ESTRO 36 Abstract Book

S782 ESTRO 36 2017 _______________________________________________________________________________________________

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 tests makes it difficult to implement and results out of tolerance are often left unexplained. Experimental designs are a robust statistical method which minimizes the number of tests to be performed and provides a

statistical analysis of the results. They were used to compare computed and measured doses for several algorithms. Material and Methods Tests were chosen using a Taguchi table L36 (2 11 x3 12 ) to enable the quantification of the influence of each parameter. Five algorithms were studied: the AAA (version 11, Varian) is used in clinical routine and the collapsed- cone convolution-superposition (CCCS) algorithm (version 1.5, Mobius Medical Systems) is used as a secondary dose calculation plan check. The AcurosXB (AXB) algorithm (version 11, Varian) was also investigated as well the pencil beam (PB) and Monte Carlo (MC) algorithms available on Iplan (version 4.5, Brainlab). Absorbed dose was first calculated in water for 72 beams with varying parameters: energy, MLC, depth, wedge angle, wedge jaw, X, Y 1 and Y 2 dimensions. Computations were then conducted for 72 beams in a CIRS Thorax phantom with varying parameters: energy, wedge angle, wedge jaw, X and Y dimensions, medium and gantry angle. Calculated doses were compared to measurements conducted on a Novalis TrueBeam STx (Varian) with a CC04 ionisation chamber (IBA). Results In water, all algorithms gave a mean difference between computed and measured doses centred on zero (within the uncertainty). No studied parameter led to statistically significant deviation. In the thorax phantom, the mean difference between computed and measured doses was - 0.7 ± 1.1 % for AAA, -1.4 ± 1.4 % for CCCS, -2.5 ± 1.0 % for AXB, 2.3 ± 2.2 % for PB and 0.3 ± 1.9 for MC. For AAA and CCCS, calculations in bone medium led to a statistically significant underestimation of the computed dose while the other parameters had no influence on the results. For MC, calculated dose was overestimated for gantry angle of 225° which was attributed to the modelization of the treatment table by the TPS. Conclusion Experimental designs were used as a statistical method to validate the AAA, CCCS and MC algorithms. The PB algorithm was rejected for clinical use because it overestimates the dose in heterogeneous medium. Results showed that the AXB algorithm systematically underestimates the dose in heterogeneous medium which could be linked to the dose to water - dose to medium conversion as referred in the literature. Further investigation is needed before its implementation in clinical routine, especially for modulated beams. The tests described by the experimental designs were also used to define the tolerance levels of the secondary plan check software and are now part of the ongoing quality assurance of the TPS . EP-1482 Signal Prediction for an On-line Delivery Verification System R. Heaton 1 , M. Farrokhkish 1 , G. Wilson 1 , B. Norrlinger 1 , D.A. Jaffray 1 , M.K. Islam 1 1 Princess Margaret Cancer Centre University Health Network, Radiation Physics, Toronto, Canada Purpose or Objective Dynamic radiation delivery techniques like VMAT introduce challenges in treatment verification. Complex treatments, as well as hypofraction and adaptive radiation therapy, require new verification approaches to ensure safe delivered. One approach is the introduction of entrance fluence monitoring device, like the Integral Quality Monitoring (IQM) System (iRT Germany), which provides a spatially encoded dose area product signal as a unique delivery fingerprint. Complementary to this measurement is the signal calculation based on the treatment plan. This work describes the calculation for the IQM system and examines the impact of selected components on clinical fields. Material and Methods