Abstract Book

S935

ESTRO 37

Material and Methods A central aspect of the RTNORM project is that experimental determinations and Monte Carlo calculations of k Q,Q0 factors are cross-validated internally in the project consortium prior to the submission of a consistent dataset to the IAEA Task Group for the update of the IAEA TRS-398. The RTNORM project aims at contributing towards this update by addressing the following objectives: 1. kV x-ray beams between 100 kV and 250 kV, targeting k Q,Q0 factors with a target standard uncertainty better than 1.0 %. 2. High-energy (MV) photon beams between 4 MV and 20 MV, including conventional flattening filter (cFF) and flattening filter free beams (FFF), targeting k Q,Q0 factors with a target standard uncertainty better than 0.7 %. Scanned proton beams between 60 MeV and 250 MeV, targeting k Q,Q0 factors with a target standard uncertainty better than 2.0 %. 3.

modelling in RayStation v5 (Collapsed Cone v 3.2 CC RS ). CT images of a polystyrene 12x12x12 cm 3 cubic slab phantom with a Gammex RMI lung equivalent insert (ρ = 0,3 g/cm 3 , diameter 6 cm) behind buildup of 2 cm polystyrene was used with an isotropic calculation grid of 2.5mm. Four 6MV beams (gantry 0°) with field sizes 16x16, 6x6, 3x3 and 1x10 cm 3 , 200 MU each, were evaluated separately. Both machines are absolutely calibrated according to NCS18. No density override and no dose scaling took place. Line dose profiles along the central axis were extracted for pairwise comparison. Results Figure 1 shows the resulting depth profiles. Extend and location of the lung insert are depicted. Overall agreement for the two large fields between CC RS and the other algorithms is apparent. However, AAA ec shows re- buildup behind the lung insert which is not visible with CC RS nor CC Pin . For the two small fields, the lower output for the Elekta Synergy machine is visible. This is due to accelerator head design with beam defining devices closer to the target, implying less head scatter and the fact that no scaling was used. In the lung cavity CC RS predicts a slightly higher dose for both AAA ec and CC Pin , even though the CC Pin algorithm is of the same type as CC RS . This effect is more pronounced for 1x10 cm 3 field. Also for the 1x10 cm 3 field, the line doses of CCRS and AAA ec are crossing. The tails of the depth profiles are in good agreement, except for the comparison of CC RS with AAA ec for the 1x10 cm 3 field.

This is achieved by (i) measurement of k Q,Q0 factors for a number of commonly used ion chambers and beam qualities in Europe, in all cases ensuring direct traceability of the measurements leading to the k Q,Q0 factors to primary standards of absorbed dose to water available in Europe; (ii) to calculate k Q,Q0 factors for these beams using several validated Monte Carlo codes; (iii) to compare the measured and calculated k Q,Q0 factors for all beam qualities and modalities. In case of medium energy x-rays a comparison will be made to compare the new absorbed dose-to- water based formalism using k Q,Q0 with a traditional air-kerma based formalism.

Results The project began in May 2017 and the earliest results include internal validation of Monte Carlo simulations in reference geometric conditions. Results of k Q,Q0 factors for the various modalities will be presented both from experiments and from Monte Carlo-calculations . Conclusion The cooperation of European national metrology institutes and research partners with industry in the EMPIR RTNORM project provides for a unique potential to join knowledge in order to provide essential and consistent datasets to be used by the European radiotherapy community. The RTNORM project is ongoing and results are being provided to TRS-398 revision task group on a regular basis. EP-1745 Differences in central line dose profiles for RayStation, Eclipse and Pinnacle, around a lung insert E. Bogaert 1 , G. Pittomvils 1 , C. De Wagter 1 1 Ghent University Hospital, Radiotherapy, Gent, Belgium Purpose or Objective Precise treatment planning in thoracic region implies correct prediction of dose in heterogeneous media. The accuracy of dose calculation depends on the performance of the calculation algorithms in these situations of electronic disequilibrium. Here, differences between algorithms become most visible for narrow high energy photon beams. In view of RayStation (RaySearch Laboratories, Sweden) being recently introduced in our department, a comparison is made with the existing Treatment Planning Systems (TPS) Eclipse and Pinnacle, regarding performance in and behind a lung insert. Material and Methods Two accelerators, Varian Clinac iX with Millennium MLC and Elekta Synergy with MLCi were modelled in Varian Eclipse v 8.9 (AAA algorithm AAA ec ) and Philips Pinnacle v 9.2 (Collapsed Cone CC Pin algorithm) respectively, before

Conclusion For low energy and for lung densities ≥ 0,2 g/cm3, both CC RS calculations are in good agreement with AAAec and CC Pin predictions, as was confirmed by Aarup et al. For these conditions Raystation dose calculation algorithm can be safely introduced. Further investigation on higher energies need to be performed. (Aarup et al. Rad. Oncol 91 (2009)) EP-1746 Influence of Cable Effect on Polarity Correction Factor of Micro Volume Ionization Chamber K. Sasaki 1 , Y. Shiota 2 , Y. Miura 2 1 Gunma Prefectural College of Health Sciences, Graduate School of Radiological Technology, Maebashi, Japan 2 Iwata City Hospital, Medical Physics, Iwata, Japan Purpose or Objective In absorbed dose measurement in radiation therapy, a micro volume ionization chamber has been used for small irradiation fields and high resolution measurements. In this study, the fundamental characteristics of micro volume ionization chamber dosimeter (PTW TM31022, Exradin A 26) with improved characteristics were investigated, compared with other ionization chamber dosimeters (PTW TM30013, PTW TM31014, PTW TM31016, Exradin A1SL) , and the influence of cable effect was investigated.