EP-1492: Influence of induced accelerator’ errors on dosimetric verification result and DVH of treatment plan

S799 ESTRO 36 _______________________________________________________________________________________________

Maximum active field size 43cm x 43cm Maximum readout speed

20 frame/second

Matrix

1280 x1280

Resolution 0.0336 cm Results Epiqa had dramatically low GA (gamma<1.0) results; less than 95% with the criterias DD:%1, DTA:1mm for field (Average: 85,03, SD:±7,8) and field+1cm (Average: 93,30, SD: ±2,4) comparison areas. There is no significant difference (p>0,05) between PD and Epiqa for GA results of DD:%3, DTA:3mm criterias. Table 2. Average and standard deviation (SD) results of each method Evaluated Method, Average Standard p field DD, DTA value deviation value Field+1cm

Conclusion This 3-class density method can be used to monitor the fraction dose in the PGs during oropharynx cancer IMRT. Small significant differences are observed for the highest dose received in the spinal cord, likely due to the bone heterogeneity. EP-1491 Verification of FFF VMAT plans with PDIP and GLAaS algorithms by using the new imager of TrueBeamSTx T. Ercan1, A. Levent2, T. Cagin3, S.M. Igdem1 1 Gayrettepe Florence Nightingale Hospital, Radiation Oncology, Gayrettepe - Istanbul, Turkey 2 Medideal Medical Projects and Solutions Inc., Medical Physics, Istanbul, Turkey 3 Liverpool School of Medicine, Department of Biostatistics, Liverpool, United Kingdom Purpose or Objective Using flattenning filter free (FFF) beams shortens the treatment time especially for stereotactic treatment techniques when the high dose rate is used. It is not possible to do quality assurance (QA) with all types of amorphous silicon (aS) detectors because of their saturation limit. In this study, verifications of volumetric modulated arc therapy (VMAT) stereotactic treatment plans were evaluated with PDIP and GLAaS algorithms by using a new unsaturated aS detector and the results were compared. Material and Methods aS1200 image detection unit (IDU) which has aS detector integrated on a Varian TrueBeam STx linac was used (Table 1). Portal Dosimetry (PD) (v.13.0.26) with PDIP algorithm (Varian Medical Systems, Palo Alto) and Epiqa (v.4.0.11) with GLAaS algorithm (Epidos s.r.o., Bratislava) QA tools were configured at SDD: 100 cm with 2400 MU/min dose rate through the detector without saturation issue to use for pre-treatment verification.10 MV FFF VMAT treatment plans (2400 Dose Rate) of 35 patients with in total 72 arcs were calculated by Anisotropic Analytical Algorithm (AAA, ver.13.0.26) in Eclipse Treatment Planning System. The verification plans were irradiated on the aS1200 imager. The evaluations for both QA tools were done with the technique of gamma analysis (GA). The GA criterias for Distance to Agreement (DTA) and Dose Difference (DD) were defined as 3%/3mm, 2%/2mm and 1%/1mm and applied for "field" (defined with jaws) and "field+1cm" areas. The results were analysed with 2 sample T-test. Table 1. Specifications of aS1200 IDU

Field

PDIP, 3mm

%3,

99.95

0.14

GLAaS, %3, 99.98 3mm

0.05

PDIP, 2mm

99.87

0.18

GLAaS, %2, 99.72 2mm

0.30

PDIP, 1mm

99.14

0.75

GLAaS, %1, 93.30 1mm

2.44

PDIP, 3mm

99.92

0.16

GLAaS, %3, 99.90 3mm

0.31

PDIP, 2mm

99.59

0.50

GLAaS, %2, 98.95 2mm

1.68

PDIP, 1mm

97.03

2.08

GLAaS, %1, 85.03 1mm

7.78

%2,

%1,

%3,

%2,

%1,

>0.06

<0.05

<0.05

>0.707

<0.05

<0.05

Conclusion We could able to detect more errors with the hardest criterias (DD:%1, DTA:1mm) as expected. Epiqa had better performance for detecting the errors. It could be the result of the differences in workflow. Epiqa compares the dose calculated with clinical algorithm and irradiated image, but PD compares the calculated dose with PDIP and irradiated image. PD and Epiqa can be used for stereotactic VMAT plans with aS1200 detector without saturation problem at SSD: 100cm reliably. If the speed is important for the clinics have high workload, PD could be prefered through being an internal software. EP-1492 Influence of induced accelerator’ errors on dosimetric verification result and DVH of treatment plan M. Kruszyna1, K. Matuszewski1 1 Greater Poland Cancer Centre, Medical Physics Department, Poznan, Poland Purpose or Objective The commonly used gamma criteria of 3% dose difference (global method) and 3 mm distance to agreement could mask clinically relevant errors. The aim of this work was to evaluate the influence of induced accelerator’s errors on 3D gamma method results with the varies criteria and on the patient’ dose distribution (DVHs).

