title: Dosimetric effect of tissue heterogeneity for (125)I prostate implants.
abstract: OBJECTIVE
  To use Monte Carlo (MC) together with voxel phantoms to analyze the tissue heterogeneity effect in the dose distributions and equivalent uniform dose (EUD) for (125)I prostate implants.
  
  
  BACKGROUND
  Dose distribution calculations in low dose-rate brachytherapy are based on the dose deposition around a single source in a water phantom. This formalism does not take into account tissue heterogeneities, interseed attenuation, or finite patient dimensions effects. Tissue composition is especially important due to the photoelectric effect.
  
  
  METHODS
  The computed tomographies (CT) of two patients with prostate cancer were used to create voxel phantoms for the MC simulations. An elemental composition and density were assigned to each structure. Densities of the prostate, vesicles, rectum and bladder were determined through the CT electronic densities of 100 patients. The same simulations were performed considering the same phantom as pure water. Results were compared via dose-volume histograms and EUD for the prostate and rectum.
  
  
  RESULTS
  The mean absorbed doses presented deviations of 3.3-4.0% for the prostate and of 2.3-4.9% for the rectum, when comparing calculations in water with calculations in the heterogeneous phantom. In the calculations in water, the prostate D 90 was overestimated by 2.8-3.9% and the rectum D 0.1cc resulted in dose differences of 6-8%. The EUD resulted in an overestimation of 3.5-3.7% for the prostate and of 7.7-8.3% for the rectum.
  
  
  CONCLUSIONS
  The deposited dose was consistently overestimated for the simulation in water. In order to increase the accuracy in the determination of dose distributions, especially around the rectum, the introduction of the model-based algorithms is recommended.
authors.affiliation: Faculdade de Ciências Médicas, Universidade Nova de Lisboa, Campo Mártires da Pátria, 130, 1169-056 Lisbon, Portugal ; Quadrantes Faro - Unidade de Radioterapia do Algarve, Rua da Associação Oncológica do Algarve, 8000-316 Faro, Portugal ; MedicalConsult, SA, Campo Grande, 56-8°A, 1700-093 Lisbon, Portugal.
authors.forename: Susana Maria
authors.initials: SM
authors.lastname: Oliveira
authors.affiliation: Faculdade de Ciências Médicas, Universidade Nova de Lisboa, Campo Mártires da Pátria, 130, 1169-056 Lisbon, Portugal ; Escola Superior de Tecnologia da Saúde de Lisboa, Instituto Politécnico de Lisboa, Av. D. João II, lote 4.69.01, 1900-096 Lisbon, Portugal.
authors.forename: Nuno José
authors.initials: NJ
authors.lastname: Teixeira
authors.affiliation: Escola Superior de Tecnologia da Saúde de Lisboa, Instituto Politécnico de Lisboa, Av. D. João II, lote 4.69.01, 1900-096 Lisbon, Portugal ; Instituto Gulbenkian de Ciência, Rua da Quinta Grande, 6, 2780-156 Oeiras, Portugal ; Centro de Biodiversidade, Genómica Integrativa e Funcional, Faculdade de Ciências, Universidade de Lisboa, Edifício ICAT, Campus FCUL, Campo Grande, 1740-016 Lisbon, Portugal.
authors.forename: Lisete
authors.initials: L
authors.lastname: Fernandes
authors.affiliation: IST/ITN, Instituto Superior Técnico, Universidade Técnica de Lisboa, Estrada Nacional 10, 2695-006 Bobadela LRS, Portugal.
authors.forename: Pedro
authors.initials: P
authors.lastname: Teles
authors.affiliation: IST/ITN, Instituto Superior Técnico, Universidade Técnica de Lisboa, Estrada Nacional 10, 2695-006 Bobadela LRS, Portugal.
authors.forename: Pedro
authors.initials: P
authors.lastname: Vaz
keywords: AAPM TG, American Association of Physicists in Medicine Task Group; Brachytherapy; CT, computerized tomography; DVH, dose–volume histogram; EBRT, external beam radiotherapy; EUD, equivalent uniform dose; HT, heterogeneous; LDRBT, low dose-rate brachytherapy; MBDCA, model-based dose calculation algorithm; MC, Monte Carlo; Model-based calculation algorithms; Monte Carlo; NTCP, normal tissue complication probability; OAR, organ at risk; PS, planning system; Prostate cancer; TCP, tumor control probability (TCP); Tissue heterogeneity; W, water; dDVH, differential dose–volume histogram