Energy constancy checking for electron beams using a wedge‐shaped solid phantom combined with a beam profile scanner

Abstract

An energy constancy checking method is presented which involves a specially designed wedge-shaped solid phantom in combination with a multiple channel ionization chamber array known as the Thebes device. Once the phantom/beam scanner combination is set up, measurements for all electron energies can be made and evaluated without re-entering the treatment room. This is also valid for the readjustment of beam energies which are found to deviate from required settings. The immediate presentation of the measurements is in the form of crossplots which resemble depth dose profiles. The evaluation of the measured data can be performed using a hand-held calculator, but processing of the measured signals through a PC-type computer is advisable. The method is insensitive to usual fluctuations in beam flatness. The sensitivity and reproducibility of the method are more than adequate. The method may also be used in modified form for photon beams.

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

“…Rosenow et al used one‐dimensional (1D) ionization chamber array combined with a simple wedge‐shaped polystyrene phantom to measure beam energies. They found that the full width at half maximum (FWHM) of the modified electron profiles correlated linearly with R 50 in the whole range of energies studied . Using the same 1D ionization chamber array and similar wedge‐shaped polystyrene phantom, Islam et al were able to reproduce depth ionization curves in polystyrene phantom and the results agree quite well within water measurements for beam energy ≤10 MeV.…”

“…Measuring changes in R 50 with an ionization chamber array combined with a double‐wedge gives equivalent result as in water scans without requiring the user to change depths between energies as well as providing the flatness and symmetry measurement simultaneously. Compared to other approaches using 1D/2D planner array, the method examined in this work has advantages: (a) the setup is easy and the baseline calibration is straightforward; (b) the Profiler software can directly report the value of R 50 ; (c) the flatness and symmetry in principal axes and the beam energy R 50 can be measured in one profile acquisition. We have also demonstrated that a modification of the calibration procedure to include the off‐axis corrections in the calibration would allow this ICA/DWP to directly determine R 50 on different beams/machine types universally.…”

Electron beam energy QA — a note on measurement tolerances

“…However, one needs to bear in mind that, while exact determination of ${\mathrm{R}}_{50}$ is required for dosimetry protocols such as TG‐51, the beam quality index for monthly QA is merely a constancy test that does not require exact determination of ${\mathrm{normalR}}_{50}$. Different methods have been proposed to determine energy constancy for QA checks 8 , 9 , 10 , 11 , 12 , 13 , 14 . A common method is to take the ratio of ion chamber measurements at two depths.…”

Quality assurance of electron and photon beam energy using the BQ‐Check phantom

“…Measurements taken from the profile underneath the wedge can be correlated with the beam energy. (5)(6)(7) Wells et al (8) introduced a double wedge technique for measuring electron beam energy that is invariant to phantom alignment in the wedge direction, and had a similar sensitivity to water tank measurements for electron energies between 6 and 20 MeV. They used a phantom with two PMMA wedges attached to a diode array to produce profiles at a fixed source-to-surface distance (SSD) and field size.…”