Abstract
Purpose:In MRI‐linac treatments, radiation dose distributions are affected by magnetic fields, especially at high‐density/low‐density interfaces. Radiobiological consequences of magnetic field dose effects are presently unknown; therefore, preclinical studies are needed to ensure the safe clinical use of MRI‐linacs. This study investigates the optimal combination of beam energy and magnetic field strength needed for preclinical murine studies.Methods:The Monte Carlo code MCNP6 was used to simulate the effects of a magnetic field when irradiating a mouse‐sized lung phantom with a 1.0cmx1.0cm photon beam. Magnetic field effects were examined using various beam energies (225kVp, 662keV[Cs‐137], and 1.25MeV[Co‐60]) and magnetic field strengths (0.75T, 1.5T, and 3T). The resulting dose distributions were compared to Monte Carlo results for humans with various field sizes and patient geometries using a 6MV/1.5T MRI‐linac.Results:In human simulations, the addition of a 1.5T magnetic field caused an average dose increase of 49% (range:36%–60%) to lung at the soft tissue‐to‐lung interface and an average dose decrease of 30% (range:25%–36%) at the lung‐to‐soft tissue interface. In mouse simulations, the magnetic fields had no effect on the 225kVp dose distribution. The dose increases for the Cs‐137 beam were 12%, 33%, and 49% for 0.75T, 1.5T, and 3.0T magnetic fields, respectively while the dose decreases were 7%, 23%, and 33%. For the Co‐60 beam, the dose increases were 14%, 45%, and 41%, and the dose decreases were 18%, 35%, and 35%.Conclusion:The magnetic field dose effects observed in mouse phantoms using a Co‐60 beam with 1.5T or 3T fields and a Cs‐137 beam with a 3T field compare well with those seen in simulated human treatments with an MRI‐linac. These irradiator/magnet combinations are suitable for preclinical studies investigating potential biological effects of delivering radiation therapy in the presence of a magnetic field.Partially funded by Elekta.
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