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.