Abstract Book

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to manage this question. The American Association of Physicists in Medicine convened a group to review the medium for reference dose calibration. This abstract highlights the thought-process and supporting data on which is the ideal medium for reference dose calibration. How this should be implemented in clinical practice is the subject of a companion abstract. Material and Methods The AAPM group evaluated three issues: First, whether dose to soft tissue in a patient should be conceptually described as dose-to-muscle or dose-to-water. Second, what the most logical workflow was for converting between calibration in water and dose calculation in patient tissue. Third, the magnitude of this difference between dose-to-muscle and dose-to-water (with the same electronic density as muscle) for different photon and electron energies (specifically: different nominal beam energies, different field sizes, and different depths). This was examined with Monte Carlo to understand the consistency of this difference. Results The AAPM group concluded that dose should be reported to muscle because it is the more accurate description of soft tissue, it is what substantial clinical trial data and patient outcomes is based upon, and it is the necessary direction in which to move to improve dose calculation accuracy in the future. To implement a dose-to-muscle framework, the ideal solution is calibration of the linac in water, and having the treatment planning system inherently account for tissue differences to calculate the dose accurately in the patient. When this solution is not possible (i.e., when the TPS does not calculate the dose to muscle in the patient), the dose is overestimated by 1%. This 1% overestimation was found to be broadly accurate (ranging from 0.6% - 1.4%) for different nominal photon and electron beam energies, different depths, and different field sizes. Conclusion Dose to muscle should be reported by the treatment planning system under reference conditions in tissue. If it does not, a multiplicative 0.99 correction factor can be applied to the reference specification in order to approximately achieve the same result as it is generally accurate across all treatment energies, photons and electrons, and depths and field sizes. Understanding which algorithms inherently calculate dose to medium versus dose to water is a complex question that must be understood before final recommendations and implementation strategies can be defined. These issues are explored in the companion abstract. OC-0405 Monte Carlo based Quality Assurance of Base Data for Beam Modeling in Treatment Planing Systems M. Kowatsch 1 , M. Söhn 2 , M. Alber 2,3 1 LKH Feldkirch, Institut of Medical Physics, Feldkirch, Austria 2 Scientific-RT, Scientific-RT, Munich, Germany 3 Universitätsklinikum Heidelberg, Klinik für RadioOnkologie und Strahlentherapie, Heidelberg, Germany Purpose or Objective Errors in beam base data (BBD) can lead to flawed beam models for the treatment planning system and thus to systematic dose computation errors. Despite a number of sophisticated guidelines, systematic BBD errors are difficult to spot and eliminate. An appropriate quality assurance (QA) of BBD would thus be an essential cornerstone to an overall high dosimetric accuracy. Here, we investigate the use of Monte Carlo (MC) simulation to verify self-consistency and physical plausibility of BBD. Material and Methods MC simulations of various field configurations offer the advantage of being inherently self-consistent if they utilize an invariant description of the phase space (PS)

above the variable collimator system. To generate such a PS, relatively few measurements are required, that can be chosen to offer optimum measurement conditions regarding field size and depth of measurement, and thereby consistent beam quality, i.e. particle spectra. Using SciMoCa (Scientific RT, Munich), a newly developed virtual MC source model with a unique commissioning method relying on pre-generated BEAMnrc template beam models (BM), 62 BBD sets of distinct beam qualities from 15 different institutions, a mix of Varian, Elekta, Siemens and Cyberknife, were analyzed. For each BBD, a SciMoCa BM was built. For analyzing the BBD, simulated depth dose curves (DDC), output factors (OF) and cross profiles were compared against the original BBD. Results The BM achieve a high accuracy of 0.2% std. dev. in abs. dose for fields ranging from 10x10 mm² up to 400x400 mm² if derived from high quality BBD and even +-1.0% if derived from flawed BBD. From the 62 BBD, only one was without systematic DDC or OF errors. Most BBD contained DDC or OF errors of up to 5% in a subset of measurements. Five BBD were so flawed that no model could be built and the entire BBD had to be re-measured. Two main sources of errors could be detected. The first kind were detector specific errors like the spectral response of e.g. diodes in depth, the calibration of the detector and other effects. The second kind were setup errors like wrong relative position of measurement, the placement of electrometer in the linac room or the alignment of the water phantom (WP). In several BDD, an over estimation of DDC for large fields/depths could be detected which came from missing water in the WP, resulting in backscatter from the WP carriage. About ¾ of BBD had severe errors that would have compromised patient dose computation. Conclusion Results show that QA of BBD is feasible by MC beam modelling with a high sensitivity. Random and systematic errors can be detected and the overall quality and self- consistency of BBD can be validated with a precision of typically +-0.5% and +-1.0% in worst cases. This leads to a huge improvement of BBD accuracy and by implication to a higher accuracy in patient dose calculation. This method could be a valuable addition to dosimetry audits, because it eliminates very specifically a frequent source of dose computation errors. OC-0406 Validation of the INTRABEAM system dosimetry with ionization chamber and EBT3 film measurements P. Watson 1 , H. Bekerat 2 , P. Papaconstadopoulos 2 , J. Seuntjens 1 1 McGIll University Health Center, Medical Physics Unit, Montreal, Canada 2 Jewish General Hospital, Medical Physics Unit, Montreal, Canada Purpose or Objective The INTRABEAM system (Carl Zeiss Meditech AG) is one of the most frequently used miniature x-ray sources for IORT. In the absence of an absorbed dose to water primary standard for this device, the INTRABEAM is calibrated by the manufacturer. The purpose of this work is to independently validate the dosimetry of the INTRABEAM system using both ionization chamber and EBT3 Gafchromic film measurements of absorbed dose. Material and Methods Ionization chamber and EBT3 film irradiations were performed in a Zeiss INTRABEAM water phantom, positioned from 5 to 30 mm from a submerged INTRABEAM source. The absorbed dose to water measured by a PTW 34013 soft x-ray chamber was calculated by three methods: the method recommended by the water phantom manual ("Zeiss" dose), the manufacturer calibration dose method ("TARGIT" dose), and our dose