KEY POINTS
- The retrospective single-centre study replanned 25 stage I NSCLC cases by comparing non-gated internal target volumes derived from all ten respiratory phases with gated volumes derived from the 30–70% end-expiratory window. Both approaches used a 5 mm ITV-to-PTV margin.
- Treatment regimens were heterogeneous: 15 patients received 48 Gy in 12 fractions, six received 54 Gy in three fractions, and four received 50 Gy in five fractions. The inclusion of 12-fraction treatment in most patients limits direct generalization to contemporary lung SBRT practice.
- Gating reduced mean ITV from 22.02 to 11.83 cm³ and mean PTV from 48.87 to 31.25 cm³, both p < 0.001. The average relative PTV reduction was 34% ± 15%.
- Target dose remained comparable. Mean PTV dose was 51.55 versus 51.87 Gy (p = 0.15), with no significant differences in D2%, D95%, D50%, conformity, or homogeneity.
- Dose fall-off improved by some measures: gradient measure decreased from 1.34 to 1.19 cm, and dose 2 cm from the PTV decreased from 30.0 to 28.1 Gy, both p < 0.001. However, R50% increased from 4.21 to 4.55, partly reflecting the smaller denominator created by PTV reduction.
- Mean lung dose fell from 3.84 to 3.08 Gy, while V20 decreased from 5.13% to 3.73%, V10 from 9.9% to 7.9%, and V5 from 17.9% to 14.3%; all comparisons were p < 0.001. Smaller but significant reductions were also reported for heart, oesophagus, bronchus, spine, and ribs.
- Gated delivery used only approximately 40% of the respiratory cycle, corresponding to an estimated treatment time about 2.5 times longer. The plans were calculated on a static CT without four-dimensional dose accumulation or interplay modeling, and the patients were actually treated without gating, so no toxicity or local-control benefit was demonstrated.
CLINICAL TAKEAWAY
End-expiratory gating can meaningfully reduce treated lung volume, particularly for mobile lower-lobe tumors or patients with limited pulmonary reserve. The benefit should be individualized against longer treatment time and motion reproducibility; these planning data alone do not establish better clinical outcomes.