In vivo verification of electron beam energy by patient exit dose and optical density of portal films.

ABSTRACT

BACKGROUND AND PURPOSE: The depth-dose curve of electron beams is mainly determined by their energy. For accelerators with scatter foils, the electron energy can, in principle, be verified by measuring the amount of the contaminating photons. This paper investigates whether exit dose measurements and evaluations of the optical density of portal films can be used to verify the energy of the electron beam in a clinically relevant setting. MATERIAL AND METHODS: During irradiation of the head and neck region of an Alderson-Rando phantom with 6- to 21-MeV electron beams, the exit dose rates behind the phantom and the dose rates at the position of a film cassette were measured. The optical density of films (EC film/EC-L Regular and EC-L Fast cassettes, Eastman Kodak Comp., Rochester, NY, USA) exposed to beams of different energies was evaluated. RESULTS: The exit and the cassette dose rates showed a steep increase with increasing electron energy. Due to its density behavior, the film with both types of cassettes failed to generate images for lower electron energies (6 and 9 MeV) but presented a strong ascent of the optical density-until reaching the saturation-with increasing electron energy. CONCLUSION: Measurements of the exit dose and evaluations of the optical density of portal films can be used to verify and document the energy of electron beams during radiotherapy.