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 wh ich reduces the required film calibration exposures to a set of films obtained for a small radiation field size and shallow depth (6 cmx6 cm at 5 cm depth). It involves modification of a compression type polyst yrene 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 sensitivit y to change with field size and depth. The proposed method is best des cribed as ''lateral scatter filtering.'' To validate the proposed meth od, central axis depth doses and isodose curves for a 4 MV photon beam were determined from films exposed within the modified phantom and th e results compared with ionization chamber measurements. When no later al filtering was used, for field sizes of 6 cmx6 cm and 25 cmx25 cm, t his comparison demonstrated up to a 65% difference between film and io nization chamber central axis depth dose measurements. When using the lateral scatter filtering technique, less than a 4% difference was obs erved for these field sizes. (C) 1997 American Association of Physicis ts in Medicine.