Temporal-Spatial Dose Pattern in Vaginal Cylinder Applicator for High Dose Rate (HDR) Brachytherapy

Abstract

High Dose Rate (HDR) brachytherapy delivery patterns have been described by the dwell energy density distribution index (DEDDI). For simplification, the DEDDI was computed for HDR Brachytherapy with vaginal cylinder applicator for GYN patient as an investigation of potential clinical advantage through optimization of dose delivery pattern. For this study, five cases of cylinder applicator GYN HDR brachytherapy were selected. Each patient was accomplished for a course of three fractions with each fraction prescription dose at 500cGy, 600cGy or 700cGy at vaginal surface and at 0.5 cm depth. The single channel cylinder diameters range to 2.5cm to 3.5cm, the delivery time ranged from 290 to 350 seconds, and the considered volumes were 100%, 90%, 80%, 50%, 30% prescription isodose volume (PIV), and the volume of cylinder with 5 mm expansion. The values of DEDDI were computed for all these PIVs by subtracting the volume of cylinder. The source decay effect for different fractions was also computed with the DEDDI to estimate the variation for time factor for treatment course. At nominal source activity, based on the volume of cylinder applicator, 100%, 90%, 80%, 50%, and 30% of PIV, the average values in Cis/cc of DEDDI were 0.741, 0.886, 0.806, 0.836, and 0.948 with standard deviation at 0.509, 0.594, 0.542, 0.578, and 0.621. The corresponding fraction average values in Cis/cc of DEDDI were 0.854, 1.030, 0.873, 1.238, and 2.039 with standard deviation at 3.8%, 3.8%, 3.8%, 3.3% and 2.4%. Quantitative values of dose delivery pattern for HDR Brachytherapy is calculable, and the simplicity of single channel cylinder applicator provides advantage of initial clinical optimization when large cohort of patient outcome data are available.