Direct reference air-kerma rate calibration of ~(192)Ir for a thimble-type ionization chamber in a cylindrical solid phantom

摘要

International and national reports strongly recommend experimental verification of the calibration of new HDR ~(192)Ir sources prior to their clinical application for afterloading brachytherapy. To guarantee traceability, calibrated transfer standards are used, e.g. the recommended well-type ionization chambers (WIC) or certain detector-phantom arrangements (DPA) consisting of a thimble-type ionization chamber with a solid phantom. In Germany, direct calibrations for ~(192)Ir were only provided for WICs. In June 2010, the PTB directly calibrated a DPA-transfer standard in the quantity reference air-kerma rate (RAKR) for ~(192)Ir photons for the University Hospital Tubingen. This direct calibration provides the advantage that the comprehensive RAKR ~(192)Ir calibration coefficient already takes all influence quantities into account--their correction factors are thus unified--except for the air-density correction k_ρ. The DPA-transfer standard described above and a WIC used as a reference were compared for acceptance tests of three GammaMedplus HDR ~(192)Ir afterloading sources. The measurement uncertainty of the WIC and of the DPA-transfer standard were ±2.6% (k = 2) and ±2.8% (k = 2) respectively. A strong correlation was found between these measurement results with a coefficient of determination of r~2 = 0.9998. Determining the RAKR of an HDR ~(192)Ir afterloading source is as simple with the DPA as it is with WIC. The direct ~(192)Ir calibrated DPA-transfer standard can therefore be used alternatively in future with the same measurement uncertainty if no WIC is available.