In recent years, MR-Linac, a radiotherapy linear accelerator (linac) equipped with magnetic resonance (MR) imaging, has been deployed in clinical facilities across Japan. Because of the magnetic field of MR-Linac, which can affect the dose distributions and dose response of ionization chambers, conventional reference dosimetry for absorbed dose to water using an ionization chamber becomes impractical. Consequently, the magnetic field effect should be considered in the reference dosimetry for MR-Linac. Although numerous studies have delved into this matter and several magnetic field correction methods have been proposed to extend the conventional formalism, a practical protocol for reference dosimetry for MR-Linac remains elusive.

The purpose of this review are as follows: (i) to summarize and evaluate literature and existing datasets as well as identify any gaps that highlight areas for the future research on this topic; (ii) to elucidate dosimetric challenges associated with ionization chamber dosimetry in magnetic fields; and (iii) to propose a formalism for reference dosimetry for MR-Linac based on available literature and datasets. This review focuses on studies based on commercially available MR-Linacs and datasets, specifically tailored for reference-class cylindrical ion chambers.

The radiotherapy is performed with the aim of delivering the optimal dose to the target volume with minimal side effect of surrounding normal tissue. For this purpose, quality assurance is essential to ensure that the target volume is correctly irradiated in the optimal geometrical arrangement, and the absorbed dose evaluation is essential to ensure that the prescribed dose is correctly delivered. The absorbed doses are generally evaluated using a small cavity ionization chamber that utilizes gas ionization. For the evaluation of absorbed dose to water using ionization chambers, the national dose and charge standards, ionization chambers and electrometer calibration systems are required. And it is also required standard dosimetry protocol that recommend conditions such as fields, depths, and optimal ionization chambers for the measurement, as well as reliable physical data. This manuscript reviews the transition of standard dosimetry of absorbed dose to water in external beam radiotherapy, including the background of dose standards, ionization chamber calibration systems, units, and physical constants.