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Objective. A robotic needle implant device for MR-guided high-dose-rate (HDR) prostate brachytherapy was developed. This study aimed to assess the feasibility and spatial accuracy of HDR brachytherapy using the robotic device, for a single intraprostatic target point. Approach. Five patients were treated from November 2019-June 2022 with the robot. The robot fits a 1.5 T MR scanner and the needle can be shifted and angulated. An intraprocedural MR scan was fused with the diagnostic MR and one preplanned needle position was selected for robotic insertion. The needle entry point and angles were set for a needle tip target point within the intraprostatic target volume. The needle was tapped stepwise towards the target point pneumatically. Final needle position was verified with MR, followed by plan optimization and dose delivery. Any remaining planned needles were inserted manually. Needle tip to geometrical target error (NTG-error) was defined as the deviation of the actual tip position relative to the predefined geometric target point, using MR-coordinates. Needle tip to treatment target error (NTT-error) was defined as the deviation of the actual tip position relative to the treatment target point, using fused MR-images pre- and post-needle implantation taking into account prostate deformation. Difference between NTT-error and NTG-error and fiducial marker shifts indicated prostate movement. For determining prostate deformation, the Jaccard index and prostate volumes were assessed. Main results. The robotic device was able to tap the needle to the planned depth for all patients. Mean robotic procedure duration was 142 min. NTG-error was 3.2 (range 1.1-6.7) mm and NTT-error 4.5 (range 2.6-9.6) mm. Marker displacements were smaller than 3 mm. No treatment-related acute toxicity was reported. Feasibility of needle placement within the prostate was considered adequate. Significance. MR-guided robotic needle insertion is feasible with a mean geometric accuracy of 3.2 mm and <3 mm prostate movement., (Creative Commons Attribution license.)

MR-guided brachytherapy offers a focal salvage treatment for the local recurrence in case of isolated locally recurrent prostate cancer in the prostate and/or seminal vesicles after primary radiotherapy. By focusing on only the local recurrence instead of the whole prostate, chances of additional toxicity of the bladder, urethra and rectum can be minimized. In almost all patients, the treatment leads to a good initial treatment response that persists in about half of patients, while others will develop progressive disease later on. For selecting suitable patients, factors such as preexistent urinary- and bowel complaints, localization and size of the recurrence, PSA doubling time and time between primary radiotherapy and development of the recurrence are relevant. MR-guided brachytherapy can provide a suitable salvage strategy, with the aims of deferring androgen deprivation therapy and a chance of cure.