Session Item

Dosimetry of small photon fields is very challenging. Without doubt, the same applies to measurements in the presence of a magnetic field, such as encountered in magnetic resonance guided radiation therapy (MRgRT). Protocol for reference dosimetry in a magnetic field using air-filled ionization chambers has been established (de Pooter et al 2021 Phys Med Biol. 66(5):05TR02). The dose response of diode-type detectors in sufficiently large (Tekin et al 2020 Med Phys 47(12):6509-6518) and small fields (Blum et al 2021 Phys Med Biol. 66(15)) was shown to exhibit strong magnetic field dependence. Nevertheless, the applicability of small- or micro-ionization chambers in a magnetic field under small field condition has not been fully explored. This investigation extends our previous study on ionization chambers up to a magnetic field B = 1.5 T (Delfs et al 2021 Med Phys. 48(8):4572-4585) to small field sizes.

Monte Carlo simulations were carried out using the EGSnrc code version 2019a together with the eemf-macro for the charged particle transport in a magnetic field. The user-code egs_chamber was used to model the Semiflex 3D 31021 and PinPoint 3D 31022 chambers from PTW Freiburg. The chamber’s perturbation correction factors P_i have been simulated between 3 and 200 mm depths by stepwise modification of the chambers’ models. Simulations were performed using 6 MV photon beam for the nominal field sizes between 10 cm x 10 cm and 0.5 cm x 0.5 cm, and up to B = 1.5 T. The chambers were positioned with their axes perpendicular to the beam’s axis and parallel to the magnetic field. The field size- and magnetic field-dependent total correction factors P_total, and additionally, the EPOM shift ∆z of the chambers were determined from the simulations.

The chamber’s perturbation correction factors for all investigated field sizes are magnetic field dependent, where the observed differences are stronger in small field sizes. In reference field size, these factors P_i, including the water-to-air stopping power ratios, can be considered constant beyond the build-up region. In small field sizes, the gradient perturbation shows depth-dependence and its changes in magnetic field also contribute strongest to the small field correction factors of the investigated ionization chambers in the presence of a magnetic field. The ∆z-shift was also shown to be reduced in a magnetic field for both large and small field sizes.

The detailed simulations performed in this work has facilitated deep understandings of the behavior of ionization chambers in a magnetic field under small field condition. The influence of all detector’s components on its magnetic field dependent dose response has been quantified, where the contribution of the gradient perturbation caused by the extended air cavity is shown to be the largest. The correction factors derived from this study can be used to identify the limit of applicability of these chambers.