Monitoring linear accelerators electron beam energy constancy with a 2D ionization chamber array and double‐wedge phantom

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 cm(2) 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 (R(50)) is calibrated for a given ICA/DWP combination in beams of know energies (R(50) values). Once this relationship is established, the ICA/DWP system will report the R(50)FWHM directly. We calibrated the ICA/DWP on a linear accelerator with energies of 6, 9, 12, 16, 20, and 22 MeV. The R(50)FWHM values of these beams and eight other beams with different R(50) values were measured and compared with the R(50) measured in water, that is, R(50)Water. Resolving changes of R(50) up to 0.2 cm with ICA/DWP was tested by adding solid‐water to shift the energy and was verified with R(50)Water measurements. To check the long‐term reproducibility of ICA/DWP we measured R(50)FWHM 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 R(50) values. For all 38 beams on same type of linac with R(50)values over a range of 2–8.8 cm, the R(50)FWHM 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 R(50)FWHM and R(50)Water for different types of linac reduced from 0.25 to 0.06 cm. Comparison of R(50)FWHM values and R(50)Water values and long‐term reproducibility of R(50)FWHM values indicates that the ICA/DWP can be used for monitoring of electron beam energy constancy well within TG‐142 recommended tolerance.