title: Pre-segmented 2-Step IMRT with subsequent direct machine parameter optimisation - a planning study.
abstract: BACKGROUND
  Modern intensity modulated radiotherapy (IMRT) mostly uses iterative optimisation methods. The integration of machine parameters into the optimisation process of step and shoot leaf positions has been shown to be successful. For IMRT segmentation algorithms based on the analysis of the geometrical structure of the planning target volumes (PTV) and the organs at risk (OAR), the potential of such procedures has not yet been fully explored. In this work, 2-Step IMRT was combined with subsequent direct machine parameter optimisation (DMPO-Raysearch Laboratories, Sweden) to investigate this potential.
  
  
  METHODS
  In a planning study DMPO on a commercial planning system was compared with manual primary 2-Step IMRT segment generation followed by DMPO optimisation. 15 clinical cases and the ESTRO Quasimodo phantom were employed. Both the same number of optimisation steps and the same set of objective values were used. The plans were compared with a clinical DMPO reference plan and a traditional IMRT plan based on fluence optimisation and consequent segmentation. The composite objective value (the weighted sum of quadratic deviations of the objective values and the related points in the dose volume histogram) was used as a measure for the plan quality. Additionally, a more extended set of parameters was used for the breast cases to compare the plans.
  
  
  RESULTS
  The plans with segments pre-defined with 2-Step IMRT were slightly superior to DMPO alone in the majority of cases. The composite objective value tended to be even lower for a smaller number of segments. The total number of monitor units was slightly higher than for the DMPO-plans. Traditional IMRT fluence optimisation with subsequent segmentation could not compete.
  
  
  CONCLUSIONS
  2-Step IMRT segmentation is suitable as starting point for further DMPO optimisation and, in general, results in less complex plans which are equal or superior to plans generated by DMPO alone.
authors.affiliation: Klinik und Poliklinik für Strahlentherapie, Universität Würzburg, Josef-Schneider-Str, 11, 97080 Würzburg, Germany. Bratengeier_K@klinik.uni-wuerzburg.de
authors.forename: Klaus
authors.initials: K
authors.lastname: Bratengeier
authors.forename: Jürgen
authors.initials: J
authors.lastname: Meyer
authors.forename: Michael
authors.initials: M
authors.lastname: Flentje
mesh_terms: D000465:Algorithms; D001943:Breast Neoplasms / Q000532:radiotherapy; D006801:Humans; D021621:Imaging, Three-Dimensional; D009369:Neoplasms / Q000532:radiotherapy*; D019047:Phantoms, Imaging; D018787:Radiation Oncology / Q000379:methods*; D011878:Radiotherapy / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D015203:Reproducibility of Results