Transit dosimetry in 192Ir high dose rate brachytherapy

[摘要] Background and purpose: Historically HDR brachytherapy treatment planning systemsignore the transit dose in the computation of patient dose. However, the total radiationdose delivered during each treatment cycle is equal to the sum of the static dose and thetransit dose and every HDR application therefore results in two radiation doses.Consequently, the absorbed dose to the target volume is more than the prescribed dose ascomputed during treatment planning. The aim of this study was to determine themagnitude of the transit dose component of two 192Ir HDR brachytherapy units and assessits dosimetric significance.Materials and Methods: Ionization chamber dosimetry systems (well-type and Farmertypeionization chambers) were used to measure the charge generated during the transit ofthe 192Ir source from a GammaMed and a Nucletron MicroSelectron HDR afterloaderusing single catheters of lengths 120 cm. Different source configurations were used forthe measurements of integrated charge. Two analysis techniques were used for transittime determination: the multiple exposure technique and the graphical solution of zeroexposure. The transit time was measured for the total transit of the radioactive source into(entry) and out of (exit) the catheters.Results: A maximum source transit time of 1.7 s was measured. The transit dose dependson the source activity, source configuration, number of treatment fractions, prescriptiondose and the type of remote afterloader used. It does not depend on the measurementtechnique, measurement distance or the analysis technique used for transit timedetermination.Conclusion: A finite transit time increases the radiation dose beyond that due to theprogrammed source dwell time alone. The significance of the transit dose would increasewith a decrease in source dwell time or a higher activity source.

[发布机构] University of the Witwatersrand