title: Dosimetric comparison of flattened and unflattened beams for stereotactic body radiation therapy: Impact of the size of the PTV on dynamic conformal arc and volumetric modulated arc therapy.
abstract: OBJECTIVE
  Flattening filter free (FFF) beams are frequently used for stereotactic body radiation therapy with various treatment modalities: conventional static fields, dynamic conformal arc (DCA) or Rapid Arc (RA). The goal of this study was to obtain some criteria to enable a conscious choice of the employment of FFF beams and of the DCA or RA technique, depending on the PTV size.
  
  
  METHODS
  24 PTVs from 1.52cm3to 445.24cm3were studied in various sites: virtual phantom, lung and liver. For each PTV, DCA and RA plans were prepared using two flattened (FF) and two unflattened photon beams. Parameters such as conformity index, gradient index, healthy-tissue and organs at risk mean doses, number of monitor units (MU), beam on time (BOT) were used to quantify obtained dose distributions. Friedman tests and Spearman's rank correlation coefficients were also performed.
  
  
  RESULTS
  No significant differences were found between FF and FFF beams for RA regarding conformity and gradient indices. For DCA, 10FFF is less suitable and forward planning becomes more challenging as PTV volume increases. FFF beams provided a better sparing of healthy-tissues except for 10FFF used with DCA. 6FFF was slightly better than 10FFF in terms of healthy-tissue mean doses. FFF beams generated significantly reduced BOTs and increased MUs. These effects were more pronounced for larger volumes and especially for RA plans.
  
  
  CONCLUSIONS
  FFF showed better results than FF beams for the considered plans. 10FFF used with DCA should be used with caution for medium and large volumes.
authors.affiliation: Institut Claudius Regaud, Institut Universitaire du Cancer de Toulouse Oncopole, Toulouse, France; INSERM, CRCT, équipe n°15, Toulouse, France; Université Paul Sabatier, Toulouse, France. Electronic address: vieillevigne.laure@iuct-oncopole.fr.
authors.forename: Laure
authors.initials: L
authors.lastname: Vieillevigne
authors.affiliation: SCM Oncologie, Montauban, France.
authors.forename: Sandra
authors.initials: S
authors.lastname: Bessieres
authors.affiliation: Institut Claudius Regaud, Institut Universitaire du Cancer de Toulouse Oncopole, Toulouse, France.
authors.forename: Monia
authors.initials: M
authors.lastname: Ouali
authors.affiliation: Institut Claudius Regaud, Institut Universitaire du Cancer de Toulouse Oncopole, Toulouse, France.
authors.forename: Christel
authors.initials: C
authors.lastname: Lanaspeze
mesh_terms: D006801:Humans; D008113:Liver Neoplasms / Q000532:radiotherapy; D008175:Lung Neoplasms / Q000532:radiotherapy; D019047:Phantoms, Imaging; D011874:Radiometry; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*
keywords: Dynamic conformal arc; Flattening filter free; Rapid arc; SBRT