Reducing the weight of the EC-L cassette

Abstract

Purpose: To model the EC-L portal film cassette to understand how its weight could be reduced without compromising image quality. Methods and Materials: The BEAM99 Monte Carlo code was used to simulate a 6-MV X-ray beam impinging on a water phantom 15 or 40 cm thick and subsequently reaching image receptors of different designs. The image receptor model included the front cassette wall, lead or copper plates 0–1.2 g/cm 2 in thickness, a Lanex Fast screen pair, and the rear cassette wall. The signal generated in the phosphor screen from primary and scattered photons and charged particles was calculated for all image receptors and for both a water phantom and a water phantom plus bone object. Subject contract was calculated using the formalism of Motz and Danos, and the detective quantum efficiency was calculated using the formalism of Swank. Experimental cassettes with copper plate thicknesses of 0.5, 0.25, and 0.0 mm were used to image patients at several anatomic sites. Results: Multiple scatter, especially at large field sizes, generates a low-energy X-ray component that can overrespond in the image receptor. The lead plate is more effective in reducing this X-ray scatter component than the copper plate. Filtering of the X-ray beam by the patient hardens the X-ray spectrum of a 6-MV X-ray beam, reducing subject contrast for thick patients. The front wall of the cassette plays an important role in contributing to the signals generated in the image receptor when the thickness of the metal plate is reduced. Over a wide variety of field sizes and patient thicknesses, subject contrast and detective quantum efficiency are relatively independent of metal plate thickness. Conclusions: The results suggest that a redesign of the EC-L cassette where the front wall of the cassette becomes part of the image receptor, and where the metal plate is changed to lead and the thickness reduced to 0.2 g/cm 2, would generate images of quality comparable to those of the existing EC-L cassette when used with a 6-MV X-ray beam. This change would reduce the weight of the EC-L cassette by 1040 g.