Room: AAPM ePoster Library
Purpose: Safety in high-dose-rate (HDR) brachytherapy requires good attention to all aspects of the process. This study was designed to evaluate total time index (TTI), and assess its relevance to radiation protection of patients, in relation to doses to organs-at-risk and assessment of brachytherapy plans.
Methods: 300 patients who had intracavitary brachytherapy (ICBT) using HDR Gynesource unit (with Cobalt-60 of 5 mm step size) were considered in this study. Evaluation of TTI in the standard ICBT plans for treatment fractions was carried out with a mathematical relationship among TTI, air kerma strength, prescription dose, total dwell time and total number of dwell positions, as documented in literature. Patients’ treatment plans involved tandem-ring and vaginal cylinder applicators created by HDRbasic planning software. Doses to reference points B of the bladder and the rectum were determined on the standard plans for the tandem-ring applicators. Evaluated values of TTI for the subsequent treatment fractions were compared with those for the previous treatments. Data analysis was performed with GraphPad Prism 7 statistical software.
Results: The mean TTI values were 0.272±0.012, 0.167±0.012 and 0.134±0.008 (Gy.m²s)/(h.Gy) for ring applicators of tandem lengths 2, 4 and 6 cm respectively. Corresponding values for cylinder applicators were 0.148±0.03, 0.201±0.039 and 0.248±0.050 (Gy.m²s)/(h.Gy) respectively for cylinder diameters 20, 25 and 30 mm. These values are greater than limits reported in literature for 2.5 mm source-step size. The TTI for each applicator type showed statistically significant differences (p=0.000) and a significant association was observed between the index and doses at the reference points.
Conclusion: Variations in TTI across brachytherapy fractions were considered acceptable being < 5 %. Deviations > 5 % would require investigation before acceding to treatment continuation. The index therefore provides a means for comparison with previous fractions in order to assess the reasonability of any current BT plan.