Tomosynthesis-based intraoperative dosimetry for low dose rate prostate brachytherapy

The aim of this study is to develop an intraoperative dose assessment procedure that can be performed after an I-125 prostate seed implantation, while the patient is still under anaesthesia. To accomplish this, we reconstruct the 3D position of each seed and co-register it with the prostate contour acquired with a transrectal ultrasound (TRUS) probe. Our seed detection method involves a tomosynthesis-based filtered reconstruction of the volume of interest requiring 7 projections acquired over an angle of 60° with an isocentric imaging system. The co-registration between the tomosynthesis-based seed positions and the TRUS-based prostate contour is based on the planned position. A phantom and a clinical study (25 patients) were carried out to validate the technique. In the patient study, the automatic tomosynthesis-based reconstruction yields a seed detection rate of 96.7% and less than 2.6% false-positive. The seed localization error obtained with a phantom study is 0.4 +/- 0.4 mm. The co-registration method based on planned seed position has proved to be not accurate enough for dosimetric purposes. We believe that this technique may be used to discover considerable underdosage and to improve the dosimetric coverage by potentially reimplanting additional seeds.