Evaluation of post-plan dosimetry using TRUS and CT after transperineal prostate seed implant

The purpose of this study was to evaluate the variability in dosimetry due to the change in prostate volume for permanent transperineal brachytherapy seed implant. This research is the beginning of an in-house quality assessment program. Nineteen cases were retrospectively evaluated. A single physician defined prostate volumes in all cases. Group A consisted of 3 cases that were treated with external-beam radiation therapy (EBRT) to 4500 cGy, followed by a brachytherapy implant boost of 10,800 cGy. Group B included 16 cases that were implant only, prescribed to 14,400 cGy. Prostate images were acquired before seed implant using transrectal ultrasound (TRUS), immediately following seed implant using TRUS, and by computed tomography (CT) acquired several weeks postimplant. The prostate images were digitized into a commercial treatment planning system for planning purposes and dosimetric evaluation for the 3 procedures. Prostate volumes were calculated by the treatment planning system. Additional data collected included the percentage of prostate receiving the prescribed dose and dose to 90% and 80% of the prostate. The dose delivered to V150 was also recorded. Overall, the postimplant ultrasound plan showed similar coverage to the ultrasound preplan, while the CT postplan revealed less than expected dosimetric coverage. The postplan CT results prompted us to evaluate our scheduling process, as well as prostate definition using TRUS and CT.