A dose verification method for high-dose-rate brachytherapy treatment plans.

AIM: To evolve a fast dose verification method for high-dose-rate (HDR) brachytherapy treatment plans and to demonstrate its applicability in different clinical cases. MATERIALS AND METHODS: We developed a software tool in VC++ for the Varisource HDR unit for HDR dosimetry plan verification using TG-43 parameters. HDR treatment dosimetry of a number clinical cases using Varisource was verified by comparison with the treatment planning system (TPS). RESULTS: A number of different types of clinical cases treated by Varisource were evaluated. TPS calculated dose values and verification code calculated dose values were found to agree to within 3% for most of the dose calculation points. CONCLUSIONS: We have validated with clinical cases a fast and independent dose verification method of the dosimetry at selected points for HDR brachytherapy treatments plan using TG-43 parameters. This can be used for the verification of the TPS calculated dose at various points. The code is written to work with Varisource, but it can conceivably be modified for other sources also by using the fitted constant of the respective source.

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.