title: The mean photon energy ĒF at the point of measurement determines the detector-specific radiation quality correction factor kQ,M in (192)Ir brachytherapy dosimetry.
abstract: The application of various radiation detectors for brachytherapy dosimetry has motivated this study of the energy dependence of radiation quality correction factor kQ,M, the quotient of the detector responses under calibration conditions at a (60)Co unit and under the given non-reference conditions at the point of measurement, M, occurring in photon brachytherapy. The investigated detectors comprise TLD, radiochromic film, ESR, Si diode, plastic scintillator and diamond crystal detectors as well as ionization chambers of various sizes, whose measured response-energy relationships, taken from the literature, served as input data. Brachytherapy photon fields were Monte-Carlo simulated for an ideal isotropic (192)Ir point source, a model spherical (192)Ir source with steel encapsulation and a commercial HDR GammaMed Plus source. The radial source distance was varied within cylindrical water phantoms with outer radii ranging from 10 to 30cm and heights from 20 to 60cm. By application of this semiempirical method - originally developed for teletherapy dosimetry - it has been shown that factor kQ,M is closely correlated with a single variable, the fluence-weighted mean photon energy ĒF at the point of measurement. The radial profiles of ĒF obtained with either the commercial (192)Ir source or the two simplified source variants show little variation. The observed correlations between parameters kQ,M and ĒF are represented by fitting formulae for all investigated detectors, and further variation of the detector type is foreseen. The herewith established close correlation of radiation quality correction factor kQ,M with local mean photon energy ĒF can be regarded as a simple regularity, facilitating the practical application of correction factor kQ,M for in-phantom dosimetry around (192)Ir brachytherapy sources. ĒF values can be assessed by Monte Carlo simulation or measurement. A technique describing the local measurement of ĒF will be published separately.
authors.affiliation: Medical Radiation Physics Group, University of Oldenburg and Pius-Hospital Oldenburg, Germany. Electronic address: cmark23@hotmail.com.
authors.forename: Ndimofor
authors.initials: N
authors.lastname: Chofor
authors.affiliation: Medical Radiation Physics Group, University of Oldenburg and Pius-Hospital Oldenburg, Germany.
authors.forename: Dietrich
authors.initials: D
authors.lastname: Harder
authors.affiliation: Medical Radiation Physics Group, University of Oldenburg and Pius-Hospital Oldenburg, Germany.
authors.forename: Hans-Joachim
authors.initials: HJ
authors.lastname: Selbach
authors.affiliation: Medical Radiation Physics Group, University of Oldenburg and Pius-Hospital Oldenburg, Germany.
authors.forename: Björn
authors.initials: B
authors.lastname: Poppe
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001918:Brachytherapy / Q000295:instrumentation* / Q000379:methods; D003198:Computer Simulation; D004867:Equipment Design; D019544:Equipment Failure Analysis; D007496:Iridium Radioisotopes / Q000032:analysis / Q000627:therapeutic use*; D018499:Linear Energy Transfer; D015233:Models, Statistical; D009010:Monte Carlo Method; D017785:Photons; D011785:Quality Assurance, Health Care / Q000379:methods*; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods*; D019275:Radiopharmaceuticals / Q000032:analysis / Q000627:therapeutic use; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: D007496:Iridium Radioisotopes; D019275:Radiopharmaceuticals
keywords: Brachytherapie-Dosimetrie; Detektor-Ansprechvermögen; Nicht-Referenzbedingungen; Radiation quality; Strahlungsqualität; brachytherapy dosimetry; detector response; mean photon energy; mittlere Photonenenergie; non-reference conditions