title: Lung stereotactic body radiotherapy with an MR-linac - Quantifying the impact of the magnetic field and real-time tumor tracking.
abstract: There are concerns that radiotherapy doses delivered in a magnetic field might be distorted due to the Lorentz force deflecting secondary electrons. This study investigates this effect on lung stereotactic body radiotherapy (SBRT) treatments, conducted either with or without multileaf collimator (MLC) tumor tracking.
  
  
  
  Lung SBRT treatments with an MR-linac were simulated for nine patients. Two different treatment techniques were compared: conventional, non-tracked deliveries and deliveries with real-time MLC tumor tracking, each conducted either with or without a 1.5T magnetic field.
  
  
  
  Slight dose distortions at air-tissue-interfaces were observed in the presence of the magnetic field. Most prominently, the dose to 2% of the skin increased by 1.4Gy on average. Regardless of the presence of the magnetic field, MLC tracking was able to spare healthy tissue, for example by decreasing the mean lung dose by 0.3Gy on average, while maintaining the target dose.
  
  
  
  Accounting for the magnetic field during treatment plan optimization allowed for design and delivery of clinically acceptable lung SBRT treatments with an MR-linac. Furthermore, the ability of MLC tumor tracking to decrease dose exposure of healthy tissue, was not inhibited by the magnetic field.
authors.affiliation: Joint Department of Physics at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, UK. Electronic address: martin.menten@icr.ac.uk.
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Menten
authors.affiliation: Joint Department of Physics at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, UK.
authors.forename: Martin F
authors.initials: MF
authors.lastname: Fast
authors.affiliation: Joint Department of Physics at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, UK.
authors.forename: Simeon
authors.initials: S
authors.lastname: Nill
authors.affiliation: Joint Department of Physics at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, UK.
authors.forename: Cornelis P
authors.initials: CP
authors.lastname: Kamerling
authors.affiliation: Joint Department of Physics at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, UK.
authors.forename: Fiona
authors.initials: F
authors.lastname: McDonald
authors.affiliation: Joint Department of Physics at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, UK. Electronic address: uwe.oelfke@icr.ac.uk.
authors.forename: Uwe
authors.initials: U
authors.lastname: Oelfke
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D004583:Electrons; D005260:Female; D006801:Humans; D008175:Lung Neoplasms / Q000532:radiotherapy*; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging; D008297:Male; D010315:Particle Accelerators; D016634:Radiosurgery; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted
keywords: Lung stereotactic body radiotherapy; MR-guided radiotherapy; MR-linac; Real-time adaptive radiotherapy; Tumor tracking