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


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.


CONCLUSIONS
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.
Geyer
Peter
P
Department of Radiotherapy and Radiooncology, University Hospital Carl Gustav Carus, Technical University Dresden, Germany. peter.geyer@mailbox.tu-dresden.de
Baus
Wolfgang W
WW
Baumann
Michael
M
D016477:Artifacts; D003720:Densitometry; D004307:Dose-Response Relationship, Radiation; D004583:Electrons; D005373:Film Dosimetry / Q000379:methods*; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D017785:Photons; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*