BERKELEY, CA (UroToday.com) - In current treatment plans for brachytherapy, the attenuation due to the composition of the catheter wall is not taken into account. Our aim was to quantify the perturbations on the dose due to the composition of catheters and their impact on the overall treatment with a high-dose-rate 192Ir source.
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In order to do so, two scenarios were investigated. The first one was an idealized case comprised of two parallel and rigid catheters, one containing the source, and another one separated by a distance that varied from 5 mm to 30 mm, called the spectator catheter. The other scenario consisted of 7 real clinical cases of prostate brachytherapy treatment involving 17 catheters. In both cases, two materials were investigated for the catheter wall, namely polyoxymethylene and stainless steel. Dose distributions were computed by a Monte Carlo GEANT4 dose calculation algorithm. The perturbations were quantified by means of dose differences with respect to the case for which the catheter wall was made of water.
We have shown that the presence of the spectator catheter causes dose differences that can reach 4.3 ± 0,1% and 1.7 ± 0.5% if the wall is respectively made of stainless steel or polyoxymethylene, at 10 mm above the source, when the two catheters are separated by a distance of 5 mm. The dose differences are greater if the distance in the z-direction is increased for both compositions of wall studied. In fact, more energy is deposited in the wall in this direction if the catheter is made of stainless steel. For the polyoxymethylene wall, more air is traversed by primary photon and thus less energy is deposited in the wall. Moreover, the magnitude of the extrema of the dose differences is reduced as the distance between the two catheters is increased in a given fixed z plane for the two materials considered. In fact, less primary photons contribute to the dose deposition as the distance between the two is larger, resulting in a lowered dose difference behind the spectator catheter.
Even though the dose perturbations are large for two parallel catheters separated by a small distance, these perturbations are less pronounced in real clinical plans. In fact, the many dwell positions and the catheters geometry result in compensating effects for the increase or decrease in the dose difference. The average dose difference in the target for the seven clinical cases studied is 1.3 ± 0.3% if stainless steel catheters are used and 0.1± 0.2% if polyoxymethylene catheters are used. The largest and smallest dose differences are 3.7 ± 0.3% and 0.003 ± 0.288%, respectively, for stainless steel, and 1.1 ± 0.4% and 0.8± 0.3% for polyoxymethylene.
Based on these observations, no clinical changes are advised in the current clinical methods. However, measurements may be appropriate if the number of catheters used in the treatment is low.
Luc Beaulieu, PhD, et al. as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
Département de Radio-oncologie et Centre de recherche du CHU de Québec, CHU de Québec, Québec, Canada; Département de physique, de génie physique et d'optique, et Centre de recherche sur le cancer de l'Université Laval, Université Laval, Québec, Canada