A new approach to film dosimetry for high energy photon beams: lateral scatter filtering.

A method of film dosimetry for high energy photon beams is proposed which reduces the required film calibration exposures to a set of films obtained for a small radiation field size and shallow depth (6 cm x 6 cm at 5 cm depth). It involves modification of a compression type polystyrene film phantom to include thin lead foils parallel to the vertical film plane at approximately 1 cm from both sides of the film emulsion. The foils act as high atomic number filters which remove low energy Compton scatter photons that otherwise would cause the film sensitivity to change with field size and depth. The proposed method is best described as "lateral scatter filtering." To validate the proposed method, central axis depth doses and isodose curves for a 4 MV photon beam were determined from films exposed within the modified phantom and the results compared with ionization chamber measurements. When no lateral filtering was used, for field sizes of 6 cm x 6 cm and 25 cm x 25 cm, this comparison demonstrated up to a 65% difference between film and ionization chamber central axis depth dose measurements. When using the lateral scatter filtering technique, less than a 4% difference was observed for these field sizes.