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

Abstract

Over the last few years, magnetic resonance image-guided radiotherapy systems have been introduced into the clinic, allowing for daily online plan adaption. While quality assurance (QA) is similar to conventional radiotherapy systems, there is a need to introduce or modify measurement techniques. As yet, there is no consensus guidance on the QA equipment and test requirements for such systems. Therefore, this report provides an overview of QA equipment and techniques for mechanical, dosimetric, and imaging performance of such systems and recommendation of the QA procedures, particularly for a 1.5T MR-linac device. An overview of the system design and considerations for QA measurements, particularly the effect of the machine geometry and magnetic field on the radiation beam measurements is given. The effect of the magnetic field on measurement equipment and methods is reviewed to provide a foundation for interpreting measurement results and devising appropriate methods. And lastly, a consensus overview of recommended QA, appropriate methods, and tolerances is provided based on conventional QA protocols. The aim of this consensus work was to provide a foundation for QA protocols, comparative studies of system performance, and for future development of QA protocols and measurement methods.

Commissioning Measurements On An Elekta Unity MR-Linac

“…Results for the various isocentre measurement techniques are presented in Table 1. All techniques showed that the isocentre was within the tolerance (≤ 0.50 mm [3]).…”

“…3(c)) images are not discernible from the nominal (no angular offset) image without further analysis of pixel intensity pro les. Obviously, visual inspection cannot be used to determine the absolute gantry angle; however, it is useful for determining if set positions are within the tolerance speci ed by Roberts et al [3] (± 0.3°). Note that the more recent publication of Woodings et al [5] suggests a speci cation of < 0.2° for gantry angle, which cannot be achieved with this methodology unless further analysis is performed on the pixel intensity pro les, discussed below.…”

“…The design of the Unity system has been comprehensively discussed by other investigators, [1][2][3][4][5] and the reader is referred to these works for further information. In short, the Elekta Unity MRL is a combination of a modi ed Philips Ingenia 1.5 T MRI with a split-coil superconducting magnet and a straight-through linear accelerator.…”

“…For all gantry angles the beam passes through the cryostat and is perpendicular to B 0 . The angular dependent beam transmission through the cryostat (aluminium annular structure containing liquid helium) is referred to as the cryostat characterisation and will vary between Unity systems mostly due to differences in construction of the cryostat annulus; however, a small component will be due to differences in helium ll [3]. A modi ed Elekta Agility® Beam Limiting Device (BLD) shapes elds ranging from 0.8 × 0.5 cm 2 to 57.4 × 22.0 cm 2 at isocentre, with a xed dose-rate at this point of 425.0 MU/min.…”

“…Published work focused on commissioning of the Elekta Unity system is limited [3][4][5]9]. Recently, the work of Roberts et al [3] provides tolerances and frequencies for common QA tests and Woodings et al [5] for acceptance procedures on the linac component. Since this technology has only recently been clinically introduced (results reported in this work were obtained with the rst Unity in the Southern Hemisphere), there is currently no region-speci c guidance for implementation and safe use of the MRL, as is the current situation for conventional treatment systems [10]; however, position papers are unlikely to vary signi cantly from the recommendations of the above investigators [3,5].…”