State of Lung SBRT across Treatment Planning Systems: Implications for Small‑Field Dosimetry
Abstract
Purpose
Lung stereotactic body radiotherapy (SBRT) is now standard of care for oligometastatic and small pulmonary lesions, where very small radiation fields are used. Although inhomogeneity correction algorithms have been well validated for larger fields, limited data exist for small fields in low‑density media. This study evaluates the performance of modern inhomogeneity correction algorithms for small fields in lung‑equivalent material, supported by measurements using detectors appropriate for small‑field dosimetry.
Methods
A lung phantom constructed from cork slabs (density: 0.25 g/cm³) was designed to approximate patient anatomy for treatment planning. Three contemporary TPS platforms (Eclipse, RayStation, Monaco) were investigated using all available dose‑calculation algorithms, alongside EGSnrc-based Monte Carlo simulations, for a 6 FFF beam. Measurements were performed in the phantom using small‑field‑appropriate detectors: microSilicon, microDiamond, and a plastic scintillation detector (PSD). Field sizes ranged from 0.5 × 0.5 cm² to 10 × 10 cm², with all data normalized to a 10 × 10 cm² reference field.
Results
When compared with EGSnrc Monte Carlo calculations, TPS algorithms performance varied substantially. For field sizes 1 cm², differences were within ±4% for all algorithms except AAA, which showed larger deviations. When inhomogeneity correction was disabled (water‑equivalent assumption), differences were observed only for fields <1 cm². The observed inaccuracies likely arise from limited or inaccurate TPS beam data for fields <2 cm². Monte Carlo‑based algorithms demonstrated better agreement than convolution‑based methods.
Conclusion
The uncertainty of modern TPS algorithms is much larger for small‑field dosimetry in low‑density lung media, with errors up to −22% to +16% for the smallest fields. Measurements in heterogeneous media also exhibit challenges related to detector positioning, orientation, and material composition. Extreme caution is warranted when using fields <1 cm² in lung SBRT, given the potential for clinically significant dose deviations.