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Prostate cancer patients with an enlarged prostate and/or excessive pubic arch interference (PAI) are generally considered non-eligible for treatment with high-dose-rate brachytherapy (HDR BT) or require hormonal therapy to down the prostate. Recently, steerable needles have been developed that allow for high targeting accuracy and curved needle trajectories (de Vries et al. PLOS One, 2021). The purpose of this study is to validate 1) the ability of these novel steerable needles for the avoidance of PAI and 2) the feasibility of implanting the steerable needles according to the treatment plan in the conventional clinical setting.

HDR BT treatment planning and implantation studies were performed with steerable needles and an anthropomorphic prostate phantom. The phantom was based on a patient dataset not directly eligible for HDR BT because of the enlarged prostate volume (Vp > 55 cm3) and excessive PAI of 10 mm. Four pre-treatment plans were generated in the Oncentra® Prostate (OCP) treatment planning system (TPS) (Elekta Instrument AB, Stockholm, Sweden) considering the dosimetric constraints: prostate V100 > 95%, urethra D0.1cm3 < 115% and rectum D1cm3 < 75%. One plan (A) did not consider PAI, functioning as the baseline plan incorporating seventeen rigid needles. The other pre-treatment plans considered PAI of which two plans (B, C) only used rigid needles and one plan (D) incorporated eleven rigid and six steerable needles to overcome PAI (Fig.1). Dose-volume histogram (DVH) outcomes evaluated the quality of the pre-treatment plans. Implantation was performed under transrectal ultrasound guidance conform the pre-treatment plan with PAI by an experienced HDR BT physician with entry-level experience in the use of the steerable needles. Postimplant DVH information evaluated the conformance to the pre-treatment plans.

Table 1 shows an increase up to 17.1% in prostate V100 in the pre-treatment plan including steerable needles in comparison with the rigid needle plans. The dose to the urethra remained below the dose criteria of 115% in all plans and the DVH values in plan D approached upon the outcomes for the baseline plan (A) where no PAI was considered. The physician was able to implant the steerable needle plan with the highest conformance to the pre-treatment plan (Table 1), with no substantial impact on implantation time, no major changes in the clinical workflow, and no changes in the clinical set-up.

The steerable needle approach allows for more flexibility in needle placement, mitigation of deflections by countersteering and the avoidance of PAI. The curved trajectories can be easily planned in the OCP TPS and implantation can be performed with excellent agreement between pre-treatment plan and postimplant treatment plan. An adequate dose distribution can be achieved without prostate downsizing, and hence, men with enlarged and/or excessive PAI can be considered to be treated with HDR BT.