Measured and Monte Carlo calculated k Q factors: Accuracy and comparison
Purpose: The journal Medical Physics recently published two papers that determine beam quality conversion factors, k Q, for large sets of ion chambers. In the first paper McEwen Med. Phys. 37, 2179-2193 (2010), k Q was determined experimentally, while the second paper Muir and Rogers Med. Phys. 37, 5939-5950 (2010) provides k Q factors calculated using Monte Carlo simulations. This work investigates a variety of additional consistency checks to verify the accuracy of the k Q factors determined in each publication and a comparison of the two data sets. Uncertainty introduced in calculated k Q factors by possible variation of W/e with beam energy is investigated further.Methods: The validity of the experimental set of k Q factors relies on the accuracy of the NE2571 reference chamber measurements to which k Q factors for all other ion chambers are correlated. The stability of NE2571 absorbed dose to water calibration coefficients is determined and comparison to other experimental k Q factors is analyzed. Reliability of Monte Carlo calculated k Q factors is assessed through comparison to other publications that provide Monte Carlo calculations of k Q as well as an analysis of the sleeve effect, the effect of cavity length and self-consistencies between graphite-walled Farmer-chambers. Comparison between the two data sets is given in terms of the percent difference between the k Q factors presented in both publications.Results: Monitoring of the absorbed dose calibration coefficients for the NE2571 chambers over a period of more than 15 yrs exhibit consistency at a level better than 0.1%. Agreement of the NE2571 k Q factors with a quadratic fit to all other experimental data from standards labs for the same chamber is observed within 0.3%. Monte Carlo calculated k Q factors are in good agreement with most other Monte Carlo calculated k Q factors. Expected results are observed for the sleeve effect and the effect of cavity length on k Q. The mean percent differences between experimental and Monte Carlo calculated k Q factors are -0.08, -0.07, and -0.23% for the Elekta 6, 10, and 25 MV nominal beam energies, respectively. An upper limit on the variation of W/e in photon beams from cobalt-60 to 25 MV is determined as 0.4% with 95% confidence. The combined uncertainty on Monte Carlo calculated k Q factors is reassessed and amounts to between 0.40 and 0.49% depending on the wall material of the chamber.Conclusions: Excellent agreement (mean percent difference of only 0.13% for the entire data set) between experimental and calculated k Q factors is observed. For some chambers, k Q is measured for only one chamber of each type-the level of agreement observed in this study would suggest that for those chambers the measured k Q values are generally representative of the chamber type.
|Keywords||dosimetry protocols, EGSnrc, k Q factors, measurements, Monte Carlo, W/e|
Muir, B.R., McEwen, M.R., & Rogers, D.W.O. (2011). Measured and Monte Carlo calculated k Q factors: Accuracy and comparison. Medical Physics, 38(8), 4600–4609. doi:10.1118/1.3600697