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SU‐FF‐T‐234: Detailed Characterization of the Water‐Equivalent Material Virtual Water in High‐Energy Photon and Electron Beams
Author(s) -
McEwen M
Publication year - 2005
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1997962
Subject(s) - imaging phantom , dosimetry , virtual water , fluence , ionization , photon , electron , scaling , materials science , range (aeronautics) , photon energy , absorbed dose , optics , computational physics , irradiation , physics , nuclear physics , radiation , geometry , nuclear medicine , mathematics , ion , composite material , medicine , ecology , water scarcity , quantum mechanics , biology , agriculture
Purpose: In reference dosimetry for high‐energy photon and electron beams a solid phantom offers a number of advantages over water as the reference material including ease of use and positioning reliability. This paper describes the characterization of the material Virtual Water (manufactured by Med‐Cal). Method and Materials: Ionization measurements were made in Co‐60, 6 & 10 MV photons and five electron beams (4–22 MeV). Two techniques were used: i) substitution ‐ Virtual Water slabs displaced water in a water phantom, and ii) direct comparison of ionization measurements in a water and Virtual Water (VW) phantom. Two formulations of Virtual Water were evaluated, having different densities. Results: Neither formulation showed exact water equivalence in photon beams — the water/VW ratio varied with the depth of measurement with a difference of over 1% at 10 cm depth. However, by using a density (range) scaling factor very good agreement (< 0.2%) between water and VW at all depths was obtained. In the case of the electron beams a range‐scaling factor was also required to match the shapes of the depth dose curves in water and Virtual Water. However, there remained a difference in the measured fluence in the two phantoms after this scaling factor had been applied. For measurements around the peak of the depth‐dose curve this difference amounted to 0.4%. Conclusion: The level of water equivalence for Virtual Water is among the best reported for epoxy‐resin based materials. The low overall uncertainty on the fluence ratio ‐ estimated to be 0.18% ‐ opens up the possibility of performing dosimetry in a solid phantom with an accuracy approaching that of measurements in water. This is particularly of interest for low energy electron beams (< 6 MeV) where positioning errors in a water phantom can contribute significantly to the overall uncertainty.