Limited Benefit of Same‑Day Adaptive Planning In Cyberknife Prostate SBRT with Real‑Time Fiducial Tracking
Abstract
Purpose
Prostate stereotactic body radiation therapy (SBRT) requires high geometric precision due to large fractional doses and tight margins. Intrafraction prostate motion as large as 7–10 mm has been reported in extreme cases. CyberKnife-based SBRT uses continuous real-time fiducial gating to actively compensate for intrafraction motion, maintaining treatment delivery accuracy of approximately 2 mm, supported by image-tracking precision of about 1 mm. The incremental clinical and dosimetric benefit of same-day adaptive planning in the presence of real-time tracking remains unclear. This study evaluates whether adaptive replanning provides meaningful dosimetric advantages over a non-adaptive, tracking-enabled workflow.
Methods
A prospective dosimetric analysis will be conducted for at least 25 patients with localized prostate cancer treated with CyberKnife SBRT using fiducial-based real-time tracking. Each patient will undergo one planning CT (PlanCT) and 1–5 daily pre-treatment CT scans. The original SBRT plan will be generated on the PlanCT using 3–5 mm margins. For each daily CT, the original plan will be recalculated following rigid fiducial-based registration, simulating delivered dose without adaptive replanning while accounting for real-time motion correction. Organs at risk (OARs), including rectum, bladder, urethra, and seminal vesicles, will be recontoured. Dose–volume histogram (DVH) metrics and normal tissue complication probability (NTCP) models will be used for comparison.
Results
Daily DVH analysis demonstrates maintained target coverage with only minor degradation in recalculated doses. Variations in bladder and rectal filling are observed but remain well below clinical tolerance. Differences in PTV coverage, hotspot magnitude, and OAR dose metrics are minimal and not clinically significant, with NTCP estimates showing only small variations.
Conclusion
For prostate SBRT delivered with CyberKnife real-time fiducial tracking, non-adaptive plans demonstrate robust dosimetric performance despite interfractional anatomical variability. Routine same-day adaptive planning may provide limited additional benefit, supporting a streamlined and efficient treatment workflow.