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Int J Radiat Oncol Biol Phys. 2001 Jul 1;50(3):829-36.

Technique charts for EC film: direct optical measurements to account for the effects of X-ray scatter.

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1
London Regional Cancer Centre, London, Ontario, Canada. peter.munro@lrcc.on.ca

Abstract

PURPOSE:

To develop a method of measuring technique charts for enhanced contrast (EC) film, to demonstrate how X-ray scatter changes the response of EC film, and to generate technique charts for general use.

METHODS AND MATERIALS:

We have developed a "digital cassette"-consisting of a metal plate/phosphor screen, a light guide, a photodiode sensor, and an electrometer-that can be used to measure the light generated in the phosphor screen of the film cassette. In turn, these measurements can be used to generate technique charts for EC film. The digital cassette has been used to measure technique charts for 4-MV and 6-MV X-ray beams for a variety of different phantom thicknesses, field sizes, and phantom-to-cassette air gaps.

RESULTS AND DISCUSSION:

We have observed that the signals generated in an ionization chamber located 9.4 cm behind a 30-cm-thick water-equivalent phantom increase by a factor of 1.9 when the field size is increased from 4 x 4 cm(2) to 40 x 40 cm(2) when irradiated by a 6-MV X-ray beam. However, the change in EC film response is a factor of 3.5 under the same conditions. Irradiations to optimally expose the EC film predicted by the digital cassette differ by up to 82% compared to those predicted by ion chamber measurements. Nevertheless, the technique charts measured using the digital cassette predict the response of the EC film to +/- 0.2 optical density. The overresponse of the EC film is most likely due to low-energy scattered photons, which interact with the high atomic number (Z = 64) phosphor screen of the enhanced contrast localization cassette. Therefore, simple solutions, such as placing a high atomic number material above the enhanced contrast localization cassette, can reduce this contribution by scattered photons to the signal generated in the cassettes.

CONCLUSIONS:

We have developed a digital cassette that can make more accurate measurements of the technique charts for EC films. Our measurements show that under some conditions, X-ray scatter can generate a large fraction of the signals recorded by the EC film. Technique charts have been generated at 4 MV and 6 MV, and these charts should have universal applicability.

PMID:
11395253
[Indexed for MEDLINE]
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