Exhibit Hall | Forum 3
Purpose: Current literature for lung SBRT shows a diversity of dosimetric parameters, with some correlation to lung toxicity and a spread in the recommended tolerances to reduce the incidence of grade 2+ Radiation Induced Pneumonitis (RP): mean lung dose (MLD)<8Gy/V20Gy<10-15%(HyTEC); mean BED<5Gy; V(BED70)<2.22%. We performed a quality management study of lung SBRT plans with the intent to verify if we meet the recommended dosimetric criteria, to assess if there is any relationship between these criteria, and to explore opportunities for improving the planning approach and potentially treatment outcome.
Methods: Fifty patients were included in this study. The prescription dose was 48-50Gy/4-5 fractions. Lungs(Lungs-GTV) MLD/mean BED/V(BED70)/V20Gy were computed and analyzed to find the best parameters which can be used to improve plan quality. Follow-up data analysis was also performed.
Results: Lungs-GTV average MLD/mean BED/V(BED70) were 2.52Gy/5.94Gy/2%. Lungs average V20Gy was 3%. We found that Lungs(Lungs-GTV) mean BED/V(BED70), and Lungs V20Gy have a linear dependence on the Lungs(Lungs-GTV) MLD. A Lungs-GTV MLD(mean BED) of 2Gy(5Gy) would result in V(BED70)[%]<2%. 20 plans with mean BED≥6Gy were re-optimized by adding an “Upper gEUD” constraint for Lungs-GTV with parameter “a”=1 to prioritize mean dose. All reoptimized plans met the same target and OARs criteria as the original plans, and showed a reduction in mean BED (with >1.0Gy/2Gy reduction in 40%/15% of plans), V(BED70), and V20Gy.
Conclusion: For 32%(74%) of cases the Lungs-GTV MLD was <2(3)Gy. This resulted in a Lungs-GTV mean BED<5(7.5)Gy and a Lungs-GTV V(BED70)<1.5(2.5)%. Based on the literature, this could decrease the grade 2+RP probability to <5-10% and potentially increase the overall survival. Three patients developed grade 2+RP: one had an MLD>8Gy, one had a secondary SBRT treatment, and one had interstitial lung disease. For such cases with retreatment/comorbidities, it is important to keep the Lungs-GTV MLD as low as reasonably feasible.