Monte Carlo aided dosimetry of the microselectron pulsed and high dose‐rate ¹⁹²Ir sources

TLDR

Few data exist on dose‑rate distributions around high‑intensity 192Ir sources used in remote afterloading devices. To calculate complete two‑dimensional dose‑rate distributions for the most common HDR and PDR 192Ir sources using Monte Carlo. A Monte Carlo photon transport code incorporating detailed source geometry was used to compute dose‑rate per unit air‑kerma strength in water over 0.15–12 cm on the transverse bisecting axis and polar profiles up to 5 cm, tabulating the results with IGWG/TG‑43 formalism. The calculated dose‑rate constants (1.115 and 1.128 cGy h⁻¹ per unit air‑kerma strength) agree with prior data, but oblique filtration by the iridium core produced 35–55 % anisotropy deviations near the longitudinal axis, and the distributions were also shown in Cartesian coordinates.

Abstract

Despite the large number of single‐stepping source pulsed and high dose‐rate (HDR) remote afterloading devices in clinical use, the published literature contain little data characterizing dose‐rate distributions around the high‐intensity (4×10 3 –4×10 4 μGy m 2 h −1 ) 192 Ir sources currently used in these devices. We have used the Monte Carlo method to calculate complete two‐dimensional dose‐rate distributions about the most widely used high dose‐rate source design, as well as the Nucletron pulsed dose‐rate (PDR) 192 Ir source. A Monte Carlo photon transport code, incorporating the detailed internal geometry of the source, was used to calculate the dose rate per unit air‐kerma strength in water medium on the transverse bisecting axis over the 0.15–12 cm distance range. In addition, polar dose profiles were calculated at distances ranging from 0.25 to 5 cm. The PDR and HDR dose‐rate distributions are tabulated using the formalism endorsed by the Interstitial Collaborative Working Group and the AAPM Task Group 43, and includes dose‐rate constant, radial dose function, anisotropy function, geometry function, and anisotropy factors. The dose‐rate constants, Λ, of the MicroSelectron/HDR and PDR sources were found to be 1.115 and 1.128 cGy h −1 per unit air‐kerma strength, respectively, in good agreement with previously published data for low dose‐rate interstitial 192 Ir sources. Oblique filtration by the high‐density iridium metal core resulted in deviations from anisotropy as large as 35%–55% near the longitudinal axis of the source. Dose‐rate distributions are also presented in Cartesian (“away” and “along”) coordinates.