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SU‐FF‐T‐120: Comparison of Detectors for Electron Depth‐Dose Measurements and Investigation of Parallel‐Plate Chamber Perturbation Factors
Author(s) -
McEwen M,
McDonald A,
Faddegon B
Publication year - 2007
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 - 10.1118/1.2760778
Subject(s) - ionization chamber , dosimetry , diode , detector , monte carlo method , ionization , physics , electron , imaging phantom , ion , bremsstrahlung , linear particle accelerator , atomic physics , optics , computational physics , nuclear physics , nuclear medicine , beam (structure) , optoelectronics , mathematics , medicine , statistics , quantum mechanics
Purpose: In all international protocols for electron beam dosimetry the primary detector of choice for measuring depth‐dose distributions is the plane‐parallel ionization chamber. Diode detectors are permitted but their performance must be checked against a ‘gold‐standard’ ion chamber before use. Recent publications have indicated that parallel‐plate ion chambers can have a significant non‐zero perturbation correction, which varies with depth. If this is not corrected for then the chamber is not a ‘gold standard’ and any comparison with a diode will be biased. This also has implications for benchmarking of Monte‐Carlo simulations. Method and Materials: A precision 1‐D scanning phantom was developed with an absolute positioning accuracy of 0.15 mm and a dose uncertainty of 0.1%. Measurements were made with two types of plane‐parallel ion chamber (NACP and Roos) and two electron diodes (Scanditronix EFD and PTW 60012) at a range of electron energies (4–22 MeV) for 4 different linac types. Results: There was very good agreement between the plane‐parallel ion chambers (R 50 values generally within 0.1 mm for all beams). i) There was a significant and consistent difference between the electron diode detectors, with a 0.7 mm difference in R 50 (independent of energy) and small differences in the build‐up and bremsstrahlung regions. ii) The ion chamber plots (converted from ionization to dose using TG‐51 and corrected for chamber perturbation using published data) showed very good agreement with the EFD diode, except for in the build‐up region where a difference of ∼1–2% persists. The corrected ion chamber and EFD diode showed good agreement with a Monte‐Carlo simulation but neither matched perfectly. Conclusion: For the highest accuracy depth‐dose measurements (e.g. TG‐51 beam calibration, MC commissioning) perturbation corrections are required for plane‐parallel ion chambers. Further work is required to determine which detector (diode or ion chamber) is a true ‘gold standard’.