Uncertainty in positioning ion chamber at reference depth for various water phantoms.

ABSTRACT

BACKGROUND: Uncertainty in the calibration of high-energy radiation sources is dependent on user and equipment type. AIM: We evaluated the uncertainty in the positioning of a cylindrical chamber at a reference depth for reference dosimetry of high-energy photon beams and the resulting uncertainty in the chamber readings for 6- and 10-MV photon beams. The aim was to investigate major contributions to the positioning uncertainty to reduce the uncertainty in calibration for external photon beam radiotherapy. MATERIALS AND METHODS: The following phantoms were used DoseView 1D, WP1D, 1D SCANNER, and QWP-07 as one-dimensional (1D) phantoms for a vertical-beam geometry; GRI-7632 as a phantom for a fixed waterproofing sleeve; and PTW type 41023 and QWP-04 as 1D phantoms for a horizontal-beam geometry. The uncertainties were analyzed as per the Guide to the Expression of Uncertainty in Measurement. RESULTS: The positioning and resultant uncertainties in chamber readings ranged from 0.22 to 0.35 mm and 0.12-0.25%, respectively, among the phantoms (using a coverage factor k = 1 in both cases). The major contributions to positioning uncertainty are definition of the origin for phantoms among users for the 1D phantoms for a vertical-beam geometry, water level adjustment among users for the phantom for a fixed waterproofing sleeve, phantom window deformation, and non-water material of the window for the 1D phantoms for a horizontal-beam geometry. CONCLUSION: The positioning and resultant uncertainties in chamber readings exhibited minor differences among the seven phantoms. The major components of these uncertainties differed among the phantom types investigated.