Room: ePoster Forums
Purpose: To identify and investigate planning techniques that can possibly reduce low-to-intermediate dose spillage (measured by R50%, and D2cm values) in lung SBRT plans.
Methods: Dose fall-off outside the target was studied in 102 SBRT VMAT plans of lung tumor. Plans having R50%, or D2cm higher than RTOG 0915, 0813 recommended tolerances were re-planned with two new optimization constraints. Deviation in D2cm can be satisfied using dose optimization constraint to a uniform 2 cm wide ring outside PTV. R50% dose constraint required creation of a ring (OptiForR50) extending 5 cm axially and 1 cm longitudinally outside PTV. Wilcoxon signed-rank test was performed to compare changes in R50% and D2cm of target, V20 to normal lungs and doses to OARs.
Results: In 102 lung SBRT plans with PTV sizes ranging from 5 to 179 cc, 32 plans with minor deviations in R50% or D2cm were re-optimized. Reductions of 4.3Â±4.8% and 0.8Â±0.4% (p-values < 0.05) were observed in the mean values of R50% and D2cm, respectively. V20 of normal lungs, mean dose to heart and aorta were significantly lowered.
Conclusion: Placing additional dose constraints on two new contours led to lower dose spillage in lung SBRT plans. All the re-optimized plans passed the RTOG-0915 criteria with no deviation.
Not Applicable / None Entered.