en Radiation Biology Radiation Biology تحقيق بديع Original Research <div style="text-align: justify;"><span style="font-size:10pt"><span style="text-justify:newspaper"><span style="text-kashida-space:50%"><span style="line-height:119%"><span style="font-family:Calibri"><span style="color:black"><span lang="en-US" style="font-size:9.0pt"><span style="font-family:Calibri"><span style="color:#1f497d"><span style="font-style:italic"><span style="font-weight:bold"><span style="language:en-US">Background:</span></span></span></span></span></span> <span lang="en-US" style="font-size:9.0pt"><span style="font-family:Calibri"><span style="language:en-US">The advent of Magnetic Resonance-guided (MR-guided) radiotherapy, facilitated by MR-guided linear accelerators (MR-LINACs), has significantly enhanced the availability of online adaptive radiotherapy treatments. Research indicates that a static magnetic field influences the amount of radiation absorbed by patients undergoing MR-guided radiotherapy. This study employed Monte Carlo simulations to assess the dose distribution of Photon, Proton, and Carbon ion beams in the absence and presence of magnetic fields. </span></span></span><span lang="en-US" style="font-size:9.0pt"><span style="font-family:Calibri"><span style="color:#1f497d"><span style="font-style:italic"><span style="font-weight:bold"><span style="language:en-US">Materials and Methods: </span></span></span></span></span></span><span lang="en-US" style="font-size:9.0pt"><span style="font-family:Calibri"><span style="language:en-US">All simulations were performed using version 9.1 of the GATE software. The experimental configuration consisted of a water phantom equipped with a one mm-thick air slit oriented parallel to the x-y plane. Energy and dose measurements from both the primary beam and secondary electrons were conducted using DoseActor. </span></span></span><span lang="en-US" style="font-size:9.0pt"><span style="font-family:Calibri"><span style="color:#1f497d"><span style="font-style:italic"><span style="font-weight:bold"><span style="language:en-US">Results: </span></span></span></span></span></span><span lang="en-US" style="font-size:9.0pt"><span style="font-family:Calibri"><span style="language:en-US">The observed magnetic field shifted the Bragg peak for both proton and carbon ion beams, with an increasing displacement correlating to higher primary beam energy levels. Furthermore, there was a positive correlation between secondary electron doses and magnetic field intensity; in scenarios involving a 7 Tesla magnetic field, photon doses at the water-air interface increased considerably. Additionally, at the air slit location, doses from photon-induced secondary electrons markedly increased at this interface. </span></span></span><span lang="en-US" style="font-size:9.0pt"><span style="font-family:Calibri"><span style="color:#1f497d"><span style="font-style:italic"><span style="font-weight:bold"><span style="language:en-US">Conclusion: </span></span></span></span></span></span><span lang="en-US" style="font-size:9.0pt"><span style="font-family:Calibri"><span style="language:en-US">The Electron Return Effect (ERE) influences dose distribution at water-air boundaries. Increased initial energies and more robust magnetic fields exacerbate these phenomena, emphasizing the critical importance of rigorous deliberation in the treatment planning process.</span></span></span></span></span></span></span></span></span></div> Comprehensive Magnetic Fields, Carbon-Ion Therapy, Monte Carlo Method, Proton Therapy, Protons. 1029 1036 http://ijrr.com/browse.php?a_code=A-10-1-1431&slc_lang=en&sid=1 S. Bagherzadeh 7900319475328460032498 7900319475328460032498 No Department of Medical Physics, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran H. Rajabi hrajabi@modares.ac.ir 7900319475328460032499 7900319475328460032499 Yes Department of Medical Physics, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran