Room: Room 209
Purpose: To study planning techniques that cause higher than acceptable values of intermediate dose spillage in lung SBRT plans based on R50%, D2cm values stated in RTOG 0915 and explore simple ways to reduce them.
Methods: RTOG recommended target dose coverage, conformity index, homogeneity index, R50% and D2cm were evaluated retrospectively in 146 lung tumor SBRT plans. Deviations in R50% and D2cm were correlated with parameters including prescription dose, tumor location, number of beams or arcs, beam configuration (coplanar or non-coplanar), type of treatment plan (IMRT or VMAT), and shortest distance to the chest wall. A chi-square or Mann-Whitney correlation statistic was performed on R50% and D2cm against the above mentioned parameters. VMAT plans with deviations in R50% and/or D2cm were re-planned by revising the couch angle to Â±10 degrees and were re-evaluated for clinical acceptability based on the RTOG metrics.
Results: All clinical plans met the target coverage, conformity index, homogeneity index and critical organ dose tolerance objectives. Significantly large number of deviations are seen in either R50% or D2cm in coplanar plans than non-coplanar plans (p-value<0.001). The deviation does not depend on any of the other mentioned variables with p-values > 0.05. When re-planned with Â±10 degree couch angle, every VMAT plan with R50% or D2cm deviations showed a decrease in R50% or D2cm.
Conclusion: This study shows that coplanarity may cause higher than acceptable dose spillage and introduction of non-coplanar beams could reduce the dose spillage in lung SBRT plans.
Not Applicable / None Entered.