Dose perturbation due to catheter materials in high-dose-rate interstitial Ir-192 brachytherapy

PURPOSE: Catheters made of either metal or plastic are currently used in brachytherapy treatment to insert radiative sources into patients. However, the radiation dose perturbations due to catheter attenuation are not taken into account in treatment planning. The purpose of this work is to quantify the effects of catheter composition on dose distribution and study their impacts on the overall treatment with high-dose-rate Ir-192 sources. METHODS AND MATERIALS: Dose perturbations are first studied in a simplified case consisting of two parallel catheters. The catheter wall is either composed of stainless steel or polyoxymethylene. The attenuations are studied as the distance between the two catheters is varied from 5 to 30 mm. Dose perturbations resulting from irradiation are evaluated with a Monte Carlo GEANT4 dose calculation algorithm. The dose differences are further investigated with seven typical high-dose-rate prostate treatment plans involving 17 catheters. RESULTS: The dose differences compared with water in the simplified case reach -4.3 +/- 0.1% for stainless steel and 1.7 +/- 0.5% for polyoxymethylene at 10 mm above the source when the catheters are separated by a distance of 5 mm. Dose perturbations are reduced in real treatment plans because of the contributions of the many dwell positions. Stainless steel and polyoxymethylene catheters induce on an average a dose difference of -1.3 +/- 0.3% and 0.1 +/- 0.2%, respectively in the target. CONCLUSIONS: The dose differences reported in this work do not warrant any changes in the clinical procedures.