Flexible intervention workflow for MRI guided robotic prostate seed brachytherapy

Low-dose-rate prostate seed brachytherapy (BT) is an established guideline-conformal treatment option for low-risk prostate cancer. Precise seed placement is essential for achieving adequate dose coverage while minimizing exposure of surrounding healthy tissue. MRI guidance offers superior soft-tissue contrast for target visualization, but technical challenges remain for accurate transrectal seed implantation under in-bore conditions.

The Remote Control Manipulator (RCM) is an MRI-compatible robotic guidance system originally designed for in-bore transrectal targeted prostate biopsies. This work investigates its feasibility for MRI-guided transrectal targeted focal prostate seed BT. We demonstrate the intervention workflow and quantify seed placement accuracy in a preclinical phantom experiment.

The proposed intervention workflow defines the setup steps, interaction of the treatment planning system (TPS) with the robot's navigation software, MR-imaging as well as seed- and needle-guide registration. We performed and analyzed the total workflow in an anthropomorphic pelvis phantom. Seed positions corresponding to the initial treatment plan, the expected positions after needle-guide alignment, and the final released seed positions were assessed in detail. Their geometric displacement was decomposed into contributions from robotic alignment, seed release, and registration uncertainty. Dosimetric consequences were evaluated using the target's V_100% dose coverage.

Robotic alignment produced a mean expected seed displacement of 0.6 ± 0.2 mm. Total deviation from planned to released seed positions was 2.6 ± 0.9 mm, with 0.7 ± 0.4 mm along-trajectory displacement. MRI-only seed registration differed from CT-verified reconstruction by 2.8 ± 1.4 mm. After two released needles, V_100% declined from 100% to 92.9% using MRI-only data and to 85.7% using CT-verified seed positions.

The workflow was successfully executed and demonstrated sub-millimeter robotic targeting precision. Seed displacement during release and limited MRI-only seed localization remain dominant sources of error impacting dose coverage. Active needle guidance during imaging and improved MRI-only seed reconstruction could enable monitoring the live dose distribution and adaptive trajectory correction.