Use of mammographic film-screen-cassette combination for portal localization imaging with a 4 MV X-ray unflattened beam

Use of mammographic film-screen-cassette combination for routine external beam radiation therapy with linear accelerators is proposed and described. The proposal is based on a special diagnostic mode developed by us on a Clinac18 (Varian Associates). A 4 MeV electron beam normally incident on the thinner of the two existing ports of the all-Cu-radiotherapeutic target produces an unflattened x-ray beam considerably enriched (up to 8.1%) with low-energy (≤ 150 keV) photons. It is proposed and proved both by Monte Carlo calculations and experimentally that such an unflattened beam can be effectively used in conjunction with a mammographic film-screen combination for the production of adequate quality localization portal images in routine radiotherapy on a daily basis. A Kodak MIN-R mammographic film-cassette combination, with a strong low-energy response, was used for this purpose. Because of its high sensitivity, (F = 4.7), 0.25 cGy doses at cassette position are adequate, when compared with 5 cGy doses usually required in megavoltage portal imaging avoiding unnecessary additional dose. The weight of the plastic MIN-R cassette is almost four times less than metal-screen cassettes used in megavoltage radiotherapy, thus providing easier handling. A technique of utilizing the unflattened portal imaging beam was proposed and developed based on the time-setting termination of the radiation beam. A special phantom for portal imaging systems calibration was used. An optical density of 1.5 was selected in order to achieve around 0.25 cGy dose at the cassette position. Portal films taken with the mammographic system in conjunction with an unflattened 4 MV x-ray beam both an anthropomorphic phantom and routine radiotherapy show the improvements in contrast and resolution.