ACPSEM position paper: dosimetry for magnetic resonance imaging linear accelerators

Abstract

Consistency and clear guidelines on dosimetry are essential for accurate and precise dosimetry, to ensure the best patient outcomes and to allow direct dose comparison across different centres. Magnetic Resonance Imaging Linac (MRI-linac) systems have recently been introduced to Australasian clinics. This report provides recommendations on reference dosimetry measurements for MRI-linacs on behalf of the Australiasian College of Physical Scientists and Engineers in Medicine (ACPSEM) MRI-linac working group. There are two configurations considered for MRI-linacs, perpendicular and parallel, referring to the relative direction of the magnetic field and radiation beam, with different impacts on dose deposition in a medium. These recommendations focus on ion chambers which are most commonly used in the clinic for reference dosimetry. Water phantoms must be MR safe or conditional and practical limitations on phantom set-up must be considered. Solid phantoms are not advised for reference dosimetry. For reference dosimetry, IAEA TRS-398 recommendations cannot be followed completely due to physical differences between conventional linac and MRI-linac systems. Manufacturers’ advice on reference conditions should be followed. Beam quality specification of TPR20,10 is recommended. The configuration of the central axis of the ion chamber relative to the magnetic field and radiation beam impacts the chamber response and must be considered carefully. Recommended corrections to delivered dose are $${k}_{{Q}_{msr}{Q}_{0}}^{{f}_{msr}{f}_{ref}}$$ k Q msr Q 0 f msr f ref , a correction for beam quality and $${k}_{\overrightarrow{B},{Q}_{msr}}^{{f}_{msr}}$$ k B → , Q msr f msr , for the impact of the magnetic field on dosimeter response in the magnetic field. Literature based values for $${k}_{\overrightarrow{B},{Q}_{msr}}^{{f}_{msr}}$$ k B → , Q msr f msr are given. It is important to note that this is a developing field and these recommendations should be used together with a review of current literature.

Magnetic field induced dose effects in radiation therapy using MR‐linacs

“…The existing data suggests that the IAEA TRS-398 63 beam quality specifier TPR 20 10 is preferred for determining the chamber beam quality conversion factor k Q , because it is insensitive to magnetic field per-turbations and the non-reference SSDs conditions of the MR-linacs. 34,64,65 A new factor, k Q, B , was introduced to correct the air-filled ionization chamber response in the magnetic field. 64,66-71 k Q, B values for Farmer chambers were determined by Monte Carlo simulations 64,66,67,72 water calorimeters, 70,73,74 graphite calorimeters, 75,76 or alanine dosimeter.…”

“…MRI‐linac dosimetry remains an important area of research and development for dosimetry standards. The existing data suggests that the IAEA TRS‐398 63 beam quality specifier $$TPR_{10}^{20}$$ is preferred for determining the chamber beam quality conversion factor $${k_Q}$$, because it is insensitive to magnetic field perturbations and the non‐reference SSDs conditions of the MR‐linacs 34,64,65 . A new factor, $${k_{Q,\ B}}$$, was introduced to correct the air‐filled ionization chamber response in the magnetic field 64,66–71 …”

Technical note: Cryostat transmission characterization for MR linac – temporal stability, clinical impact and change implementation

“…Although the MRI is designed as zero boil‐off system 3 and the helium level is fixed, during some service operations it may change and/or require a top‐up. To avoid output measurement inconsistency due to change in the partial covering of the annulus with liquid helium, the initial vendor recommendations involved absolute dose calibration at gantry angle of 90°/270° and a fill cut‐off level of 65% (despite the MR scanner being able to operate with fill levels as low as 30%) to ensure liquid helium is always present in the radiation beam 6,7 …”

“…To avoid output measurement inconsistency due to change in the partial covering of the annulus with liquid helium, the initial vendor recommendations involved absolute dose calibration at gantry angle of 90 • /270 • and a fill cut-off level of 65% (despite the MR scanner being able to operate with fill levels as low as 30%) to ensure liquid helium is always present in the radiation beam. 6,7 Considering that the initial helium fill levels for newly installed machines are further from the annulus and to avoid setup uncertainties associated with the measurement of absolute dose at gantry angle 90 • (i.e., through the tank wall), the vendor is currently transitioning to the calibration conditions recommendation using gantry angle 0 • . Following a helium top-up on our system and in order to align to the recent vendor recommendations, we have recently updated the absolute dose calibration conditions and the associated TPS model.…”