title: Determination of the optimal tolerance for MLC positioning in sliding window and VMAT techniques.
abstract: OBJECTIVE
  Several authors have recommended a 2 mm tolerance for multileaf collimator (MLC) positioning in sliding window treatments. In volumetric modulated arc therapy (VMAT) treatments, however, the optimal tolerance for MLC positioning remains unknown. In this paper, the authors present the results of a multicenter study to determine the optimal tolerance for both techniques.
  
  
  METHODS
  The procedure used is based on dynalog file analysis. The study was carried out using seven Varian linear accelerators from five different centers. Dynalogs were collected from over 100,000 clinical treatments and in-house software was used to compute the number of tolerance faults as a function of the user-defined tolerance. Thus, the optimal value for this tolerance, defined as the lowest achievable value, was investigated.
  
  
  RESULTS
  Dynalog files accurately predict the number of tolerance faults as a function of the tolerance value, especially for low fault incidences. All MLCs behaved similarly and the Millennium120 and the HD120 models yielded comparable results. In sliding window techniques, the number of beams with an incidence of hold-offs >1% rapidly decreases for a tolerance of 1.5 mm. In VMAT techniques, the number of tolerance faults sharply drops for tolerances around 2 mm. For a tolerance of 2.5 mm, less than 0.1% of the VMAT arcs presented tolerance faults.
  
  
  CONCLUSIONS
  Dynalog analysis provides a feasible method for investigating the optimal tolerance for MLC positioning in dynamic fields. In sliding window treatments, the tolerance of 2 mm was found to be adequate, although it can be reduced to 1.5 mm. In VMAT treatments, the typically used 5 mm tolerance is excessively high. Instead, a tolerance of 2.5 mm is recommended.
authors.affiliation: Department of Medical Physics, Hospital Sant Joan de Reus, IISPV, Tarragona 43204, Spain.
authors.forename: V
authors.initials: V
authors.lastname: Hernandez
authors.affiliation: Department of Medical Physics, Hospital Sant Joan de Reus, IISPV, Tarragona 43204, Spain.
authors.forename: R
authors.initials: R
authors.lastname: Abella
authors.affiliation: Department of Radiation Oncology, Hospital Quirón, Barcelona 08023, Spain.
authors.forename: J F
authors.initials: JF
authors.lastname: Calvo
authors.affiliation: Department of Medical Physics, Institut Català d'Oncologia, Girona 17007, Spain.
authors.forename: D
authors.initials: D
authors.lastname: Jurado-Bruggemann
authors.affiliation: Department of Medical Physics, Institut Català d'Oncologia, L'Hospitalet de Llobregat 08908, Spain.
authors.forename: I
authors.initials: I
authors.lastname: Sancho
authors.affiliation: Department of Medical Physics, Hospital de la Santa Creu i Sant Pau, Barcelona 08041, Spain.
authors.forename: P
authors.initials: P
authors.lastname: Carrasco
mesh_terms: D005240:Feasibility Studies; D010315:Particle Accelerators / Q000295:instrumentation*; D011874:Radiometry; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation* / Q000379:methods*; D012984:Software