Accurate Monte Carlo simulation of megavoltage beams coupled to external magnetic fields

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A novel technology integrating magnetic resonance imaging to megavoltage radiotherapy machines promises unique advantages for image guidance over x-ray based techniques, with high-contrast for target definition and no imaging radiation dose to the patient. With the advent of such technologies, the clinical routine of conventional radiotherapy must be completely adapted to assure optimal treatment and patient safety. One important topic that needs to be addressed is radiation dosimetry. At strengths used in magnetic resonance imaging, magnetic fields can have a significant impact on the energy deposition due to the force acting charged particles set in motion by the beam. Modeling these effects to obtain dose distributions in heterogeneous geometries requires using the Monte Carlo method, since other techniques commercially available are obsolete in these conditions. One main effect of magnetic fields on energy deposition is a spatial distortion of the dose distribution, which among other factors varies with the geometry and the orientation of the beam with respect to the field. It is recognized that the Monte Carlo method has the potential to reproduce the energy deposition of megavoltage photon beams in the presence of magnetic fields to an accuracy level better than clinical requirements. The main purpose of the research program is to improve and validate Monte Carlo algorithms for simulating the transport and energy deposition of megavoltage radiation in heterogeneous geometries in the presence of external magnetic fields. Over the next decade, MRI-integrated radiotherapy is expected to become widely used in clinics. Therefore this research program anticipates significant interest and impact in radiotherapy, achieving accurate radiation dosimetry modeling of megavoltage beams coupled to magnetic fields. The outcomes of the program will contribute to meeting the same standards in beam calibration, beam modeling and patient dose calculation as currently achieved with the technology and methods currently available for modalities used in the absence of magnetic fields, such as conventional radiotherapy linear accelerators.