Accurate measurement of transit time of the HDR brachytherapy source of a remote after-loading unit is necessary to calculate the total radiation dose given to the treatment volume. Presently, most of the HDR brachytherapy treatment planning systems neglect the transit time in the computation of dose. The aim of this investigation is to use a well type ionization chamber to measure the transit time during the source movement between two dwell positions. As well type ionization chamber and a precision electrometer (manufacturer CD instruments, Bangalore) were used to measure the charge generated during the movement of the Ir-192 source of a Gammamed HDR brachytherapy unit with an interstitial needle. Effective transit time and effective speed were determined on the basis of methodology described by Sahoo . Corrections were done on the basis of relative sensitivity values for varaious dwell position in the ionization chamber. In the present study the variation of effective speed with interdwell distance was minimal as compared with that of Sahoo . The effective transit times were 0.129, 0.182, 0.301, 0.402, 0.701, and 0.993 seconds for 1, 2, 4, 6, 8 and 10 cm interdwell separations respectively. The effective transit times in the present study were higher than those of Sahoo . Software modification accounting for the dynamic dose should be incorporated into all HDR planning systems. Such an improvement would enhance the safety and accuracy of HDR brachytherapy.
Financed by the National Centre for Research and Development under grant No. SP/I/1/77065/10 by the strategic scientific research and experimental development program:
SYNAT - “Interdisciplinary System for Interactive Scientific and Scientific-Technical Information”.