ESTRO 2020 Abstract book

S746 ESTRO 2020

et al (2008) 1 proposed a new formalism for small and non- standard field dosimetry, introducing a new correction factor which correlates the differences between the clinical field size and the machine-specific reference field size . Afterwards, the 483 protocol 2 provided correction factors for different detectors and small field sizes. The purpose of this work is to present results of OF using a MicroDiamond from PTW, a Razor diode and a Razor NanoChamber from IBA and compare the experimental results with Monte Carlo Simulation, using the Penelope 3 code system and the Ulisses 4 geometry, tracking and scoring routines. Since the correction factor for the NanoChamber is not on the 483 protocol list, a study of it was made. Material and Methods Measurements of OF, with a reference field size of 3x3 cm 2 , were performed in three Varian Machine: TrueBeam and Edge with high definition MLC and TrueBeam with Millennium 120 MLC, using the MicroDiamond, Razor and NanoChamber detectors. Beam collimation included were: 1) cones of different diameter (7.5 mm, 10 mm, 12.5 mm, 15 mm and 17.5 mm) with jaws opening at 5x5 cm 2 ; and 2) square field size of 3x3, 2x2, 1x1 and 0.5x0.5 cm 2 defined by the MLC. Monte Carlo Simulation were performed for the same square field size defined by the MLC. The energies studies were 6X (FF and FFF) and 10X-FFF. Results Our preliminar results indicate 1) OF differences less than 1% between the three different Varian machines; 2) OF differences between different detectors increase with the decreasing of the field size 3) OF differences between measurements and Monte Carlo Simulation around 2% for 6X (FF and FFF) and 1% for 10X-FFF (Fig.1); 4) the need of correction factor for NanoChamber for equivalent square field size (S clin ) below 1.11 cm and 1.55 cm for 6X (FF and FFF) and 10X-FFF, respectively (Table1).

consequence, with a range of dose per pulse between 8.5 mGy/pulse and 40 mGy/pulse. At least three measurements were performed for voltages 100 V, 400 V and - 400 V. k pol was estimated according to TRS-398 and k sat according to Laitano et al ( Phys. Med. Biol 51, 2006). The final value of k pol and k sat for each chamber and each energy was obtained as the average and standard deviation of the three groups of measurements. Results Type A uncertainty (k=1) of k sat was on average 7·10 -5 , 5·10 - 5 , 7·10 -5 and 10 -4 for 6, 8, 10 and 12 MeV, while dispersion between chambers was about one order of magnitude higher. In the case of k pol , uncertainty on average was 4·10 - 4 , 2·10 -4 , 2·10 -4 , 10 -3 while again the dispersion between chambers was about one order of magnitude higher. Type B uncertainties were not analyzed.

Conclusion Dispersion between chambers for both k pol was higher than the associated uncertainty due to charge measurement (Type A). For this reason, individual evaluation of k pol and k sat for the Advanced Marcus chamber could be adequate when used for measurements on high dose-per-pulse electron beams. The maximum deviation between chambers was found at the extreme energies: 0.18 % for k sat at 12 MeV and 0.25 % for k pol at 6 MV showing the robustness of the chamber. PO-1323 Comparison of Output Factor measurements with Monte Carlo Simulation for Small Field Dosimetry D. Mateus 1,2 , C. Greco 1 , L. Peralta 3 1 Fundação Champalimaud, Radiotherapy, Lisboa, Portugal ; 2 Mercurius Health, Radiotherapy, Lisbon, Portugal ; 3 Faculty of Sciences of Lisbon University, Physics, Lisbon, Portugal Purpose or Objective The measurement of field output factors (OF) for MV photon small fields are subjected to large uncertainties, due to the challenging of the small field dosimetry, which involves the lack of electronic equilibrium, source occlusion and volume effect of the used detector. Alfonso and k sat

Conclusion Our results demonstrate good agreement between Monte Carlo Simulation and measured data for OF, although Monte Carlo simulation still need improvements in the statistical fluctuations of the results. The results further demonstrate the importance of applying correction factors for NanoChamber to compensate volume averaging and perturbations effects. 1 R. Alfonso et al., “A new formalism for reference dosimetry of small and nonstandard fields”, Phys. Med., Vol. 35, No, 11, 5179-5186, 2008; 2 IAEA TRS-483 protocol: Dosimetry of small static fields used in external beam radiotherapy – An international code of practice for reference and relative dose determination, International Atomic Energy Agency (IAEA) , 2017; 3 NEA., “Penelope- 2014: A code System for Monte Carlo Simulation of Electron and Photon Transport”, NEA, 2005; 4 L. Peralta and A. Louro , AlfaMC: A fast alpha particle transport Monte Carlo code, , Nucl. Instr. and Meth. in Phys. Res. A, Vol 737 (2014) 163-169