ESTRO 36 Abstract Book

S791 ESTRO 36 _______________________________________________________________________________________________

Material and Methods Fourteen H&N and ten prostate VMAT plans were created and their respective QA plans were developed using Monaco 5.1 treatment planning system and delivered by an Elekta Synergy Linac equipped with an Agility 160 MLC system. The cylindrical phantom Octavius 4D was used to measure the dose distribution. The 2D-array Octavius 729 consists of 729 vented ionization chambers arranged in a 27x27 matrix with a spatial resolution of 10mm.The chamber volume is 0.125cm 3 . The Octavius 1500 array has the same layout and dimensions but with 1405 ionization chambers with a chamber volume of 0.06cm 3 . In order to reconstruct and analyze the measured 3D dose from each plan, the PTW VeriSoft 6.2 patient plan verification software was used and a volumetric 3D gamma index analysis (max dose of calculated volume, suppress dose below 10% of max dose of calculated volume) for both 3%3mm and 2%2mm criteria was performed to compare and evaluate the measured and calculated doses for both Table I summarizes the results for both cases. The mean pass rate of global 3D gamma index for all prostate cases was superior to 99% with 3%3mm and 95% with 2%2mm criteria. The Octavius1500 achieves higher results for both criteria. The mean difference was 2.9% for the gamma 2%2mm and 0.6% for the 3%3mm. For the H&N cases, the mean passing rate was lower than prostate cases. Similarly, the Octavius1500 obtain better results for both criteria. The mean difference was 4.2% for the gamma 2%2mm and 0.8% for the 3%3mm. arrays. Results

EP-1480 Patient-specific QA for CyberKnife MLC plans using Monte Carlo P.H. Mackeprang 1 , D. Vuong 1 , W. Volken 1 , D. Henzen 1 , D. Schmidhalter 1 , M. Malthaner 1 , S. Mueller 1 , D. Frei 1 , D.M. Aebersold 2 , M.K. Fix 1 , P. Manser 1 1 Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital- Bern University Hospital- and University of Bern, Bern, Switzerland 2 Department of Radiation Oncology, Inselspital- Bern University Hospital- and University of Bern, Bern, Switzerland Purpose or Objective To implement and validate a Monte Carlo (MC) based dose calculation framework to perform patient-specific quality assurance (QA) for multi-leaf collimator (MLC) based CyberKnife treatment plans. Material and Methods In order to calculate dose distributions independently from the treatment planning system (TPS), an independent dose calculation (IDC) framework was developed based on the EGSnrc MC transport code. The framework uses XML-format treatment plan input and DICOM format patient CT data, an MC beam model using phase space files, CyberKnife MLC beam modifier transport using the EGS++ class library, a beam sampling and coordinate transformation engine and dose scoring using DOSXYZnrc. Validation of the framework was performed against dose profiles of single beams with varying field sizes measured with a diode detector in a water tank in units of cGy / monitor unit (MU) and against a two-dimensional dose distribution of a full treatment plan measured with Gafchromic EBT3 (Ashland Advanced Materials, Bridgewater NJ) film in a homogeneous solid water slab phantom. The film measurement was compared to IDC by gamma analysis using 1% (global) / 1 mm criteria and a 10% global low dose threshold. Finally, the dose distribution of a clinical prostate treatment plan was calculated and compared to dose calculated by the TPS finite size pencil beam algorithm by gamma analysis using either 2% (global) / 2 mm or 1% (global) / 1 mm criteria and a 10% global low dose threshold. Results Dose profiles calculated with the developed framework in a homogeneous water phantom agree within 3% or 1 mm to measurements for all field sizes. 87.1% of all voxels pass gamma analysis comparing film measurement to calculated dose. Gamma analysis comparing dose calculated by the framework to TPS calculated dose for the clinical prostate plan showed 99.9% passing rate for 2% / 2 mm criteria and 85.4% passing rate for 1% / 1 mm, respectively. Dose differences of up to ±10% were observed in this case near bony structures or metal fiducial markers. Conclusion An MC based modular IDC framework was successfully implemented and validated against measurements and is now available to perform patient-specific QA by independent dose calculation. EP-1481 Testing algorithms in water and heterogeneous medium using experimental designs S. Dufreneix 1 , A. Barateau 1 , M. Bremaud 1 , C. Di Bartolo 1 , C. Legrand 1 , J. Mesgouez 1 , D. Autret 1 1 Institut de Cancérologie de l'Ouest, Medical Physics, Angers, France Purpose or Objective The IAEA Tecdoc 1580 and 1583 suggest several beam configurations for testing, commissioning and ongoing quality assurance of TPS. However, the large number of

Expectedly, the greatest differences between Octavius 1500 and 729 are shown in the gamma 2%2mm criteria. Figure 1 shows a comparison between the gamma 2%2mm for both detectors for a H&N case.

Conclusion As we expected, the Octavius 1500 achieves a better result for pre-treatment VMAT plan verification and the results are more remarkable for the gamma 2%2mm criteria. In addition, plans with a higher complexity such H&N can benefit from the superior results of the Octavius 1500. Moreover, the Octavius 1500 present the possibility to increase the spatial sampling frequency and the coverage of a dose distribution by merging two measurements. We can conclude that Octavius 1500 outperformed Octavius 729 for VMAT pre-treatment QA.