ESTRO 2020 Abstract Book

S698 ESTRO 2020

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 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).

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 PO-1324 Small field output factors measurement with EBT3 gafchromic film in water A. Obesso 1 , C. Ferrer 1 , C. Huertas 1 , E. Corredoira 1 1 Hospital universitaria La Paz, Radiofísica y Radioprotección, Madrid, Spain Purpose or Objective This work investigates the possibility of small fields output factors (S cp ) measurement with radiochromic film directly in water, and compare them with those measured with different detectors. Material and Methods All measurements were performed on the IBA Blue Phantom in an Elekta Infinity Linear Accelerator. The detectors used were the PFD3G and EFD3G diodes, and ionization chambers CC01, CC04 and CC13 (IBA Dosimetry) with a Scanditronix-Wellhofer electrometer. Measurements were performed with GafChromic RTQA2 radiochromic film, and analyzed with FilmQA Pro 2016 software. The films were placed at 100 cm source surface distance and 10 cm depth. The field sizes measured were 1x1cm 2 , 2x2 cm 2 , 3x3 cm 2 and 5x5 cm 2 . In addition, 0.5x0.5cm 2 field size was only measured with radiochromic film, as the source occlusion, the loss of the lateral electronic equilibrium and the volume effect in the chambers causes several uncertainties. The output factors measured with cameras and diodes were corrected by the S cp 5x5 CC13 of the 5x5cm field in the CC13 camera and by the correction factor provided by TG 483 ( k ), following the expression: Where Q is the charge in the corresponding field and Q 5x5 de charge in the 5x5 cm 2 field. Results The measures with cameras or diodes are very similar to those with film. The output factors corresponding to the field size 1x1cm 2 , 2x2cm 2 and 3x3 cm 2 have a difference of <3.5% in all cases related to the measurement collected with the diode without shielding, EFD3G. A greater discrepancy is observed in field size 5x5cm 2 (<4.3%) probably due to the uncertainties within the placement process, and will required further investigations. The point that best fits all measuring equipment is 2x2 cm 2 with a maximum difference of <1%. The resulting value for the output factor in the 0.5x0.5cm field is 0.43.

Conclusion Our results demonstrate good agreement between Monte Carlo Simulation and measured data for OF, although