ESTRO 38 Abstract book

S144 ESTRO 38

standard uncertainty of 0.37 % in the 7 MV photon beam of an MRI linac in a 1.5 T magnetic field. Material and Methods Measurements were done with a magnetic field of 1.5 T and, after ramp-down, without magnetic field. Methods were developed for measurement of detector depth, distance and beam output. A thermodynamic description was used to demonstrate potential temperature effects due to the magneto-caloric effect (MCE). To evaluate the primary standard on a fundamental basis, realisation of D w at 1.5 T was evaluated parameter by parameter. Results It was shown that the measurement of D T w , are either independent of, or can be determined in, a magnetic field. The chemical heat defect, h , was considered zero within its stated uncertainty, as for 0 T. Evaluation of the MCE and measurements done during magnet ramp-down, indicated no changes in the specific heat capacity of water. However, variation of the applied transmission monitor in the bore was the limiting factor for output normalization. The table shows the calculated correction factors for heat conduction, k C measured correction factors for perturbation of the calorimeter HPC both with and without magnetic field, k HPC and the correction for beam non-uniformity k nu . Hardly any significant effect of the magnetic field on the correction factors is observed. The figure shows the corrected calorimetric D w measurements in a 0 T magnetic field of an Elekta MRI- linac (left) and in the same machine with 1.5 T magnetic field (right). Both graphs show D w measurements normalized to the monitor in the bore with the HPC perpendicular and in parallel direction to the bore. Note that values on the vertical axes cannot be compered due to the magnetic field dependence of the applied monitor. Conclusion This study confirmed that the uncertainty for measurement of D w with a water calorimeter in a 1.5 T magnetic field is estimated to be the same as under conventional reference conditions at 0 T. The VSL water calorimeter can therefore be applied as a primary standard for D w in magnetic fields and is currently the only primary standard operable in a magnetic field that provides direct access to the international traceability framework.

Purpose or Objective Reference dosimetry in a strong magnetic field is made more complex due to (i) the change in dose deposition and (ii) the change in sensitivity of the detector. Potentially it is also influenced by thin air layers, interfaces between media, orientations of field, chamber and radiation, and minor variations in ion chamber stem or electrode construction. The PTW30013 and IBA FC65-G detectors are waterproof Farmer-type ion chambers that are suitable for reference dosimetry. The magnetic field correction factors have previously been determined for these chamber types. The aim of this study was to assess the chamber-to- chamber variation and determine whether generic chamber type-specific magnetic field correction factors ( k B,M,Q ) can be applied for each of the PTW30013 and FC65- G -type ion chambers when they are oriented parallel (//) to, and perpendicular (|-) to, the magnetic field. Material and Methods The experiment was conducted with 12 PTW30013 and 13 FC65-G chambers The k B,M,Q were measured using the practical method of van Asselen et al. (2018, PMB, 63 p125008) that can be performed in the clinic. In this study each chamber was cross-calibrated against the local standard chamber twice; with and without magnetic field. Measurements with 1.5 T magnetic field were performed with the 7 MV FFF beam of the MRI- linac. Measurements without magnetic field (0 T) were performed with the 6 MV conventional beam of an Elekta Agility linac.

A prototype MR-compatible PTW MP1 phantom was used along with a prototype holder that facilitated measurements with the chamber aligned 90 degrees counter-clockwise (|-) and 180 degrees (//) to the direction of the magnetic field. A monitor chamber was also mounted on the holder and all measurements were normalized so that variations in the output of each linac were removed. Measurements with the local standard chamber were repeated during the experiment to quantify the experimental uncertainty. Recombination was measured in the 6 MV beam. Generic beam quality correction factors were applied. The differences in these factors between beams is constant within each chamber type (dependent on beam quality and dose-per-pulse).

OC-0290 Consistency of PTW30013 and FC65-G ion chamber magnetic field correction factors S. Woodings 1 , B. Van Asselen 1 , T. Van Soest 1 , L. De Prez 2 , J. Lagendijk 1 , B. Raaymakers 1 , J. Wolthaus 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands ; 2 VSL, Dutch Metrology Institute, Delft, The Netherlands