Generation of portal film charts for 10-MV x rays.

An analytical method to generate portal film charts for 10-MV photon beams, which takes into account the presence of the cassette front screen is presented. The selection of the best film-screen combination was based on the new AAPM recommendations for radiotherapy portal imaging [AAPM Rep. No. 24, AAPM Task Group No. 28 (1987)]: 2-g/cm2-thick copper front screen and 0.4-g/cm2-thick copper rear screen, and X-OMAT TL Kodak unwrapped film. Doses at the film position were measured as a function of patient thickness, field size, air gap, and the results compared well with the doses derived from the analytical method: within +/- 15%. An optical density of 1.6 was selected for construction of the portal film charts. The application of the method for routine treatment planning quality assurance allows a quick and precise determination of the best exposures to the portal films.