Quality assurance of electron-beam energy using an aluminum wedge.

An aluminum wedge was designed to facilitate evaluation of the constancy of high-energy electron teletherapy beams (6-18 MeV). Ready-packed film was exposed with the wedge on top, and optical density distribution was recorded with a scanning densitometer. Energy-dependent statistics were obtained using the practical-range extrapolation method. The procedure takes approximately 15 minutes and requires less than one minute of therapy machine time. Repeated daily measurements on a medical linear accelerator at 12 MeV showed a variation of +/- 0.37 MeV (3 sigma). This technique has proved useful for prompt machine evaluation following malfunctions and has been incorporated into routine quality assurance testing.