The influence of a rectal ultrasound probe on the separation between prostate and rectum in high-dose-rate brachytherapy

PURPOSE:The aim of this study was to compare the distance between prostate and rectum as well as rectal dose-volume histogram (DVH) parameters for high-dose-rate (HDR) prostate brachytherapy (BT) with and without a transrectal ultrasound (US) probe in place during delivery. METHODS AND MATERIALS:The study included 20 patients with high-risk prostate cancer treated consecutively with combined external beam radiotherapy (EBRT) and MRI-based HDR-BT. The MRI-based HDR-BT dose plan and prostate gland contour were transferred to the US images after rigid MRI/US coregistration, followed by delineation of the rectum on US images acquired with a transrectal US probe. The prostate-rectum separation was estimated at the apex, reference, and base plane on the US (with rectal probe) and MR images (without rectal probe). Rectal DVH parameters for EBRT + HDR-BT given in equivalent 2 Gy fractionation doses were estimated and compared for US-based and MRI-based HDR-BT dose planning. RESULTS:The median (and range) prostate-rectum separation increased on MR images (without rectal probe) as compared with on US images (with rectal probe) by 10 mm (-5, 18) at the base, 1 mm (-2, 3) at the reference and decreased at the apex by 2 mm (-5, 11). The rectal D5.0cm3, D2.0cm3, and D0.1cm3 decreased by a median of 4 Gy (-1, 10), 4 Gy (-2, 13), and 7 Gy (-4, 26), respectively. CONCLUSIONS:MRI-based HDR-BT without a rectal US probe in place as compared with US-based BT with the probe in place demonstrated a significant increase in the prostate-rectum separation, with a potential of reducing rectal dose.