CORRECTION FACTORS FOR WATERPROOFING SLEEVES IN KILOVOLTAGE X-RAY-BEAMS

This paper investigates the effect of the waterproofing sleeve on the calibration of kilovoltage photon beams (50-300 kV). The sleeve effect correction factor, p(s), has been calculated using the Monte Carlo me thod as the ratios of the air kerma in an air cavity of a cylindrical chamber without the waterproofing sleeve to that with a sleeve. Three sleeve materials have been studied, PMMA, nylon and polystyrene. The c alculations were carried out using the EGS4 (Electron Gamma Shower ver sion 4) code system with the application of a correlated-sampling vari ance-reduction technique. The results show that the sleeve correction factor for a I-mm thick PMMA waterproofing sleeve surrounding a Farmer -type cylindrical chamber at 2 cm depth in water is about 0.991 at 50 kV (1.04 mm Al HVL) and 1.000 at 300 kV (21.5 mm Al HVL). For I-mm thi ck nylon and polystyrene sleeves, p(s) varies from 0.992 to 1.000 and from 0.981 to 1.000, respectively, for the same beam quality range. Th e p(s) factor varies with sleeve thickness, beam quality and phantom d epth. No significant dependence of the p(s) factor on field size and s ource-surface distance has been found. Measurements for PMMA, nylon an d polystyrene sleeves of various thicknesses have also been carried ou t and show excellent agreement with the Monte Carlo calculations. (C) 1997 American Association of Physicists in Medicine.