Time-resolved in vivo dosimetry for source tracking in brachytherapy.

The purpose of this article is to demonstrate that brachytherapy source tracking can be realized with in vivo dosimetry. This concept could enable real-time treatment monitoring.In vivo dosimetry was incorporated in the clinical routine during high-dose-rate prostate brachytherapy at Aarhus University Hospital. The dosimetry was performed with a radioluminescent crystal positioned in a dedicated brachytherapy needle in the prostate. The dose rate was recorded every 50-100 ms during treatment and analyzed retrospectively. The measured total delivered dose and dose rates for each dwell position with dwell times >0.7 s were compared with expected values. Furthermore, the distance between the source and dosimeter, which was derived from the measured dose rates, was compared with expected values. The measured dose rate pattern in each needle was used to determine the most likely position of the needle relative to the dosimeter.In total, 305 needles and 3239 dwell positions were analyzed based on 20 treatments. The measured total doses differed from the expected values by -4.7 ± 8.4% (1SD) with range (-17% to 12%). It was possible to determine needle shifts for 304 out of 305 needles. The mean radial needle shift between imaging and treatment was 0.2 ± 1.1 mm (1SD), and the mean longitudinal shift was 0.3 ± 2.0 mm (1SD).Time-resolved in vivo dosimetry can be used to provide geometric information about the treatment progression of afterloading brachytherapy. This information may provide a clear indication of errors and uncertainties during a treatment and, therefore, enables real-time treatment monitoring.