title: Dose specification for ¹⁹²Ir high dose rate brachytherapy in terms of dose-to-water-in-medium and dose-to-medium-in-medium.
abstract: Dose calculation in high dose rate brachytherapy with (192)Ir is usually based on the TG-43U1 protocol where all media are considered to be water. Several dose calculation algorithms have been developed that are capable of handling heterogeneities with two possibilities to report dose: dose-to-medium-in-medium (Dm,m) and dose-to-water-in-medium (Dw,m). The relation between Dm,m and Dw,m for (192)Ir is the main goal of this study, in particular the dependence of Dw,m on the dose calculation approach using either large cavity theory (LCT) or small cavity theory (SCT). A head and neck case was selected due to the presence of media with a large range of atomic numbers relevant to tissues and mass densities such as air, soft tissues and bone interfaces. This case was simulated using a Monte Carlo (MC) code to score: Dm,m, Dw,m (LCT), mean photon energy and photon fluence. Dw,m (SCT) was derived from MC simulations using the ratio between the unrestricted collisional stopping power of the actual medium and water. Differences between Dm,m and Dw,m (SCT or LCT) can be negligible (<1%) for some tissues e.g. muscle and significant for other tissues with differences of up to 14% for bone. Using SCT or LCT approaches leads to differences between Dw,m (SCT) and Dw,m (LCT) up to 29% for bone and 36% for teeth. The mean photon energy distribution ranges from 222 keV up to 356 keV. However, results obtained using mean photon energies are not equivalent to the ones obtained using the full, local photon spectrum. This work concludes that it is essential that brachytherapy studies clearly report the dose quantity. It further shows that while differences between Dm,m and Dw,m (SCT) mainly depend on tissue type, differences between Dm,m and Dw,m (LCT) are, in addition, significantly dependent on the local photon energy fluence spectrum which varies with distance to implanted sources.
authors.affiliation: Instituto de Pesquisas Energéticas e Nucleares-IPEN-CNEN/SP, São Paulo, Brazil. Department of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology, Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands.
authors.forename: Gabriel Paiva
authors.initials: GP
authors.lastname: Fonseca
authors.forename: Åsa Carlsson
authors.initials: ÅC
authors.lastname: Tedgren
authors.forename: Brigitte
authors.initials: B
authors.lastname: Reniers
authors.forename: Josef
authors.initials: J
authors.lastname: Nilsson
authors.forename: Maria
authors.initials: M
authors.lastname: Persson
authors.forename: Hélio
authors.initials: H
authors.lastname: Yoriyaz
authors.forename: Frank
authors.initials: F
authors.lastname: Verhaegen
mesh_terms: D000465:Algorithms; D001918:Brachytherapy / Q000379:methods*; D007496:Iridium Radioisotopes / Q000627:therapeutic use*; D011834:Radiation Monitoring / Q000379:methods* / Q000592:standards; D019275:Radiopharmaceuticals / Q000627:therapeutic use*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D014867:Water / Q000737:chemistry
chemical_list: D007496:Iridium Radioisotopes; D019275:Radiopharmaceuticals; D014867:Water