Detection of electron beam energy variations using a computed radiography system

Abstract

A method to evaluate the electron beam energy constancy by employing the computed radiography (CR) system has been developed. In this method, a right triangular plastic wedge is used to produce a curve of the CR storage phosphor plate signal versus the wedge thickness. The curve which resembles the percentage depth ionization curve of the clinical electron beams can be used to derive the energy constancy metric EC50. The sensitivity of the method was tested using polystyrene sheets of variable thicknesses. For electron energies up to 12 MeV energy changes induced by 1.5 mm thick polystyrene can be detected, while a 2.3 mm thick polystyrene sheet is required for higher energies. The measurements were carried out over a two years period. The results showed a good reproducibility with the use of the same CR plate and cassette, and without the requirement of calibration procedures. The overall range of the EC50 was within the 99% confidence intervals, and the standard deviation of the EC50 was measured to be from 0.3 to 0.4 mm for different beam energies. This technique provides an efficient and accurate method to perform the electron beam energy check and could be used by centers equipped with the CR system without requiring additional detection devices.