In-air calibration of an HDR 192Ir brachytherapy source using therapy ion chambers.

The Gammamed Plus 192Ir high dose rate brachytherapy sources were calibrated using the therapy level ionization chambers (0.1 and 0.6 cc) and the well-type chamber. The aim of the present study was to assess the accuracy and suitability of use of the therapy level chambers for in-air calibration of brachytherapy sources in routine clinical practice. In a calibration procedure using therapy ion chambers, the air kerma was measured at several distances from the source in a specially designed jig. The room scatter correction factor was determined by superimposition method based on the inverse square law. Various other correction factors were applied on measured air kerma values at multiple distances and mean value was taken to determine the air kerma strength of the source. The results from four sources, the overall mean deviation between measured and quoted source strength by manufacturers was found -2.04% (N = 18) for well-type chamber. The mean deviation for the 0.6 cc chamber with buildup cap was found -1.48 % (N = 19) and without buildup cap was 0.11% (N = 22). The mean deviation for the 0.1 cc chamber was found -0.24% (N = 27). Result shows that probably the excess ionization in case of 0.6 cc therapy ion chamber without buildup cap was estimated about 2.74% and 1.99% at 10 and 20 cm from the source respectively. Scattered radiation measured by the 0.1 cc and 0.6 cc chamber at 10 cm measurement distance was about 1.1% and 0.33% of the primary radiation respectively. The study concludes that the results obtained with therapy level ionization chambers were extremely reproducible and in good agreement with the results of the well-type ionization chamber and source supplier quoted value. The calibration procedure with therapy ionization chambers is equally competent and suitable for routine calibration of the brachytherapy sources.

Dose uniformity assessment of intraluminal brachytherapy using HDR 192Ir stepping source.

PURPOSE: The aim of this study is to achieve dose uniformity for intraluminal implants by assessment of dose distributions for single catheter generated by using various combinations of source stopping spacing and optimization mode. MATERIALS AND METHODS: A dose distribution was generated using HDR 192Ir stepping source on single straight catheter of fixed length used for Intraluminal brachytherapy. The various combinations of source position spacing and optimization mode were used and these dose distributions were evaluated by using three different parameters. The source position spacings were 0.2, 0.5, 1.0, 1.4, 2.0, 2.5, 3.0 and 3.3 cm. Three different optimization modes that compute the source stopping times along the catheter were used. The parameters used for assessment of dose distributions were statistical analysis of doses to dose reference points, area under natural dose-volume histogram and the dose non-uniformity ratio. RESULTS: None of the combinations of source position spacing and optimization mode was able to generate the desired optimum uniform dose distribution. However in a discrete manner, comparatively higher uniform dose distribution was found with short (0.2 cm) and longer (1.5 to 2.0 cm) source spacing. Optimization mode of Iterative correction was found to be suitable for the single catheter used in intraluminal brachytherapy. CONCLUSION: The applicator dimension and irradiation target volume should be taken in to consideration while selecting either higher or smaller source position spacing for the single catheter intraluminal brachytherapy. The Anisotropy factor of the source has some role in the variation of the dose uniformity over the target volume.