High Resolution Radiation Therapy Dosimetry in Magnetic Fields using Novel Silicon Array Dosimeters: A Pilot for MRI-linac Applications

The integration of online magnetic resonance imaging with linear accelerators presents an exciting new era for radiation therapy. A hybrid MRI-linac machine provides improved soft-tissue contrast enabling the target to be visualised at the time of treatment rather than a surrogate, as is currently common practice. With this advancement comes a number of unique challenges related to the presence of a magnetic field during treatment, one of which is the change in the dose distribution. The magnetic field’s associated force vector, the Lorentz force, influences the trajectory of charged particles within it. Dose-depositing secondary electrons are no exception. Using Monte Carlo simulations, point spread dose arrays have been modelled to demonstrate on a fundamental level how dose distributions are perturbed by magnetic fields as a function of magnetic field strength, orientation with respect to the primary beam and the density of the medium. For the case of inline magnetic fields, the point spread distribution maintains its symmetry, but lengthens and becomes more narrow laterally.