Reducing Errors In Prostate Tracking With An Improved Fiducial Implantation Protocol For Stereotactic Body Radiotherapy (Sbrt)

SBRT is an established technique for treating prostate cancer. Robotic SBRT requires implantation of fiducial markers for target tracking. Fiducials must be easily identified on images used for treatment planning and by the orthogonal KV X-ray imaging system. The spatial distribution of fiducials must allow accurate calculation of a 3D transformation that describes the position of the prostate within the reference frame of the planning CT scan. When we began our robotic prostate SBRT program, we were unable to accurately perform 3D tracking in 23% of patients. As a result we developed a modified fiducial implantation protocol. 54 patients with prostate adenocarcinoma were treated with robotic SBRT. For treatment planning purposes, CT planning scans were accurately co-registered to T1 weighted gradient echo and T2 weighted turbo spin echo MR image sequences using transrectally implanted platinum fiducial markers. Due to differences in magnetic susceptibility, platinum markers are better visualized on MRI than gold. For the first 26 patients, fiducials were implanted under ultrasound guidance according to the manufacturer’s fiducial placement guidelines (cohort 1). Unfortunately our initial tracking error rate was high (23%). In Oct 2016 we developed a more rigorous protocol and treated a second cohort of patients accordingly (cohort 2, 28 patients). In the protocol, 4 platinum fiducials were implanted in the postero-lateral peripheral zone in a single coronal plane. In cohort 1, patients had a mean age of 64 years (50 - 74), PSA of 6.6mcg/L (1.1 – 14.7), and prostate volume of 56cc (22 - 125), while in cohort 2 they had a mean age of 65 years (53 - 75), PSA of 6.2 mcg/L (1 - 12) and prostate volume of 47cc (21 - 106). In all 54 patients’ fiducials were easily visualized and there were no cases of urosepsis related to fiducial implantation. In 6 of 26 patients (23%) from cohort 1 only translational mapping without accurate spatial rotations could be calculated. Accurate 3D tracking (accounting for translations and rotations) was possible in all 28 patients from the protocol group. Adopting a formal fiducial placement protocol improved our ability to identify the position of the prostate during the treatment in the CT planning frame from 77% to 100% reliability.