ESTRO 2020 Abstract book

S747 ESTRO 2020

PO-1325 Measurement and MC simulation of magnetic field correction factors of compact ionization chambers I. Büsing 1 , B. Delfs 1 , T. Tekin 1 , A. Schönfeld 1 , B. Poppe 1 , H.K. Looe 1 1 University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital - Carl von Ossietzky University, Oldenburg, Germany Purpose or Objective Farmer-type ionization chambers are commonly used for reference dosimetry at MR linacs. The alteration of their dose response in magnetic field and the underlying mechanisms have been frequently studied and reported. However, compact ionization chambers are also used for measurements in situations where high spatial resolution is required, such as small fields and lateral profiles. Therefore, it is of the same importance to study their magnetic field dependent dose response, for which reports in the literature are still rare. Furthermore, the correct modelling of the effective sensitive volume of an ionization chamber in Monte Carlo simulation has been shown to be of high importance. Since this might play a more essential role in smaller chambers due to their more compact design, this aspect was also investigated in this work. Material and Methods Six compact ionization chambers from two manufacturers (PTW 31021, PTW 31022, PTW 31023, PTW 31016, IBA CC01-G, IBA CC003-S) were investigated. Measurements were performed using a 4 cm x 4 cm field of a 6 MV photon beam to evaluate the magnetic field correction factors k B,Q of these chambers. An electromagnet was positioned in the beam, with which the magnetic field can be varied up to 1.4 T. The chambers were positioned with their reference points axially and radially in 5 cm water depth. For selected chambers, measurements were repeated with smaller field sizes of 2 cm x 2 cm and 0.5 cm x 0.5 cm to assess the possible field size dependence of the correction factors. Monte Carlo simulations were performed using the EGSnrc code. For fully-guarded chambers, the effective sensitive volumes were evaluated using a high-resolution proton microbeam measurements that were then compared to computation results obtained from finite element analysis. The role of effective sensitive volume was studied. Results The modification of dose responses of the investigated ionization chambers shows strong dependence on the chamber’s design, that is, the shape of sensitive volume including the presence of non-active air volume; and the components surrounding the air volume such as the chamber’s stem. Additionally, the positioning of the chambers with respect to the beam’s axis and magnetic field; and the magnetic field strength also play an essential role. Nevertheless, all chambers show increase or decrease of dose response in magnetic field by not more than 5% up to 1.5 T. The correction factors derived from Monte Carlo simulations with detailed chamber models considering their effective sensitive volumes show agreement better than 1% to measurements. Conclusion A comprehensive study on the dose response of compact chambers in magnetic field has been carried out by measurements and Monte Carlo simulations. By comparing the results of chambers with different designs, the underlying mechanisms of the magnetic field dependent dose response of these compact chambers have been revealed.

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 It has been proven, that it is possible to measure the output factors directly in water with radiochromic film without any protection. This practice reveals itself very useful when measuring small field’s beams.