KEY POINTS
- The study analyzed 50 lung lesions from 41 patients and evaluated 76 prescription strategies, varying CT representation, prescribed volume, dose metric and calculation algorithm. Delivered dose was approximated by recalculating GTV dose across all 10 phases of the respiratory cycle.
- The lesions were small and mobile: median GTV was 1.9 cm³, median ITV 3.3 cm³, and median respiratory-motion amplitude 1.0 cm. Prescription strategies used single-phase mid-respiratory CT, average-intensity-projection CT or maximum-intensity-projection CT with GTV, ITV or PTV dose metrics.
- The most accurate strategy was GTV D50% on a mid-respiratory single-phase CT. More broadly, GTV or ITV prescriptions using D50%, mean dose or D2% on single-phase or average CT produced <2% systematic bias and <1.5% interpatient variability relative to the 4D delivered GTV dose.
- The main exception among PTV-based strategies was PTV D2%, which showed approximately 2% bias with <1.5% variability. Monte Carlo and collapsed-cone calculations showed no statistically significant difference across the evaluated dose indicators.
- Conventional PTV D95% and D98% prescriptions performed substantially worse. Figure 1 shows biases of approximately −15% to −18% for representative PTV coverage prescriptions, while Figure 2 demonstrates markedly greater interpatient variability than GTV/ITV-based approaches.
- The poor precision was partly explained by PTV density. For PTV D98%, the difference between prescribed and delivered GTV dose correlated strongly with PTV density on both single-phase and average CT (R≈0.8), meaning two patients receiving nominally the same prescription could receive meaningfully different tumor doses.
- Maximum-intensity-projection-based PTV prescriptions could show apparently good precision for selected metrics—with roughly 5.3% correctable bias and 1.5% precision for PTV D50%—but MIP performed poorly for representing GTV D2% and D98%. Historical type-A pencil-beam calculations were inaccurate across virtually all prescription approaches.
CLINICAL TAKEAWAY
A lung SBRT prescription such as “PTV D95%” may not describe the biological tumor dose as consistently as clinicians assume. Reporting median or mean GTV/ITV dose alongside PTV coverage could make clinical studies, dose-response analyses and interinstitutional comparisons substantially more interpretable.