External beam profiles measured with film or ionization chamber

Radiation therapy beam profiles measured with an ionization chamber in a scanning water phantom are compared with profiles measured with a standard film dosimetry technique. An 18 cm field was filmed with 4 MV photons in a polystyrene phantom at depths of 1, 7, and 13 cm. The film was scanned with a scanning densitometer and the resulting profiles were converted to dose profiles with a sensitometric curve. To determine the effect of backscatter on the film a special phantom was constructed to eliminate the backscatter from the film measurement. This provided a comparison with the solid film phantom where full backscattering was present. The beam profiles measured with the film differed by less than 3% from the ionization chamber profiles and there was less than 1% difference between the film techniques.