Abstract
Validate that a two‐dimensional (2D) ionization chamber array (ICA) combined with a double‐wedge plate (DWP) can track changes in electron beam energy well within 2.0 mms as recommended by TG‐142 for monthly quality assurance (QA). Electron beam profiles of 4–22 MeV were measured for a 25 × 25 cm2 cone using an ICA with a DWP placed on top of it along one diagonal axis. The relationship between the full width half maximum (FWHM) field size created by DWP energy degradation across the field and the depth of 50% dose in water (R50) is calibrated for a given ICA/DWP combination in beams of know energies (R50 values). Once this relationship is established, the ICA/DWP system will report the R50FWHM directly. We calibrated the ICA/DWP on a linear accelerator with energies of 6, 9, 12, 16, 20, and 22 MeV. The R50FWHM values of these beams and eight other beams with different R50 values were measured and compared with the R50 measured in water, that is, R50Water. Resolving changes of R50 up to 0.2 cm with ICA/DWP was tested by adding solid‐water to shift the energy and was verified with R50Water measurements. To check the long‐term reproducibility of ICA/DWP we measured R50FWHM on a monthly basis for a period of 3 yr. We proposed a universal calibration procedure considering the off‐axis corrections and compared calibrations and measurements on three types of linacs (Varian TrueBeam, Varian C‐series, and Elekta) with different nominal energies and R50 values. For all 38 beams on same type of linac with R50values over a range of 2–8.8 cm, the R50FWHM reported by the ICA/DWP system agreed with that measured in water within 0.01 ± 0.03 cm (mean ± 1σ) and maximum discrepancy of 0.07 cm. Long‐term reproducibility results show the ICA/DWP system to be within 0.04 cm of their baseline over 3 yr. With the universal calibration the maximum discrepancy between R50FWHM and R50Water for different types of linac reduced from 0.25 to 0.06 cm. Comparison of R50FWHM values and R50Water values and long‐term reproducibility of R50FWHM values indicates that the ICA/DWP can be used for monitoring of electron beam energy constancy well within TG‐142 recommended tolerance.
Highlights
Periodic quality assurance (QA) of a clinical linear accelerator is important for dose accuracy during the treatment series of fractions
We examined the uncertainties and stability of the R50FWHM values for known and unknown electron beam energies measured with the ionization chamber array (ICA)/double‐wedge plate (DWP) and compared with measurements made with water scans
Measured R50FWHM compared to the measured R50Water showed good agreement for all electron beams in the study We demonstrated that the ICA/DWP can be used for monitoring of electron beam energy constancy with high accuracy (
Summary
Periodic quality assurance (QA) of a clinical linear accelerator (linac) is important for dose accuracy during the treatment series of fractions. Deviations from baseline values acquired during commissioning[1,2] or from the treatment planning system (TPS) should not exceed the TG or MPPG tolerance levels Both AAPM Task Group 1422 and AAPM Medical Physics Practice Guideline (MPPG8.a)[3] recommend that electron beam energy constancy be evaluated monthly and annually. The electron beam energy quality metric, “R50Water,” represents the depth at which the dose absorbed in water along the central beam axis, is reduced to 50% of its maximum value under full scatter conditions.[4] Electron beam energy constancy is traditionally measured with solid water slabs during monthly QA This procedure can be time consuming and becomes tedious on a linac with multiple electron energies as different thicknesses of solid water slabs are required to characterize each electron energy. A recent study has shown that the tolerances for electron beam energy checks using a two‐depth method are highly nonlinear due to the differences in gradient of the percent depth dose (PDD) falloff region.[5]
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