title: Respiratory gating based on internal electromagnetic motion monitoring during stereotactic liver radiation therapy: First results.
abstract: BACKGROUND
  Intrafraction motion may compromise the target dose in stereotactic body radiation therapy (SBRT) of tumors in the liver. Respiratory gating can improve the treatment delivery, but gating based on an external surrogate signal may be inaccurate. This is the first paper reporting on respiratory gating based on internal electromagnetic monitoring during liver SBRT.
  
  
  METHODS
  Two patients with solitary liver metastases were treated with respiratory-gated SBRT guided by three implanted electromagnetic transponders. The treatment was delivered in end-exhale with beam-on when the centroid of the three transponders deviated less than 3 mm [left-right (LR) and anterior-posterior (AP) directions] and 4mm [cranio-caudal (CC)] from the planned position. For each treatment fraction, log files were used to determine the transponder motion during beam-on in the actual gated treatments and in simulated treatments without gating. The motion was used to reconstruct the dose to the clinical target volume (CTV) with and without gating. The reduction in D95 (minimum dose to 95% of the CTV) relative to the plan was calculated for both treatment courses.
  
  
  RESULTS
  With gating the maximum course mean (standard deviation) geometrical error in any direction was 1.2 mm (1.8 mm). Without gating the course mean error would mainly increase for Patient 1 [to -2.8 mm (1.6 mm) (LR), 7.1 mm (5.8 mm) (CC), -2.6 mm (2.8mm) (AP)] due to a large systematic cranial baseline drift at each fraction. The errors without gating increased only slightly for Patient 2. The reduction in CTV D95 was 0.5% (gating) and 12.1% (non-gating) for Patient 1 and 0.3% (gating) and 1.7% (non-gating) for Patient 2. The mean duty cycle was 55%.
  
  
  CONCLUSIONS
  Respiratory gating based on internal electromagnetic motion monitoring was performed for two liver SBRT patients. The gating added robustness to the dose delivery and ensured a high CTV dose even in the presence of large intrafraction motion.
authors.affiliation: a Department of Oncology , Aarhus University Hospital , Denmark.|b Institute of Clinical Medicine, Aarhus University , Denmark.
authors.forename: Per Rugaard
authors.initials: PR
authors.lastname: Poulsen
authors.affiliation: c Department of Medical Physics , Aarhus University Hospital , Denmark.
authors.forename: Esben Schjødt
authors.initials: ES
authors.lastname: Worm
authors.affiliation: c Department of Medical Physics , Aarhus University Hospital , Denmark.
authors.forename: Rune
authors.initials: R
authors.lastname: Hansen
authors.affiliation: d Department of Radiology , Aarhus University Hospital , Denmark.
authors.forename: Lars Peter
authors.initials: LP
authors.lastname: Larsen
authors.affiliation: a Department of Oncology , Aarhus University Hospital , Denmark.|b Institute of Clinical Medicine, Aarhus University , Denmark.
authors.forename: Cai
authors.initials: C
authors.lastname: Grau
authors.affiliation: a Department of Oncology , Aarhus University Hospital , Denmark.|b Institute of Clinical Medicine, Aarhus University , Denmark.
authors.forename: Morten
authors.initials: M
authors.lastname: Høyer
mesh_terms: D000368:Aged; D055590:Electromagnetic Phenomena; D005240:Feasibility Studies; D005260:Female; D006801:Humans; D008113:Liver Neoplasms / Q000556:secondary / Q000601:surgery*; D008297:Male; D016343:Monitoring, Intraoperative / Q000295:instrumentation*; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D012119:Respiration