Transit dosimetry in 192Ir high dose rate brachytherapy
Date
2010-12-02
Authors
Ade, Nicholas
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Abstract
Background and purpose: Historically HDR brachytherapy treatment planning systems
ignore the transit dose in the computation of patient dose. However, the total radiation
dose delivered during each treatment cycle is equal to the sum of the static dose and the
transit 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 as
computed during treatment planning. The aim of this study was to determine the
magnitude of the transit dose component of two 192Ir HDR brachytherapy units and assess
its dosimetric significance.
Materials and Methods: Ionization chamber dosimetry systems (well-type and Farmertype
ionization chambers) were used to measure the charge generated during the transit of
the 192Ir source from a GammaMed and a Nucletron MicroSelectron HDR afterloader
using single catheters of lengths 120 cm. Different source configurations were used for
the measurements of integrated charge. Two analysis techniques were used for transit
time determination: the multiple exposure technique and the graphical solution of zero
exposure. 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 depends
on the source activity, source configuration, number of treatment fractions, prescription
dose and the type of remote afterloader used. It does not depend on the measurement
technique, measurement distance or the analysis technique used for transit time
determination.
Conclusion: A finite transit time increases the radiation dose beyond that due to the
programmed source dwell time alone. The significance of the transit dose would increase
with a decrease in source dwell time or a higher activity source.
Description
Keywords
transit/time/dose, HDR brachytherapy, HDR planning system, afterloader, ionization chamber dosimetry system, source configuration, multiple exposure technique