Benchmarking FLUKA and Penelope main codes for characterizing magnetic field effects on dose distributions in MRI-Linac technique

External radiotherapy is based on megavoltage electron or photon beams produced by clinical linear accelerators. Image guided radiotherapy has been proposed and clinically incorporated during the last decades aimed at using imaging techniques during radiation therapy to improve treatment accuracy by equipping linear accelerators with imaging technology to allow patient scanning during treatment and comparisons with previous reference images enable to adjust more precisely tumor targeting. As known, magnetic resonance imaging is one of the more accurate imaging techniques capable of suitable distinction among different soft tissues, however it is based on strong magnetic fields that may represent non negligible drawbacks if considered to be coupled with linear accelerators. Incorporating magnetic resonance scanners to clinical linear accelerators may constitute a high precision online image guide radiotherapy strategy. This work provides a framework to investigate the effects of strong magnetic fields in dose distributions due to incident photon beams, typically used in clinics. The proposed approach consists of investigating and characterizing two independent well-validated Monte Carlo simulation main codes, FLUKA and Penelope. Effects on charged particle track have been carefully investigated and inter-compared between both main codes, remarking excellent agreement among them as well as when compared with analytical relativistic formulation. Additionally, dosimetry variations/distortions due to the presence of magnetic fields as required for MRI-Linac technique have been obtained using dedicated phantoms including interfaces and inhomogeneities. (author)