KEY POINTS
- The retrospective planning study evaluated 16 cardiac targets in 14 patients previously included in prospective ventricular-tachycardia studies. For every target, three VMAT plans were generated: no probe, apical probe placement and parasternal probe placement.
- STAR was planned as 25 Gy in a single fraction using three coplanar 6-MV FFF arcs. Probe-sparing plans used a 2.5-cm planning-risk margin and modified gantry trajectories to avoid direct irradiation.
- The ultrasound probe could be spared in all 16 targets, with maximum D0.035cc remaining below 1 Gy. Median probe dose was only 0.2 Gy with apical placement and 0.4 Gy with parasternal placement.
- Target coverage was essentially preserved. Median PTV D98 was 24.1 Gy without the probe, 24.1 Gy with apical placement and 24.2 Gy with parasternal placement, with no significant differences in target dose-volume metrics.
- The trade-off was a broader intermediate-dose distribution. Median CI50 increased from 4.0 without the probe to 5.0 with apical and 4.5 with parasternal placement, indicating a less steep dose gradient.
- Normal-tissue exposure increased modestly but generally remained well within guidance values. Heart-minus-PTV V12.5 rose from 5.9% to 8.2% and 7.6%, while left-lung V10 increased from 3.1% to 4.2% and 3.4% with apical and parasternal placement, respectively.
- Plan complexity also increased slightly, particularly for apical placement: median monitor units rose from 7,744 to 8,802, although the authors estimate this would add less than one minute of beam-on time under typical FFF delivery.
CLINICAL TAKEAWAY
A hands-free echocardiographic probe can be incorporated into photon STAR planning without materially compromising target coverage, making real-time cardiac motion monitoring technically plausible. The study remains purely a planning feasibility analysis: actual ultrasound-guided gating, device durability under irradiation, patient-specific QA and motion-margin reduction still need clinical validation.