To investigate recommendations for reference dosimetry of electron beams and gradient effects for the NE2571 chamber and to provide beam quality conversion factors using Monte Carlo simulations of the PTW Roos and NE2571 ion chambers.
The EGSnrc code system is used to calculate the absorbed dose-to-water and the dose to the gas in fully modeled ion chambers as a function of depth in water. Electron beams are modeled using realistic accelerator simulations as well as beams modeled as collimated point sources from realistic electron beam spectra or monoenergetic electrons. Beam quality conversion factors are calculated with ratios of the doses to water and to the air in the ion chamber in electron beams and a cobalt-60 reference field. The overall ion chamber correction factor is studied using calculations of water-to-air stopping power ratios.
The use of an effective point of measurement shift of 1.55 mm from the front face of the PTW Roos chamber, which places the point of measurement inside the chamber cavity, minimizes the difference betweenR 50, the beam quality specifier, calculated from chamber simulations compared to that obtained using depth-dose calculations in water. A similar shift minimizes the variation of the overall ion chamber correction factor with depth to the practical range and reduces the root-mean-square deviation of a fit to calculated beam quality conversion factors at the reference depth as a function of R 50. Similarly, an upstream shift of 0.34 r cav allows a more accurate determination of R 50 from NE2571 chamber calculations and reduces the variation of the overall ion chamber correction factor with depth. The determination of the gradient correction using a shift of 0.22 r cav optimizes the root-mean-square deviation of a fit to calculated beam quality conversion factors if all beams investigated are considered. However, if only clinical beams are considered, a good fit to results for beam quality conversion factors is obtained without explicitly correcting for gradient effects. The inadequacy of R 50 to uniquely specify beam quality for the accurate selection of k Q factors is discussed. Systematic uncertainties in beam quality conversion factors are analyzed for the NE2571 chamber and amount to between 0.4% and 1.2% depending on assumptions used.
The calculated beam quality conversion factors for the PTW Roos chamber obtained here are in good agreement with literature data. These results characterize the use of an NE2571 ion chamber for reference dosimetry of electron beams even in low-energy beams.
The authors thank Malcolm McEwen of NRC for experimental work to establish the BEAMnrc model of the Elekta Precise linac and useful discussions regarding practical electron beam dosimetry, Jörg Wulff of Varian for helpful discussions regarding egs_chamber simulations and chamber models, and Daren Sawkey of Varian for confirming the applicator model currently used by Varian accelerators. Work supported by an OGSST and an NSERC CGS held by B. R. Muir, and by NSERC, the CRC program, CFI, and OIT.
I.A. Review of electron beam dosimetry with the TG-51 protocol
II.A. Monte Carlo calculations as a function of depth
II.A.1. Beam quality conversion factors, k Q
II.A.2. Source models
II.B. Individual correction factors at the reference depth
II.C. Variation of overall chamber correction factor with depth
II.D. Systematic uncertainties in calculated k Q factors
III. RESULTS AND DISCUSSION
III.A. Comparison of R 50 calculated with D w to that determined from D ch
III.B. Beam quality conversion factors
III.B.1. Roos chamber
III.C. Comparison of individual correction factors to other publications
III.D. Calculations of k ecal
III.D.1. PTW Roos
III.E. Shifts to minimize variation of the overall ion chamber correction factor with depth
III.F. Uncertainty in calculated k Q factors for the NE2571 chamber
- Electron beams
- Ionization chambers
- Collisional energy loss
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