Dosimetric implications of shifts in linear accelerator electron beam energy detected in routine constancy checks: a scanning film densitometry detection method.

The effects of change in electron beam energy are primarily manifest by changes in the range parameters of the depth ionisation/dose curve. Even for a change of up to 10% in the mean energy at the surface, E0, the dose to the depth of maximum on the central axis changes by less than 1%. Using as a limit of acceptability that the change in the therapeutic range (R85) should not be more than +/- 1.5 mm, the precision required by beam energy checking is that a change of 0.4 MeV in E0 should be detectable for all electron beams provided by the accelerator. To satisfy this criterion a routine method is proposed that uses therapy verification film exposed to the electron beam under a perspex wedge. The automatically processed film is then scanned with the densitometer of a beam data acquisition system (BDAS). The optical density versus distance plot is analysed using the BDAS computer that converts it to a quasi-depth dose curve and then calculates E0 and Ep,0 from the range parameters. The results for electron beams from "console" energies of 5 to 14 MeV show that the test criterion is within the capability of the method, and that the method is very practical for routine use in a quality assurance program.