title: Synchrotron stereotactic radiotherapy: dosimetry by Fricke gel and Monte Carlo simulations.
abstract: Synchrotron stereotactic radiotherapy (SSR) consists in loading the tumour with a high atomic number element (Z), and exposing it to monochromatic x-rays from a synchrotron source (50-100 keV), in stereotactic conditions. The dose distribution results from both the stereotactic monochromatic x-ray irradiation and the presence of the high Z element. The purpose of this preliminary study was to evaluate the two-dimensional dose distribution resulting solely from the irradiation geometry, using Monte Carlo simulations and a Fricke gel dosimeter. The verification of a Monte Carlo-based dosimetry was first assessed by depth dose measurements in a water tank. We thereafter used a Fricke dosimeter to compare Monte Carlo simulations with dose measurements. The Fricke dosimeter is a solution containing ferrous ions which are oxidized to ferric ions under ionizing radiation, proportionally to the absorbed dose. A cylindrical phantom filled with Fricke gel was irradiated in stereotactic conditions over several slices with a continuous beam (beam section = 0.1 x 1 cm2). The phantom and calibration vessels were then imaged by nuclear magnetic resonance. The measured doses were fairly consistent with those predicted by Monte Carlo simulations. However, the measured maximum absolute dose was 10% underestimated regarding calculation. The loss of information in the higher region of dose is explained by the diffusion of ferric ions. Monte Carlo simulation is the most accurate tool for dosimetry including complex geometries made of heterogeneous materials. Although the technique requires improvements, gel dosimetry remains an essential tool for the experimental verification of dose distribution in SSR with millimetre precision.
authors.affiliation: INSERM-U647 Rayonnement synchrotron et recherche médicale and ID17 biomedical beamline of the European Synchrotron Radiation Facility, Grenoble, France.
authors.forename: Caroline
authors.initials: C
authors.lastname: Boudou
authors.forename: Marie-Claude
authors.initials: MC
authors.lastname: Biston
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Corde
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Adam
authors.forename: Claudio
authors.initials: C
authors.lastname: Ferrero
authors.forename: François
authors.initials: F
authors.lastname: Estève
authors.forename: Hélène
authors.initials: H
authors.lastname: Elleaume
mesh_terms: D002138:Calibration; D004307:Dose-Response Relationship, Radiation; D005782:Gels; D007477:Ions; D009682:Magnetic Resonance Spectroscopy; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons; D011829:Radiation Dosage; D011874:Radiometry; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D017356:Synchrotrons; D014867:Water; D014965:X-Rays
chemical_list: D005782:Gels; D007477:Ions; D014867:Water