Room: Exhibit Hall | Forum 7
Purpose: D2cm and R50% defines falloff gradient beyond PTV extending into normal tissue. In this study we attempt to characterize dose spillage using dose gradient in the first 5 mm from the PTV edge.
Methods: We retrospectively analyzed 40 clinically implemented RapidArc lung SBRT plans that met RTOG 813 and RTOG 915 protocol. Seven 5 mm rings that are concentric with PTV were used to obtain the maximum and mean dose in each ring. Evaluating maximum dose in rings were very relevant since RTOG constraints R50% and D2cm are dependent on maximum doses away from the edge of PTVs.
Results: The prescriptions are either 48Gy or 50Gy in 4 fx. The maximum dose always appear on the inner edge of the ring closer to PTV and a more meaningful predictor of dose falloff gradient A dose falloff matrix of equivalent PTV diameter versus distance away from PTV edge in 5mm increment was generated. The dose falloff gradient systematically varied from 144cGy/mm for a 2.2 cm equivalent diameter PTV to 67 cGy/mm for a 5.5 cm equivalent diameter PTV in the first 5 mm away from the PTV edge.
Conclusion: RTOG dose spillage constraints D2cm and R50% are the bare minimum that can be achieved in VMAT Lung SBRT. A better predictor of dose spillage outside PTV is dose falloff gradient in the first 5 mm from the edge of the PTV. The average dose falloff gradient of 100cGy/mm in the first 5 mm sets the tone for plan quality that objectively meets or well below RTOG constraints. In VMAT planning there is plenty of potential to improve normal tissue sparing beyond D2cm and R50%.