Dosimetric characterization of round HDR 192Ir accuboost applicators for breast brachytherapy

The AccuBoost brachytherapy system applies HDR 192Ir beams peripherally to the breast using collimating applicators. The purpose of this study was to benchmark Monte Carlo simulations of the HDR 192Ir source, to dosimetrically characterize the round applicators using established Monte Carlo simulation and radiation measurement techniques and to gather data for clinical use.

Dosimetric measurements were performed in a polystyrene phantom, while simulations estimated dose in air, liquid water, polystyrene and ICRU 44 breast tissue. Dose distribution characterization of the 4-8 cm diameter collimators was performed using radiochromic EBT film and air ionization chambers.

The central axis dose falloff was steeper for the 4 cm diameter applicator in comparison to the 8 cm diameter applicator, with surface to 3 cm depth-dose ratios of 3.65 and 2.44, respectively. These ratios did not considerably change when varying the phantom composition from breast tissue to polystyrene, phantom thickness from 4 to 8 cm, or phantom radius from 8 to 15 cm. Dose distributions on the central axis were fitted to sixth-order polynomials for clinical use in a hand calculation spreadsheet (i.e., nomogram). Dose uniformity within the useful applicator apertures decreased as depth-dose increased.

Monte Carlo benchmarking simulations of the HDR 192Ir source using the MCNP5 radiation transport code indicated agreement within 1% of the published results over the radial/angular region of interest. Changes in phantom size and radius did not cause noteworthy changes in the central axis depth-dose. Polynomial fit depth-dose curves provide a simple and accurate basis for a nomogram.