Reference dosimetry in MRI-linacs: evaluation of available protocols and data to establish a Code of Practice

Abstract

With the rapid increase in clinical treatments with MRI-linacs, a consistent, harmonized and sustainable ground for reference dosimetry in MRI-linacs is needed. Specific for reference dosimetry in MRI-linacs is the presence of a strong magnetic field. Therefore, existing Code of Practices (CoPs) are inadequate. In recent years, a vast amount of papers have been published in relation to this topic. The purpose of this review paper is twofold: to give an overview and evaluate the existing literature for reference dosimetry in MRI-linacs and to discuss whether the literature and datasets are adequate and complete to serve as a basis for the development of a new or to extend existing CoPs. This review is prefaced with an overview of existing MRI-linac facilities. Then an introduction on the physics of radiation transport in magnetic fields is given. The main part of the review is devoted to the evaluation of the literature with respect to the following subjects: • beam characteristics of MRI-linac facilities; • formalisms for reference dosimetry in MRI-linacs; • characteristics of ionization chambers in the presence of magnetic fields; • ionization chamber beam quality correction factors; and • ionization chamber magnetic field correction factors. The review is completed with a discussion as to whether the existing literature is adequate to serve as basis for a CoP. In addition, it highlights subjects for future research on this topic.

MR‐guided proton therapy: Impact of magnetic fields on the detector response

“…beams were recently summarized [21], revealing deviations up to 9% as compared to the response in absence of a magnetic field. In our study, the impact of magnetic fields on chamber readings was considerably lower as compared to photons.…”

“…For photons, a significant amount of resources was invested into developing suitable dosimetry procedures and ionizing-radiation detector correction factors for reliable reference and relative dosimetry in the presence of magnetic fields [6,7,[16][17][18][19][20][8][9][10][11][12][13][14][15]. A detailed overview of the current status of reference dosimetry in MR linacs was recently presented by de Pooter [21] Proton therapy would be ideally suited to profit from such an advanced imaging modality, especially due to its higher conformity and increased sensitivity to anatomical changes [22][23][24][25][26][27][28];…”

Machine QA for the Elekta Unity system: A Report from the Elekta MR‐linac consortium

“…Several publications have considered individual aspects of QA on MR-linac devices. Measurement equipment performance, [10][11][12][13][14][15][16][17][18] reference 19 and relative 15,[20][21][22][23] dosimetry within the presence of a magnetic field, treatment planning system, [24][25][26][27][28] Linac, 22,[29][30][31] MR device commissioning, 32,33 and routine QA 32,[34][35][36][37] are the broad range of areas addressed. While not considerably different to conventional RT QA, there are several aspects that need considering.…”

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

“…To do this, must be calculated by Monte Carlo simulations for the respective beam quality of the MR-linac, as done in [13] . The spread in the results for magnetic field correction factors that were determined by different methods is approximately 1% [15] .…”

“…As the beam quality of conventional linacs differs from the beam quality of the two clinically available MR-linacs, that is 6 MV flattening filter free (FFF) for the 0.35 T MR-linac [16] and 7 MV FFF for the 1.5 T MR-linac [17] , the determination in a setup using a conventional linac is problematic. Due to the fact that the effects of the magnetic field can be explained by a change of the secondary electron’s trajectories due to the Lorentz force [15] , a change in the energy spectrum of these secondary particles might result in a change of .…”