title: Influence of the jaw tracking technique on the dose calculation accuracy of small field VMAT plans.
abstract: OBJECTIVE
  The aim of this study was to evaluate experimentally the accuracy of the dose calculation algorithm AcurosXB in small field highly modulated Volumetric Modulated Arc Therapy (VMAT).
  
  
  METHODS
  The 1000SRS detector array inserted in the rotational Octavius 4D phantom (PTW) was used for 3D dose verification of VMAT treatments characterized by small to very small targets. Clinical treatment plans (n = 28) were recalculated on the phantom CT data set in the Eclipse TPS. All measurements were done on a Varian TrueBeamSTx, which can provide the jaw tracking technique (JTT). The effect of disabling the JTT, thereby fixing the jaws at static field size of 3 × 3 cm2and applying the MLC to shape the smallest apertures, was investigated for static fields between 0.5 × 0.5-3 × 3 cm2and for seven VMAT patients with small brain metastases. The dose calculation accuracy has been evaluated by comparing the measured and calculated dose outputs and dose distributions. The dosimetric agreement has been presented by a local gamma evaluation criterion of 2%/2 mm.
  
  
  RESULTS
  Regarding the clinical plans, the mean ± SD of the volumetric gamma evaluation scores considering the dose levels for evaluation of 10%, 50%, 80% and 95% are (96.0 ± 6.9)%, (95.2 ± 6.8)%, (86.7 ± 14.8)% and (56.3 ± 42.3)% respectively. For the smallest field VMAT treatments, discrepancies between calculated and measured doses up to 16% are obtained. The difference between the 1000SRS central chamber measurements compared to the calculated dose outputs for static fields 3 × 3, 2 × 2, 1 × 1 and 0.5 × 0.5 cm2collimated with MLC whereby jaws are fixed at 3 × 3 cm2and for static fields shaped with the collimator jaws only (MLC retracted), is on average respectively, 0.2%, 0.8%, 6.8%, 5.7% (6 MV) and 0.1%, 1.3%, 11.7%, 21.6% (10 MV). For the seven brain mets patients was found that the smaller the target volumes, the higher the improvement in agreement between measured and calculated doses after disabling the JTT.
  
  
  CONCLUSIONS
  Fixing the jaws at 3 × 3 cm2and using the MLC with high positional accuracy to shape the smallest apertures in contrast to the JTT is currently found to be the most accurate treatment technique.
authors.affiliation: Radiotherapy Department, Maastro Clinic, 6229 ET, Maastricht, The Netherlands.
authors.forename: Ans C C
authors.initials: AC
authors.lastname: Swinnen
authors.affiliation: Radiotherapy Department, Maastro Clinic, 6229 ET, Maastricht, The Netherlands.
authors.forename: Michel C
authors.initials: MC
authors.lastname: Öllers
authors.affiliation: Radiotherapy Department, Maastro Clinic, 6229 ET, Maastricht, The Netherlands.
authors.forename: Erik
authors.initials: E
authors.lastname: Roijen
authors.affiliation: Radiotherapy Department, Maastro Clinic, 6229 ET, Maastricht, The Netherlands.
authors.forename: Sebastiaan M
authors.initials: SM
authors.lastname: Nijsten
authors.affiliation: Radiotherapy Department, Maastro Clinic, 6229 ET, Maastricht, The Netherlands.
authors.forename: Frank
authors.initials: F
authors.lastname: Verhaegen
mesh_terms: D000465:Algorithms; D001932:Brain Neoplasms / Q000601:surgery*; D006801:Humans; D007574:Jaw Relation Record; D058958:Organs at Risk / Q000528:radiation effects; D010347:Patient Care Planning; D019047:Phantoms, Imaging; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*
keywords: calculation algorithm; dosimetry; jaw tracking technique; small fields