title: Radiotherapy beyond cancer: target localization in real-time MRI and treatment planning for cardiac radiosurgery.
abstract: OBJECTIVE
  Atrial fibrillation (AFib) is the most common cardiac arrhythmia that affects millions of patients world-wide. AFib is usually treated with minimally invasive, time consuming catheter ablation techniques. While recently noninvasive radiosurgery to the pulmonary vein antrum (PVA) in the left atrium has been proposed for AFib treatment, precise target location during treatment is challenging due to complex respiratory and cardiac motion. A MRI linear accelerator (MRI-Linac) could solve the problems of motion tracking and compensation using real-time image guidance. In this study, the authors quantified target motion ranges on cardiac magnetic resonance imaging (MRI) and analyzed the dosimetric benefits of margin reduction assuming real-time motion compensation was applied.
  
  
  METHODS
  For the imaging study, six human subjects underwent real-time cardiac MRI under free breathing. The target motion was analyzed retrospectively using a template matching algorithm. The planning study was conducted on a CT of an AFib patient with a centrally located esophagus undergoing catheter ablation, representing an ideal case for cardiac radiosurgery. The target definition was similar to the ablation lesions at the PVA created during catheter treatment. Safety margins of 0 mm (perfect tracking) to 8 mm (untracked respiratory motion) were added to the target, defining the planning target volume (PTV). For each margin, a 30 Gy single fraction IMRT plan was generated. Additionally, the influence of 1 and 3 T magnetic fields on the treatment beam delivery was simulated using Monte Carlo calculations to determine the dosimetric impact of MRI guidance for two different Linac positions.
  
  
  RESULTS
  Real-time cardiac MRI showed mean respiratory target motion of 10.2 mm (superior-inferior), 2.4 mm (anterior-posterior), and 2 mm (left-right). The planning study showed that increasing safety margins to encompass untracked respiratory motion leads to overlapping structures even in the ideal scenario, compromising either normal tissue dose constraints or PTV coverage. The magnetic field caused a slight increase in the PTV dose with the in-line MRI-Linac configuration.
  
  
  CONCLUSIONS
  The authors' results indicate that real-time tracking and motion compensation are mandatory for cardiac radiosurgery and MRI-guidance is feasible, opening the possibility of treating cardiac arrhythmia patients completely noninvasively.
authors.affiliation: Radiation Physics Laboratory, Sydney Medical School, The University of Sydney, Sydney, New South Wales 2006, Australia and Institute for Robotics and Cognitive Systems, University of Luebeck, Luebeck 23562, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Ipsen
authors.affiliation: Department of Radiation Oncology, University of Luebeck and University Medical Center Schleswig-Holstein, Campus Luebeck, Luebeck 23562, Germany.
authors.forename: O
authors.initials: O
authors.lastname: Blanck
authors.affiliation: Illawarra Cancer Care Centre (ICCC), Wollongong, New South Wales 2500, Australia and Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, New South Wales 2500, Australia.
authors.forename: B
authors.initials: B
authors.lastname: Oborn
authors.affiliation: Medical Department II, University of Luebeck and University Medical Center Schleswig-Holstein, Campus Luebeck, Luebeck 23562, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Bode
authors.affiliation: Ingham Institute for Applied Medical Research, Liverpool Hospital, Liverpool, New South Wales 2170, Australia.
authors.forename: G
authors.initials: G
authors.lastname: Liney
authors.affiliation: Department of Radiology and Nuclear Medicine, University of Luebeck and University Medical Center Schleswig-Holstein, Campus Luebeck, Luebeck 23562, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Hunold
authors.affiliation: Department of Radiation Oncology, University of Luebeck and University Medical Center Schleswig-Holstein, Campus Luebeck, Luebeck 23562, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Rades
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Luebeck, Luebeck 23562, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Schweikard
authors.affiliation: Radiation Physics Laboratory, Sydney Medical School, The University of Sydney, Sydney, New South Wales 2006, Australia.
authors.forename: P J
authors.initials: PJ
authors.lastname: Keall
mesh_terms: D000465:Algorithms; D001281:Atrial Fibrillation / Q000473:pathology / Q000503:physiopathology / Q000601:surgery; D006348:Cardiac Surgical Procedures / Q000379:methods*; D003198:Computer Simulation; D006321:Heart / Q000000981:diagnostic imaging / Q000503:physiopathology; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D060526:Magnetic Fields; D053783:Magnetic Resonance Imaging, Interventional / Q000379:methods*; D008297:Male; D009010:Monte Carlo Method; D009038:Motion; D009206:Myocardium / Q000473:pathology; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012119:Respiration; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed