MRI-LINAC dosimetry approach by Monte Carlo methods coupling charged particle radiation transport with strong magnetic fields

A. GAYOL; M. VALENTE

Kuala Lumpur

Congreso; 15th International Symposium of Radiation Physics - ISRP; 2021

Sunway University - Int. Soc. rad. Physics

MRI-LINAC devices incorporate magnetic resonance imaging systems to therapeutic procedures with megavoltage photon beams produced by linear clinical accelerators. MRI-LINAC devices are considered to represent a new type of technology with great expectations for high precision radiotherapy since it allows treatment guidance by means of in-situ images of the patient. The strong magnetic fields used in these equipments influence the trajectories of secondary electrons, and charged particles in general, by means of the Lorentz force. Thus, modifying the ionizing radiation field, which may cause local variations in the absorbed dose distribution. The proper implementation of the external electromagnetic field coupling in Monte Carlo simulation codes is a key issue to confirm the feasibility of using such a tool to describe this type of complex applications along with the subsequent dosimetric effects. This work provides a framework to carefully investigate the effects of magnetic fields on charged particle tracks and on dose distributions, due to incident photon beams typically used in clinics. PENELOPE and FLUKA simulation main codes were used to study the deflection of the electron trajectories, while evaluating dosimetric effects, considering simplified arrangements to complex clinical setups. Comparisons of the numerical results achieved for the radius of curvature of the trajectories with an analytic relativistic formulation was satisfactory, and dosimetry distortions due to the presence of strong magnetic field, as required for MRI-LINAC technique, have been obtained using dedicated phantoms including interfaces and inhomogeneities as well as preliminary approaches for patient-specific clinical cases.