SU-E-T-98: An Analysis of TG-51 Electron Beam Calibration Correction Factor Uncertainty

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Abstract

Purpose:

To analyze the uncertainty of the TG-51 electron beam calibration correction factors for farmer type ion chambers currently used by institutions visited by IROC Houston.

Methods:

TG-51 calibration data were collected from 181 institutions visited by IROC Houston physicists for 1174 and 197 distinct electron beams from modern Varian and Elekta accelerators, respectively. Data collected and analyzed included ion chamber make and model, nominal energy, ND,w, I5₀, R5₀, k'R5₀, dref, Pgr and pdd(dref). k'R5₀ data for parallel plate chambers were excluded from the analysis.

Results:

Unlike photon beams, electron nominal energy is a poor indicator of the actual energy as evidenced by the range of R5₀ values for each electron beam energy (6–22MeV). The large range in R5₀ values resulted k'R5₀ values with a small standard deviation but large range between maximum value used and minimum value (0.001–0.029) used for a specific Varian nominal energy. Varian data showed more variability in k'R5₀ values than the Elekta data (0.001–0.014). Using the observed range of R5₀ values, the maximum spread in k'R5₀ values was determined by IROC Houston and compared to the spread of k'R5₀ values used in the community. For Elekta linacs the spreads were equivalent, but for Varian energies of 6 to 16MeV, the community spread was 2 to 6 times larger. Community Pgr values had a much larger range of values for 6 and 9 MeV values than predicted. The range in Varian pdd(dref ) used by the community for low energies was large, (1.4–4.9 percent), when it should have been very close to unity. Exradin, PTW Roos and PTW farmer chambers ND,w values showed the largest spread, ≥11 percent.

Conclusion:

While the vast majority of electron beam calibration correction factors used are accurate, there is a surprising spread in some of the values used.

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