Volume 34, Issue 1, January 2024, Pages 129-134Author links open overlay panel

The introduction of MR-guided treatment machines into the radiation oncology clinic has provided unique challenges for the radiotherapy QA program. These MR-linac systems require that existing QA procedures be adapted to verify linac performance within the magnetic field environment and that new procedures be added to ensure acceptable image quality for the MR system. While both high and low-field MR-linac options exist, this chapter is intended to provide a structure for implementing a QA program within the low-field MR environment. This review is divided into three sections. The first section focuses on machine QA tasks including mechanical and dosimetric verification. The second section is concentrated on the procedures implemented for imaging QA. Finally, the last section covers patient specific QA tasks including special considerations related to the performance of patient specific QA within the framework of online adaptive radiotherapy.

Section snippetsMR-Guided Radiotherapy Quality Assurance for Low-Field MR-Linac

The purpose of quality assurance in radiation therapy is to ensure that the patient is treated safely and accurately in a manner consistent with the irradiation treatment goals prescribed by the physician. In order to accomplish this, a quality assurance program must ensure that every step along the radiation therapy workflow operates within acceptable tolerances. While MR-guided radiotherapy (MRgRT) is new to radiation therapy, the major components of a quality assurance program for MRgRT can

System Description of Low-Field MR-Linac

The first MRgRT system to receive regulatory approval and enter the market comprised a low-field (0.345 T), split-bore, superconducting MRI and a tri-Cobalt 60 delivery system where each of the 3 heads was equally spaced on a ring-like gantry structure that was placed in the 28 cm gap between the halves of the MRI bore.1 In 2017, this version was replaced by a combined MR-linear accelerator system.2 While the basic design of the MRI remained the same, the tri-Cobalt heads were replaced by the

Machine QA

One of the first steps in establishing a machine QA program is to identify the equipment that is available for use in the MRgRT. Most radiotherapy dosimeters were developed for use outside the presence of a magnetic field and are therefore not suitable for use in an MR environment.6 With the initiation of MRgRT, several vendors have developed MR-compatible dosimeters to meet the needs for this emerging technology. For the low-field MR-linac, several MR-compatible dosimeters have been tested and

Imaging QA

Many imaging QA tasks involve verifying the traditional image quality metrics that are common to all MRI imaging systems, and the tools for testing these have already been developed within the diagnostic imaging workspace. However, due to the different focuses of the diagnostic and therapeutic groups, it is important that radiotherapy QA pays special attention to verify the geometric integrity and verification of the method for utilizing MR images for electron density information. Additionally,

Patient-Specific QA

Treatment planning for the low-field MR system utilizes step-and-shoot intensity modulated radiotherapy (IMRT) with the field shaped by a double-stacked, double-focused multileaf collimator (MLC) system utilizing 138 nonferromagnetic tungsten alloy leaves, with each individual leaf having a thickness of 0.83 cm and a leaf height of 5.5 cm. The 2 stacks of MLCs are off-set by half the width of an MLC leaf so that the effective MLC leaf width at isocenter is 0.415 cm, which is suitable for

Declaration of Competing Interest

Joshua Kim has served as a consultant for ViewRay.

Funding

The author was supported in part by an Internal Mentored Grant.

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