title: A dosimetric characterization of a novel linear accelerator collimator.
abstract: OBJECTIVE
  The aim of this work is to characterize a new linear accelerator collimator which contains a single pair of sculpted diaphragms mounted orthogonally to a 160 leaf multileaf collimator (MLC). The diaphragms have "thick" regions providing full attenuation and "thin" regions where attenuation is provided by both the leaves and the diaphragm. The leaves are mounted on a dynamic leaf guide allowing rapid leaf motion and leaf travel over 350 mm.
  
  
  METHODS
  Dosimetric characterization, including assessment of leaf transmission, leaf tip transmission, penumbral width, was performed in a plotting tank. Head scatter factor was measured using a mini-phantom and the effect of leaf guide position on output was assessed using a water phantom. The tongue and groove effect was assessed using multiple exposures on radiochromic film. Leaf reproducibility was assessed from portal images of multiple abutting fields.
  
  
  RESULTS
  The maximum transmission through the multileaf collimator is 0.44% at 6 MV and 0.52% at 10 MV. This reduced to 0.22% and 0.27%, respectively, when the beam passes through the dynamic leaf guide in addition to the MLC. The maximum transmission through the thick part of the diaphragm is 0.32% and 0.36% at 6 and 10 MV. The combination of leaf and diaphragm transmission ranges from 0.08% to 0.010% at 6 MV and 0.10% to 0.14% depending on whether the shielding is through the thick or thin part of the diaphragm. The off-axis intertip transmission for a zero leaf gap is 2.2% at 6 and 10 MV. The leaf tip penumbra for a 100 × 100 mm field ranges from 5.4 to 4.3 mm at 6 and 10 MV across the full range of leaf motion when measured in the AB direction, which reduces to 4.0-3.4 mm at 6 MV and 4.5-3.8 mm at 10 MV when measured in the GT direction. For a 50 × 50 mm field, the diaphragm penumbra ranges from 4.3 to 3.7 mm at 6 MV and 4.5 to 4.1 mm at 10 MV in the AB direction and 3.7 to 3.2 mm at 6 MV and 4.2 to 3.7 mm when measured in the GT direction. The tongue and groove effect observed from exposure of a radiochromic film to two abutting fields is an underdose of 25%. The head scatter factor at both 6 and 10 MV is similar to that from the MLCi2 collimator to within 0.8%. The uncertainty in the leaf position reproducibility is 0.05 mm (2σ).
  
  
  CONCLUSIONS
  The Agility collimator is a low leakage, high definition collimator where both the MLC and the sculpted diaphragm have been optimized for dynamic treatments.
authors.affiliation: Leeds Cancer Centre, Bexley Wing, St. James' University Hospital, Leeds LS9 7TF, United Kingdom.
authors.forename: C M
authors.initials: CM
authors.lastname: Thompson
authors.affiliation: Leeds Cancer Centre, Bexley Wing, St. James' University Hospital, Leeds LS9 7TF, United Kingdom.
authors.forename: S J
authors.initials: SJ
authors.lastname: Weston
authors.affiliation: Leeds Cancer Centre, Bexley Wing, St. James' University Hospital, Leeds LS9 7TF, United Kingdom.
authors.forename: V C
authors.initials: VC
authors.lastname: Cosgrove
authors.affiliation: Institute of Medical Physics, School of Physics, University of Sydney, Sydney NSW 2006, Australia and Division of Medical Physics, Leeds Institute of Genetics, Health and Therapeutics, University of Leeds, Leeds LS2 9JT, United Kingdom.
authors.forename: D I
authors.initials: DI
authors.lastname: Thwaites
mesh_terms: D004867:Equipment Design; D005373:Film Dosimetry / Q000379:methods*; D006801:Humans; D009038:Motion; D010315:Particle Accelerators / Q000295:instrumentation*; D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D015203:Reproducibility of Results