Film dosimetry of small electron beams for routine radiotherapy planning

The characteristics of very small fields, 1 X 1 and 2 X 2 cm, of electron beams of nominal energies, 5, 7, 10, 12, 15, and 18 MeV have been studied and compared to a 10 X 10 cm field. A parallel-plate ion chamber and film have been used to obtain various dose parameters. The central axis depth dose measurements, field flatness, uniformity index, and relative output factors are presented. It was found that satisfactory results for determining the relative output factor can be obtained from film data using a scanning densitometer. It is our conclusion that film dosimetry is acceptable in determining the necessary clinical parameters needed to treat patients with fields as small as 2 X 2 cm. For the 1 X 1 cm field size and for the electron energies greater than 10 MeV, there was substantial disagreement between the ion chamber and film data in the buildup region as well as the regions beyond the depth of maximum dose to the depth of 90% dose.