title: Incorrect dosimetric leaf separation in IMRT and VMAT treatment planning: Clinical impact and correlation with pretreatment quality assurance.
abstract: OBJECTIVE
  Dynamic treatment planning algorithms use a dosimetric leaf separation (DLS) parameter to model the multi-leaf collimator (MLC) characteristics. Here, we quantify the dosimetric impact of an incorrect DLS parameter and investigate whether common pretreatment quality assurance (QA) methods can detect this effect.
  
  
  METHODS
  16 treatment plans with intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) technique for multiple treatment sites were calculated with a correct and incorrect setting of the DLS, corresponding to a MLC gap difference of 0.5mm. Pretreatment verification QA was performed with a bi-planar diode array phantom and the electronic portal imaging device (EPID). Measurements were compared to the correct and incorrect planned doses using gamma evaluation with both global (G) and local (L) normalization. Correlation, specificity and sensitivity between the dose volume histogram (DVH) points for the planning target volume (PTV) and the gamma passing rates were calculated.
  
  
  RESULTS
  The change in PTV and organs at risk DVH parameters were 0.4-4.1%. Good correlation (>0.83) between the PTVmean dose deviation and measured gamma passing rates was observed. Optimal gamma settings with 3%L/3mm (per beam and composite plan) and 3%G/2mm (composite plan) for the diode array phantom and 2%G/2mm (composite plan) for the EPID system were found. Global normalization and per beam ROC analysis of the diode array phantom showed an area under the curve <0.6.
  
  
  CONCLUSIONS
  A DLS error can worsen pretreatment QA using gamma analysis with reasonable credibility for the composite plan. A low detectability was demonstrated for a 3%G/3mm per beam gamma setting.
authors.affiliation: Radiotherapy Research Unit, Department of Oncology, Herlev & Gentofte Hospital, University of Copenhagen, Denmark. Electronic address: maria.sjoelin@regionh.dk.
authors.forename: Maria
authors.initials: M
authors.lastname: Sjölin
authors.affiliation: Radiotherapy Research Unit, Department of Oncology, Herlev & Gentofte Hospital, University of Copenhagen, Denmark.
authors.forename: Jens Morgenthaler
authors.initials: JM
authors.lastname: Edmund
mesh_terms: D016477:Artifacts; D055615:Electrical Equipment and Supplies; D006801:Humans; D010315:Particle Accelerators; D011785:Quality Assurance, Health Care; D011874:Radiometry / Q000295:instrumentation*; D011880:Radiotherapy Planning, Computer-Assisted / Q000592:standards*; D050397:Radiotherapy, Intensity-Modulated
keywords: Dosimetry; MLC; QA; VMAT