title: Application of monomer/polymer gel dosimetry to study the effects of tissue inhomogeneities on intensity-modulated radiation therapy (IMRT) dose distributions.
abstract: OBJECTIVE
  When planning an intensity-modulated radiation therapy (IMRT) treatment in a heterogeneous region (e.g. the thorax), the dose computation algorithm of a treatment planning system may need to account for these inhomogeneities in order to obtain a reliable prediction of the dose distribution. An accurate dose verification technique such as monomer/polymer gel dosimetry is suggested to verify the outcome of the planning system.
  
  
  METHODS
  The effects of low-density structures: (a) on narrow high-energy (18 MV) photon beams; and (b) on a class-solution IMRT treatment delivered to a thorax phantom have been examined using gel dosimetry. The used phantom contained air cavities that could be filled with water to simulate a homogeneous or heterogeneous configuration. The IMRT treatment for centrally located lung tumors was delivered on both cases, and gel derived dose maps were compared with computations by both the GRATIS and Helax-TMS planning system.
  
  
  RESULTS
  Dose rebuildup due to electronic disequilibrium in a narrow photon beam is demonstrated. The gel measurements showed good agreement with diamond detector measurements. Agreement between measured IMRT dose maps and dose computations was demonstrated by several quantitative techniques. An underdosage of the planning target volume (PTV) was revealed. The homogeneity of the phantom had only a minor influence on the dose distribution in the PTV. An expansion of low-level isodoses in the lung volume was predicted by collapsed cone computations in the heterogeneous case.
  
  
  CONCLUSIONS
  For the class-solution described, the dose in centrally located mediastinal tumors can be computed with sufficient accuracy, even when neglecting the lower lung density. Polymer gel dosimetry proved to be a valuable technique to verify dose calculation algorithms for IMRT in 3D in heterogeneous configurations.
authors.affiliation: Division of Radiotherapy, Ghent University Hospital, De Pintelaan 185, 9000, Gent, Belgium.
authors.forename: Koen
authors.initials: K
authors.lastname: Vergote
authors.forename: Yves
authors.initials: Y
authors.lastname: De Deene
authors.forename: Filip
authors.initials: F
authors.lastname: Claus
authors.forename: Werner
authors.initials: W
authors.lastname: De Gersem
authors.forename: Bart
authors.initials: B
authors.lastname: Van Duyse
authors.forename: Leen
authors.initials: L
authors.lastname: Paelinck
authors.forename: Eric
authors.initials: E
authors.lastname: Achten
authors.forename: Wilfried
authors.initials: W
authors.lastname: De Neve
authors.forename: Carlos
authors.initials: C
authors.lastname: De Wagter
mesh_terms: D000465:Algorithms; D003196:Computer Graphics; D005782:Gels / Q000528:radiation effects; D006801:Humans; D008175:Lung Neoplasms / Q000532:radiotherapy; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011108:Polymers / Q000528:radiation effects; D011874:Radiometry; D011880:Radiotherapy Planning, Computer-Assisted; D020266:Radiotherapy, Conformal / Q000379:methods*
chemical_list: D005782:Gels; D011108:Polymers