title: Monte Carlo simulation of electron dose from (131)I-targeted tumor cells within a heterogeneous tumor.
abstract: OBJECTIVE
  In internal radiotherapy, the variable distribution of target receptors within the tumoral tissue, and the variable ranges of electrons may be responsible for a heterogeneous dose distribution at the cellular level. The aim of the present study was to use Monte Carlo simulations to assess (131)I electron dose in a model of heterogeneous tumor containing multiple clusters of cancer cells, targeted by (131)I-labeled molecules.
  
  
  METHODS
  The model consisted of 150-μm-diameter spherical tumor cell clusters, in which (131)I was homogeneously distributed. Clusters were placed 24 μm apart, separated by septa of nonradioactive connective tissue. The electron dose distribution to tumor cells in a single cluster was first assessed. Then was assessed the dose increase to these targets after adding multiple layers of neighboring clusters (total number of clusters = 15,624).
  
  
  RESULTS
  Dose distribution within a single isolated cluster follows a decreasing gradient, the dose for the outermost cell layer being about half that at the center. When radioactive neighbors were added, the dose to the central cluster increased. The most important contribution was given by the nearest neighbors, whereas the contribution from neighbors beyond a distance of 1 mm was only for 5% of the final dose. If the central cluster was unlabeled, the absorbed dose to the outermost cell layer of this cluster was reduced by 27%, and that at the center by 45%.
  
  
  CONCLUSIONS
  The electron cross-dose of (131)I falls rapidly as a function of distance and becomes negligible after just 1 mm. Small clusters of tumor cells that are not radiolabeled may receive a very small dose. Therefore, in internal radiotherapy it is important to aim at targeting tumor cells as homogeneously as possible, rather than relying on the cross-dose to achieve a therapeutic effect.
authors.affiliation: Laboratoire de Physique Moléculaire et des Collisions, Université Paul Verlaine-Metz, Institut de Physique, Metz, France.
authors.forename: Paolo
authors.initials: P
authors.lastname: Zanotti-Fregonara
authors.forename: Christophe
authors.initials: C
authors.lastname: Champion
authors.forename: Maria Cristina
authors.initials: MC
authors.lastname: Marzola
authors.forename: Domenico
authors.initials: D
authors.lastname: Rubello
authors.forename: Pierre-Alexandre
authors.initials: PA
authors.lastname: Just
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Moretti
authors.forename: Elif
authors.initials: E
authors.lastname: Hindié
mesh_terms: D003198:Computer Simulation; D004583:Electrons; D007456:Iodine Isotopes / Q000737:chemistry / Q000494:pharmacology*; D008954:Models, Biological; D009010:Monte Carlo Method; D009369:Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage
chemical_list: D007456:Iodine Isotopes