Build-up modification of commercial diodes for entrance dose measurements in ‘higher energy’ photon beams
Identifieur interne : 000E33 ( Main/Merge ); précédent : 000E32; suivant : 000E34Build-up modification of commercial diodes for entrance dose measurements in ‘higher energy’ photon beams
Auteurs : Dietmar Georg [Belgique] ; Bie De Ost [Belgique] ; Marie-Thérèse Hoornaert [Belgique] ; Pierre Pilette [Belgique] ; Jan Van Dam [Belgique] ; Michel Van Dycke [Belgique] ; Dominique Huyskens [Belgique]Source :
- Radiotherapy and Oncology [ 0167-8140 ] ; 1999.
English descriptors
Abstract
Background and Purpose: Several commercially available p-type diodes do not provide sufficient build-up for in-vivo dosimetry in ‘higher’ energy photon beams, and only limited information could be found in the literature describing the correction factor variation and/or the achievable accuracy for in-vivo dosimetry methods in this energy range. The first aim of this study is to assess and analyze the variation of diode correction factors for entrance dose measurements at higher photon energies. In a second step the total build up thickness of the diode has been modified in order to minimize the correction factor variation. Materials and Methods: Diode correction factors accounting for non-reference conditions (field size, source surface distance, tray, wedge, and block) are determined in 18–25 MV photon beams provided by different treatment units for Scanditronix p-type diodes recommended for higher energy photon beams: old type and new type EDP-20, and EDP-30 diodes. Hemispherical build-up caps of different materials (copper, iron, lead) are used to increase the total build-up thickness. Perturbation effects with and without additional build-up caps are assessed for the three diode types. Results: For unmodified diodes field size correction factors (CFS) vary between 1.7% and 6%, dependent on diode type and treatment unit. For example, for an old type EDP-20 the CFS variation at 18 MV is much higher on a GE linac (5%) as compared to the Philips machine (1.7%). Depending on diode type, this variation can be reduced to 1–2% when adding additional build-up. The variation of source to surface distance correction factors is almost independent of build-up thickness. By adding additional build-up the influence of trays and blocks can be almost eliminated. Conclusions: The correction factor variation of unmodified diodes reflects the variation of the electron contamination with treatment geometry. A total build-up thickness of 3O mm is found to be the ‘best compromise’ for the three types of diodes investigated when measuring entrance doses in the energy range between 18 and 25 MV.
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DOI: 10.1016/S0167-8140(99)00058-4
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Box 23, 1950 Kraainem</wicri:regionArea><wicri:noRegion>1950 Kraainem</wicri:noRegion></affiliation></author><author><name sortKey="Pilette, Pierre" sort="Pilette, Pierre" uniqKey="Pilette P" first="Pierre" last="Pilette">Pierre Pilette</name><affiliation wicri:level="1"><country xml:lang="fr">Belgique</country><wicri:regionArea>Belgian Hospital Physicist Association, P.O. Box 23, 1950 Kraainem</wicri:regionArea><wicri:noRegion>1950 Kraainem</wicri:noRegion></affiliation></author><author><name sortKey="Van Dam, Jan" sort="Van Dam, Jan" uniqKey="Van Dam J" first="Jan" last="Van Dam">Jan Van Dam</name><affiliation wicri:level="1"><country xml:lang="fr">Belgique</country><wicri:regionArea>Belgian Hospital Physicist Association, P.O. Box 23, 1950 Kraainem</wicri:regionArea><wicri:noRegion>1950 Kraainem</wicri:noRegion></affiliation></author><author><name sortKey="Van Dycke, Michel" sort="Van Dycke, Michel" uniqKey="Van Dycke M" first="Michel" last="Van Dycke">Michel Van Dycke</name><affiliation wicri:level="1"><country xml:lang="fr">Belgique</country><wicri:regionArea>Belgian Hospital Physicist Association, P.O. Box 23, 1950 Kraainem</wicri:regionArea><wicri:noRegion>1950 Kraainem</wicri:noRegion></affiliation></author><author><name sortKey="Huyskens, Dominique" sort="Huyskens, Dominique" uniqKey="Huyskens D" first="Dominique" last="Huyskens">Dominique Huyskens</name><affiliation wicri:level="3"><country xml:lang="fr">Belgique</country><wicri:regionArea>Department of Oncology, Section Radiation Physics, U.Z. 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The first aim of this study is to assess and analyze the variation of diode correction factors for entrance dose measurements at higher photon energies. In a second step the total build up thickness of the diode has been modified in order to minimize the correction factor variation. Materials and Methods: Diode correction factors accounting for non-reference conditions (field size, source surface distance, tray, wedge, and block) are determined in 18–25 MV photon beams provided by different treatment units for Scanditronix p-type diodes recommended for higher energy photon beams: old type and new type EDP-20, and EDP-30 diodes. Hemispherical build-up caps of different materials (copper, iron, lead) are used to increase the total build-up thickness. Perturbation effects with and without additional build-up caps are assessed for the three diode types. Results: For unmodified diodes field size correction factors (CFS) vary between 1.7% and 6%, dependent on diode type and treatment unit. For example, for an old type EDP-20 the CFS variation at 18 MV is much higher on a GE linac (5%) as compared to the Philips machine (1.7%). Depending on diode type, this variation can be reduced to 1–2% when adding additional build-up. The variation of source to surface distance correction factors is almost independent of build-up thickness. By adding additional build-up the influence of trays and blocks can be almost eliminated. Conclusions: The correction factor variation of unmodified diodes reflects the variation of the electron contamination with treatment geometry. A total build-up thickness of 3O mm is found to be the ‘best compromise’ for the three types of diodes investigated when measuring entrance doses in the energy range between 18 and 25 MV.</div></front></TEI></record>Pour manipuler ce document sous Unix (Dilib)
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