Study of the correlation between rectal wall in vivo dosimetry performed with MOSkins and implant modification during TRUS-guided HDR prostate brachytherapy

In transrectal-US guided HDR prostate brachytherapy, modifications of the geometry of the needle implant can arise in the time lapse between image acquisition and dose delivery. The aim of the present study was to investigate whether in vivo dosimetry measurements could be directly correlated to possible intra-fraction modifications of the implants. During 14 HDR prostate brachytherapy treatments, in vivo dosimetry measurements were performed with an array of two MOSkin detectors integrated onto the longitudinal axis of the transrectal-US probe. The implant displacement was quantified as the difference between the barycenters of the needle location on images acquired prior to treatment planning and after treatment delivery (ΔIB). A further parameter that takes into account also the needle distance from the dosimeters and the planned source dwell times was calculated (ΔwIB). Correlation analysis was performed to indicate any possible relationship between measured and calculated dose discrepancies (ΔDDPPvsTPS) and both ΔIB and ΔwIB. ΔDDPPvsTPS, ranged from a minimum of −19.5% to a maximum of 15.8%. Analysis of the implant barycentre shifts demonstrated a clear systematic trend of the needle implant in the direction of the rectum, with a consequent increase of the dose to the rectal wall. A moderate correlation of r = 0.64 was found between ΔDDPPvsTPS and ΔIB in the antero-posterior direction (ΔIBy), with r increasing to 0.74 if ΔwIBy was considered. The modification in dose to the rectal wall, as a consequence of implant migration that possibly happens in the time lapse between image acquisition and dose delivery, was correctly identified by IVD performed with MOSkins. This study is a further demonstration on how the application of IVD systems may facilitate improvements in quality for HDR prostate brachytherapy