Correction factors for water-proofing 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 method 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 calculations were carried out using the EGS4 (Electron Gamma Shower version 4) code system with the application of a correlated-sampling variance-reduction technique. The results show that the sleeve correction factor for a 1-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 1-mm thick 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. The P(s) factor varies with sleeve thickness, beam quality and phantom depth. No significant dependence of the P(s) factor on field size and source-surface distance has been found. Measurements for PMMA, nylon and polystyrene sleeves of various thicknesses have also been carded out and show excellent agreement with the Monte Carlo calculations.