Transit dose of an Ir-192 high dose rate brachytherapy stepping source

Clinical dosimetry for high dose rate (HDR) brachytherapy with a single ste pping source generally neglects the transit dose. This study investigates t he effects of the transit dose in the target volume of an HDR brachytherapy stepping source. A video method was used to analyse the entrance, exit and the interdwell transit speed of the source for different path lengths and step sizes ranging from 2.5 mm to 995 mm. The transit speed was found to va ry with the step size and path length. For the travelled distances of 2.5, 5.0, 10.0, 230 and 995 mm, the average transit speeds were 54, 72, 233, 385 and 467 mm s(-1) respectively. The results also show that the manufacturer has attempted to compensate for the effects of interdwell transit dose by reducing the actual dwell time of the source. A well-type chamber was used to determine the dose differences between two sets of measurements, one bei ng the stationary dose only and the other being the sum of stationary and t ransit doses. Single catheters of active lengths of 20 and 40 mm, different dwell times of 0.5, 1, 2 and 5 s and different step sizes of 2.5, 5 and 10 mm were used in the measurements with the well-type chamber. Most of the m easured dose differences between stationary and stationary plus interdwell source movement were within 2%. The additional dose due to the source trans it can be as high as 24.9% for the case of 0.5 s dwell time, 10 mm step siz e and 20 mm active length. The dose difference is mainly due to the entranc e and exit source movement but not the interdwell movement.