Session Item

Robotic Stereotactic Body Radiotherapy (SBRT) employs radiopaque fiducial markers implanted near the tumor for real-time tracking. Fiducials are usually placed before simulation in a dedicated interventional radiology platform. This represents a limitation to treatment availability and may result in potential treatment delay. In our Institution, an in-house percutaneous CT-guided fiducial placement procedure was implemented for pelvic SBRT. The aim of our study is to evaluate the performance and side effects of in-house fiducial placement.

Patients underwent percutaneous fiducial insertion with a 18 G needle under CT guidance, using a radiopaque skin marker to calculate the depth of target location from body surface (Figure 1). Careful skin asepsis and local anaesthesia was obtained at the insertion site. A >2 cm spacing between fiducials was advised. One week after placement, simulation CT and orthogonal X-ray imaging were acquired to verify fiducial usability for SBRT tracking. Data from a consecutive cohort of patients treated with fiducial-guided, robotic-arm pelvic SBRT were collected from January 2018 to September 2021. Success rate was defined as the implanted/tracked fiducials ratio. Kruskal Wallis-test was used to compare median success rate over time.

In the observed time frame, 282 patients underwent CT-guided fiducial placement, accounting for 883 implanted fiducials (median 3, range 1-4). Target sites were the prostate bed, extra-spinal bones and pelvic lymph nodes in 158 (56%), 37 (13%) and 87 (31%) patients, respectively. Side effects consisted of minor bleeding at the insertion site and transient pain requiring medication after 24 hours in 5 patients (2%). No grade >2 toxicity was observed, particularly infectious or haemorrhagic complications.

Overall success rate was 86% (719/833); median success rate per procedure was 100% (range 50-100%). Among the 114 fiducials rejected for tracking, failure was due to migration in 63 cases (55%) and misplacement in 51 cases (45%). In the subset of patients receiving ≥3 fiducials (n=161), at least 3 fiducials were used for tracking in 80% (n=129) of patients.  Overall success rate increased across the observed time window from 2018 (53/73, 74%) to 2019 (245/293, 84%) to 2020 (246/272, 90%) to 2021 (175/195, 90%) (Figure 2). A consistent, statistically significant improvement in median success rate was observed over time from 2018 (75%, Interquartile Range, IQR 67-100%) to 2019 (100%, IQR 75-100%)   to 2020 (100%, IQR 75-100%)   to 2021 (100%, IQR 100-100%) : p= 0.0008.

Our in-house percutaneous CT-guided fiducial placement is a safe procedure requiring minimal standard equipment, resulting in success rates comparable with published experiences performed in a dedicated interventional radiology setting. A consistent improvement in median success rate was observed over 4 years, suggesting the need for appropriate interventions to shorten the learning curve.