ESTRO 37 Abstract book

S943

ESTRO 37

EP-1757 Validation of Independent IMRT and VMAT Dose Calculation System based on Geant4 Monte Carlo Toolkit H.J. Choi 1 , H. Park 1 , W.G. Shin 1 , J.I. Kim 2 , C.H. Min 1 1 Yonsei Univ., Radiation convergence engineering, Wonju, Korea Republic of 2 Seoul National University Hospital, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective Although the commercial treatment planning system (TPS) has been improved for accurately estimating the patient dose, there are still limitations in multi-leaf collimator (MLC) geometry and dose calculation in heterogeneous region such as lung cases with moving target, dummy shield cases, and surgical implant cases. The most reliable method to overcome these limitations is independent full Monte Carlo (MC) simulation. We developed an IMRT and VMAT dose calculation system with automated DICOM-RT interface based on Geant4 toolkit which possible to precisely design the MLC and calculate dose with 4D simulation and multithreading features. The aim of this study is to accurately evaluate diverse clinical cases using the independent MC-based patient dose calculation system for better treatment quality. Material and Methods The MC commissioning of 6 MV Varian Clinac 2300 IX was performed based on the measurement data. For the MLC validation, the Geant4 dose distributions were compared with the measured one using the film with the in-house designed bar pattern opening. DICOM-RT interface was developed for the beam condition setting and the phantom modeling automatically. Dose scaling factor was calculated by comparing maximum depth dose in the water phantom with 10 x 10 cm 2 field beam. Finally, ten IMRT and VAMT plans of heterogeneous and homogeneous cases were compared between our system and TPS using the Analytical Anisotropic Algorithm (AAA). Results In MC commissioning, the maximum local dose difference in each PDD and lateral profile was less than 1.6%. In MLC validation study, dose distributions were well-matched between Geant4 and film measurement within 1% for leakage dose and lateral profile differences. However, there was about 20% overestimation in TPS compared to Geant4 due to difference in MLC leaf tip modeling. Nevertheless, overall IMRT and VMAT dose distributions in the water phantom showed fairly good agreement between in-house system and TPS within 1.9% mean dose difference. In the five abdomen case studies, PTV doses were matched well within 1.37% difference of prescribed dose, however, there were about 3% differences in small normal tissue located in areas with high ratio of leaf pairs with short distances. In the five lung case studies, Geant4 dose distributions were more heterogeneous with respect to TPS. The mean PTV doses were varied up to 3.6% of prescribed dose, while the dose of large normal tissues was quite similar within 1% difference of prescribed dose. Conclusion The current validation study of in-house system shows possibility to accurately verify the patient dose of IMRT and VMAT based on full MC simulations. We found that the TPS dose still showed different tendency in heterogeneous media from the MC dose. In the future, we will evaluate more patient cases for highly heterogeneous media with considering the moving target with 4D CT images. EP-1758 A correlation study between gamma index passing rate and clinical dose volume histogram L. Szczurek 1 1 International Oncology Centre Affidea Poznan, Medical Physics, Poznan, Poland

Purpose or Objective To evaluate methods of the pre-treatment Volumetric Arc Therapy (VMAT), analysis of the plans, based on the percentage gamma passing rate (%GP) of two-dimensional (2D) and three-dimensional (3D) dosimetric verifications and their correlation and sensitivity with percentage dosimetric errors (%DE) between the planned dose volume histogram (DVH) and the patient’s predicted DVH calculated by Compass and OmniPro system (IBA Dosimetry, Schwarzenbruck, Germany ). Material and Methods Two groups of patients, with prostate after prostatectomy (11 plans) and rectal cancer (16 plans), treated with VMAT technique were analyzed. Pre- treatment verifications were performed for all plans by acquiring the planar dose distribution with matrix detector. %GP of 2D and 3D with different acceptance criteria: 1%1mm, 2%2mm, 3%3mm, was obtained by OmniPro and Compass software. Additionally, %DE were calculated from planned dose volume histogram created in the treatment planning system (TPS) Monaco (Elekta) and the patient’s predicted DVH was calculated with Compass system. Analysis was performed for target volume PTV and some typical organs at risk (OAR) in pelvic region. D 1% , D 98% , D mean for target and dose in OAR, recommended by QUANTEC group and ICRU, were analyzed. Statistical correlation between %GP and %DE was verified with Pearson’s correlation coefficient. Sensitivity was calculated, based on the receiver operating characteristics (ROCs), to account for the incidence of false negatives, obtained base on the gamma index method. Results The t-test results between the planned and estimated DVH values for prostatectomy and rectal cancer group for PTV, bladder, rectum, femoral head, showed that mean values obtained from histograms were comparable (p>0.05). The %DE between -2.66 and 1.26 for prostatectomy, and from -1.18 to 0.80 for rectal cancer group were observed. For criterion 3%3mm the average %GP were acceptable in both groups, with average rates of 99.89% for 2D and 98.88% for 3D, respectively. The number of correlations was higher for 3D vs 2D verifications, but still poor for all analyzed data. Mean Pearson’s R-values for prostatectomy and rectal cancer group were < 0.29 and < 0.20, respectively. The average area under the curve (AUC) of ROCs was 0.22 for prostatectomy and 0.55 for rectal cancer group. Conclusion Analysis of the %GP vs %DE values revealed only weak correlations in 2D and 3D verifications. DVH results obtained in Compass system will provide more helpful analysis and confirm that analyzed plans respected dosimetric constrains. EP-1759 Logfile based detection and dosimetric effects of deviations between TPS and linac M. Meinschad 1 , M. Pasler 2 , G. Leitold 1 , P. Szeverinski 1 , M. Kowatsch 1 , T. Künzler 1 1 Landeskrankenhaus Feldkirch, Medical Physics, Feldkirch, Austria 2 Gemeinschaftspraxis für Strahlentherapie Singen- Friedrichshafen, Medical Physics, Friedrichshafen, Germany Purpose or Objective With conventional phantom based QA, deviations between planned and delivered dose distribution are detectable. The underlying reason for these deviations is not identifiable. With the use of log files one can discern between different error sources, and can also quantify their effect on the dose distribution. Errors occur due to mispositioning of the beam shaping parts of the linac, as well as to simplifications and interpolation between control points (CPs) in the TPS. There are several studies