The Early Experience of Synchrony in Ultra-Hypofractionation for Prostate Cancer: A Case Series.

Ultra-hypofractionated radiotherapy has emerged as an effective treatment for localized prostate cancer, offering comparable oncologic outcomes to conventional fractionation while significantly reducing treatment duration. However, the delivery of high doses per fraction demands exceptional precision to minimize toxicity risks, particularly in the context of intrafractional prostate motion. Real-time tracking systems, such as Synchrony (Accuray, Sunnyvale, CA, USA), aim to address this challenge by continuously monitoring and correcting for target displacement during beam delivery. This study evaluates the technical feasibility and early clinical outcomes of ultra-hypofractionated radiotherapy (36.25 Gy in five fractions) delivered with Synchrony real-time tracking, with specific analysis of prostate specific antigen (PSA) kinetics, treatment delivery parameters, and safety. Five consecutive patients received fiducial-based radiotherapy with continuous motion tracking. PSA levels were monitored during the first two months post-treatment. Treatment efficiency metrics (including beam interruptions and motion events) and acute toxicity (Common Terminology Criteria for Adverse Events (CTCAE) v5.0) were prospectively recorded. A rapid biochemical response was observed, with a mean PSA decline rate of -4.27 ng/mL/month. System performance demonstrated stable rigid-body tracking with a median deviation of 1.04 mm (IQR: 0.74 to 1.39 mm), and clinically acceptable posterior displacement in the Z-axis with a median of -1.54 mm (IQR: -2.80 to -0.38 mm). The composite 3D target offset had a median of 2.52 mm (IQR: 1.53 to 3.30 mm). Automatic pauses occurred when motion exceeded safety thresholds, increasing mean treatment duration by 13.9% (actual: 586.5 seconds vs. planned: 514.9 seconds). Despite systematic posterior tracking offsets, toxicity was minimal (one grade 1 GI event). Synchrony tracking enabled precise ultra-hypofractionated delivery with submillimeter accuracy and minimal toxicity. The modest increase in treatment time is a justifiable trade-off for real-time motion adaptation. These early results support further investigation into Synchrony-optimized prostate ultra-hypofractionated radiotherapy.