ESTRO 2022 - Abstract Book

S1357

Abstract book

ESTRO 2022

Figure 1. TL intensity versus dose for BaSO 4 :Eu dosimeter following g – or proton – irradiation.

Conclusion The radiation quality dependence study shows that the TL output per dose to water of the BaSO 4 :Eu nanophosphor is dependent on radiation type (g-rays vs protons) and photon energy (g-rays vs X-rays). The strong X-ray energy dependence may be explained by the high mass energy absorption coefficient of the BaSO 4 compound. However, the simple glow curve structure, excellent reusability and low fading indicates favorable properties for dose determination in radiotherapy, as long as the samples are calibrated in the user beam.

PO-1574 Perturbation correction factors of ionization chambers in small photon fields in the magnetic field

T. Tekin 1 , I. Blum 1 , B. Delfs 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 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. Materials and Methods 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.

Results