Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy

Stefan Pojtinger; Marcel Nachbar; Ralf-Peter Kapsch; Daniela Thorwarth

doi:10.1016/j.phro.2020.10.005

Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy

Phys Imaging Radiat Oncol. 2020 Oct 27:16:95-98. doi: 10.1016/j.phro.2020.10.005. eCollection 2020 Oct.

Authors

Stefan Pojtinger^{1

2}, Marcel Nachbar², Ralf-Peter Kapsch¹, Daniela Thorwarth²

Affiliations

¹ Physikalisch-Technische Bundesanstalt, Braunschweig, Germany.
² Section for Biomedical Physics, Department of Radiation Oncology, University of Tübingen, Germany.

Abstract

Correction factors for reference dosimetry in magnetic resonance (MR) imaging-guided radiation therapy ( $k_{\vec{B}, M, Q}$ ) are often determined in setups that combine a conventional 6 MV linac with an electromagnet. This study investigated whether results based on these measurements were applicable for a 7 MV MR-linac using Monte Carlo simulations. For a Farmer-type ionization chamber, $k_{\vec{B}, M, Q}$ was assessed for different tissue-phantom ratios ( ${TPR}_{20, 10}$ ). $k_{\vec{B}, M, Q}$ differed by $0.0029 (43)$ between ${TPR}_{20, 10} = 0.6790 (23)$ (6 MV linac) and ${TPR}_{20, 10} = 0.7028 (14)$ (7 MV MR-linac) at $1.5 T$ . The agreement was best in an orientation in which the secondary electrons were deflected to the stem of the ionization chamber.

Keywords: Dosimetry; EGSnrc; Ionization chambers; MR-linac; Magnetic field correction factors; Monte Carlo.