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Material and Methods In the treatment prostate plan with VMAT highfractionated (2x7.5Gy), FFF technique the errors of dose (differences ±1%; 2%; 3%; 5% 7%, 10%), collimator angle (rotations in both directions: 0.5; 1.0; 1.5; 2.0; 2.5; 3.0) and MLC shifts were introduced. For each modified plan, the pre-treatment verification plan was created and measured with 2D-arrays: 729 and SRS 1000 with rotational phantom Octavius® 4D and Verisoft 6.1 software with DVH option (PTW, Freiburg, Germany). Measured (with errors) and calculated (reference plan) dose distributions were analyzed with 3D gamma evaluation method for various tolerance parameters DTA [mm] and DD [%] 1.0; 1.5; 2.0; 2.5; 3.0, by global and local dose methods with a 5% threshold. To detect errors, the achieved score should be less than the assumed tolerance of 95%. Additional the DVHs from error-induced and reference plan were analyzed for CTV D50, D98, D2, and D25, D50 for OARs. Results For 12 error-induced plan with dose discrepancies, proper detection for 729 and SRS 1000 were obtained as follows: 3/12 and 6/12 (G3%/3mm); 8/12 and 6/12 (L3%/3mm); 8/12 and 7/12 (G2%/2mm); 8/12 and 8/12 (L2%/2mm). The rotations of collimator were detected >3° for 729 and >2° for SRS 1000. The MLC errors were discovered for plans with 1 leaf (MLC1) and 1 pair of leaves (MLC2) blocked, for all leaves shifted about 0.05cm (MLC3) misalignment weren’t indicated so obvious. The clinical relevance of plan with MLC errors and chosen discrepancies for collimator rotation (3°) and dose differences (+5%) were presented in the table 1.

Conclusion To more sophisticated analysis the gamma criteria should be less than 3%/3mm or/and local dose method should be used. The resolution of used detector is crucial and should be high for better interpretation of results. Gamma method presents some statistic data, for scrutiny analysis the clinical interpretation should be assessed. EP-1493 Machine record parameters or Epid based data for ART QA. A comparison of two scenarios. P. Haering1, C. Lang1, M. Splinter1 1 DKFZ, E040, heidelberg, Germany Purpose or Objective Using machine record files and Epid based dosimetry is popular for machine and patient related QA, as this may also work for adaptive treatment approaches. The Siemens Artiste treatment machine used here, allows a comparison of both methods in one session. Exit images and all relevant machine parameters are included in the image header collected during treatment. Here we present results of a comparison between QA dose recalculations based on the two sources, exit images and machine recorded parameters. Material and Methods A software tool was developed that allows for the extraction of the relevant parameters (MLC-positions, MU,

etc.) from the machine records as well as from the Epid measured exit fluencies. While machine data had to undergo a reformat to be used for recalculation, the exit fluencies need more attention. Here both, the delivered fluence as well as the absorption in the patient do play a role. Therefore both have to be separated to receive reliable MLC positions. The algorithm used first generates an image containing only absorption information for the beam using this to remove this influence on the MLC positions. MUs were used from the parameter file, as the fluence uncertainties on the EPID images have shown to be to large to be used for that purpose. The extracted parameters are then inserted in a newly generated Dicom RT-Plan file that then can be used in the treatment planning system (here Raystation, Raysearch) to recalculate the dose. Dose distributions (Epid based, parameter file based and originally planned) are then compared. Results Measuring exit doses with the EPID was a simple task and could be done for all coplanar field sets. The software tool made it simple to extract all the needed parameter from the files and images resulting in 2 new Dicom plan files. Dose recalculation was done by just importing the new plan files to Raystation. Comparing the original dose distribution to the machine file based one showed almost no difference at all (< 0.7%), as MU and leaf position differences where quite small. This might also be grounded in the used calculation grid of 2mm size. MLC positions derived from EPID images show much larger differences. Here detection uncertainties, EPID positioning and the resulting image resolution of 0.3mm do play a major role. This resulted in in noticeable differences in the dose gradients regions. Absolute dose differences where below 1.5%. Conclusion Recalculating doses based on EPID and machine based parameters is a possible way for QA in an adaptive treatment approach. As QA parameters are taken from information that is given anyway or that can be easily generated, it does not complicate the procedure of frequent replanning. Results are as expected quite good for the machine file approach while higher discrepancies were found using EPID data. Main problem we face here is that especially for the machine file based version we do not have full independent data sources. EP-1494 The MedAustron proton gantry: nozzle design recommendations based on Monte Carlo simulations H. Fuchs1,2, L. Grevillot2, A. Elia2, A. Carlino2,3, J. Osorio2, V. Letellier2, R. Dreindl2, M. Stock2, S. Vatnitsky2 1 Medizinische Universität Wien Medical University of Vienna, Department of Radiation Oncology & Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria 2 MedAustron lon Therapy Center, Department of Medical Physics, Wr. Neustadt, Austria 3 University of Palermo, Department of Physics and Chemistry, Palermo, Italy Purpose or Objective MedAustron is equipped with one vertical and three horizontal fixed beam lines and one proton gantry based on the PSI gantry 2 design for patient treatments. This work focuses on simulations and design considerations for the proton gantry nozzle, allowing an optimization of beam delivery properties at isocenter. Material and Methods Different gantry nozzle designs were evaluated using Gate/Geant4 Monte Carlo (MC) simulations: air filled nozzle, helium filled nozzle, full vacuum nozzle, moving snout, compacting of nozzle elements (vacuum window and monitors). Design considerations were based on the