used as reference for comparing LT and MCS. Furthermore log files were converted with an in-house Matlab script to .tel files to recalculate the irradiated plans with Monaco 5.0 TPS (Elekta, Crawley). For recalculation a grid size of 3mm and an uncertainty of 1% per control point were used resulting in a final uncertainty of roughly 0.1%. Isodose and DVH comparison were performed to evaluate equality of recalculated and originally calculated plans. Results The difference for leaf travel between SG and DLL to the Dicom-RT file was between -9.5% to 2.7% and -0.4% to 6.2%, respectively and between SG and DLL from -2.8 to - 11.3%. The differences of the MCI between the two log files was -0.4% to 0.3% and up to 20% compared to the DICOM file (see Table 1). The difference of 20% for plan 6 originates from the definition of LT. In this case, 2 beams with 2 arcs were evaluated. For SG and DLL all beams were evaluated as a single beam, the Dicom RT files were evaluated beam-by-beam. The maximum LT for a particular leaf between 2 control points (CP) showed big discrepancies and was in one case 20.1 mm for the SG and 32.6 mm for the DLL. The differences originate from writing errors between CPs in the SG and these errors are still inexplicable.Random dose errors in DVH up to +-0.5 Gy can be seen by recalculation of both log files for the entire plan. For linac parameter statistics (LT, LS, LP) the SG cannot be used because of random writing errors.

Conclusion Both file types are accurate for dose recalculation. The 4 Hz resolution and writing errors of the Servicegraph log are limiting a robust statistical analysis of linac parameters. Dynamic linac logs allow for dose recalculation and for a more detailed statistical analysis of the linac. Both types of log files can be taken for patient QA to decrease the workload of measurements and for recalculation of delivered dose to the planning CT. PO-0796 Optimisation of plan robustness to sinus filling in a magnetic field. A. Pollitt 1 , R. ChuteR 1 , P. Whitehurst 1 , R. MacKay 1 , M. Van Herk 2 , A. McWilliam 2 1 Christie Hospital NHS, Radiotherapy, Manchester, United Kingdom 2 University of Manchester, Division of Molecular and Clinical Cancer Science, Manchester, United Kingdom Purpose or Objective The MR Linac (Elekta AB, Stockholm, Sweden) will provide on-treatment MR imaging allowing for excellent soft tissue imaging. Such a machine will become an integral part of the drive towards daily online adaptive radiotherapy. However, the presence of the magnetic field results in the Lorentz force and will cause an increase or decrease in dose to superficial tissues (Raaijmakers et al. 2007). This is particularly pertinent for sinus cancers, of which 60% are squamous cell carcinoma’s and primarily on the surface layer of the nasal cavity. Recent studies (Bol et al 2015, Uilkema et al. 2015) have been performed to determine the effect of the Lorentz force on low density cavities in the body. This abstract aims to investigate the effect of the magnetic field on plan quality and optimisation for varying sinus filling and emptying states. Material and Methods Ten patients with PTV’s overlapping the sinus cavity were selected from the clinical archive. For each patient four plans were optimised at 60Gy in 30 fractions, 2 with no B- field and 2 with the 1.5T B-field present. For each, 1 plan assumed full sinuses with the volume overridden to 1gcm - 3 and the other assumed empty sinuses with the volume overridden to 0gcm -3 . All plans were created using Monaco (v5.19.07, Elekta AB Stockholm, Sweden) and met the departmental constraints for Target and OAR doses. To investigate the effect of a change in sinus filling, plans were recalculated on their opposite filling state, i.e plan optimised on a full sinus was recalculated on an empty sinus. The difference in dose between the two plans for target coverage and OARs was calculated. This comparison will determine the magnitude of the effects from sinus filling in each scenario. Investigating the range of dose differences will provide information on how to optimise these plans to minimise the effect of the Lorentz force. Results The change in dose to the Target for the different filling and magnetic field combinations can be seen in Figure 1. Several of the dose differences for plans optimized on an empty cavity, for both with and without B-field show a shift of the mean of the distribution which is greater than 2% (considered potentially clinically significant). i.e. mean Dose = 2.36%; V 50% = 2.26%; V 5% = 3.12%; V 2% = 3.21%. An OAR which also saw a difference greater than 2% was the Brainstem PRV 1cc max = 2.16 %.