Abstract Book

S942

ESTRO 37

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

calculated with Monte-Carlo algorithm (MC) with uncertainty equal to 1%, while the MLC one was calculated using FSPB. The dose was prescribed at 80% isodose level relative to maximum dose. The coverage of prescription dose to target and the new conformity index (nCi) were within 99-100% and below 1.15 respectively. In doing absolute dose verification, an orthogonal pieces of radiochromic EBT3 films (Ashland) were placed in the Ball cube, mini Ball cube of the head phantom and lung ball cube of XLT Phantom for each treatment plan irradiation. After irradiation the films were analyzed and compared with exported corresponding dose planes from the treatment planning system. Results For the MLC-based treatment plans in the skull and spine, the difference between the FSPB calculation and the EBT3 films measurement was within 3% in the whole target. For the lung treatment using MLC, the difference was within 5% at the center region of the target, but the calculated dose overestimated more than 5% in the peripheral region of the target (The peripheral region was defined as at most 5 mm from the target surface). Such difference was increased to 10.3 ± 1.2% at the target surface. For the Iris-based treatment plans to the skull, spine and lung, the differences between the MC calculation and the EBT3 film measurement were all within 3% in the whole target. Conclusion The accuracy of FSPB calculation algorithm in MLC based treatment plan is acceptable for treating target in the skull and spine. However the algorithm is not recommended for lung treatment, especially for a lung target in the lung center since the calculation can overestimate the dose by up to 10.3 ± 1.2% in the peripheral region of target. 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).