The absorbed dose distribution at sub-millimeter distances from the Best single 192Ir intravascular brachytherapy seed was measured using a high spatial resolution gel dosimetry system. Two gel phantoms from the same batch were used; one for the seed irradiation and one for calibration. Since the response of this gel is energy independent for photons between 20 keV and 1250 keV, the gel was calibrated using a narrowly collimated 60Co gamma-ray beam (cross sectional area ~ 1 cm2). A small format laser computed tomography scanner was used to acquire the data. The measurements were performed with a spatial-resolution of 100 µm in all dimensions. The seed was calibrated at NIST in terms of air kerma strength. The absorbed dose rate as well as the radial dose function, gL(r), was measured for radial distances between 0.6 mm and 12.6 mm from the seed center. The dose-rate constant was measured, yielding a value of lambda = (1.14 ± 0.05) cGyh-1U-1, which agrees with published data within the measurement uncertainty. For distances between 0.6 mm and 1.5 mm, gL(r) decreases from a maximum value of 1.12 down to 1.05; between 1.5 mm and 6 mm, an enhancement is clearly observed with a maximum value around 1.23 and beyond 6 mm, gL(r) has an approximately constant value around 1.0, which suggests that this seed can be considered as a point source only at distances larger than 6 mm. This latter observation agrees with data for the same seed reported previously using Gafchromic film MD-55-2. Additionally, published Monte Carlo (MC) calculations have predicted the observed behavior of the radial dose function resulting from the absorbed dose contributions of beta particles and electrons emitted by the 192Ir seed. Nonetheless, in the enhancement region, MC underestimates the dose by approximately 20 %.
Citation: Physics in Medicine and Biology
Pub Type: Journals
3D dosimetry, BANG del dosimetry, High-spatial-resolution, Ir-192, optical-CT-scanner