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A Planning Technique to Lower the Dose Spillage in Lung SBRT Plans Based On RTOG-0915 Metrics

G Narayanasamy1*, E Galhardo1 , D Desai2 , X Zhang1 , S Morrill1 , T Kim1 , S Maraboyina1 , L Peacock1 , J Penagaricano1 , (1) University of Arkansas for Medical Sciences, Little Rock, AR, (2) Northside Hospital, Atlanta, GA


(Sunday, 7/29/2018) 5:05 PM - 6:00 PM

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 ±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 ±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.


TH- External beam- photons: extracranial stereotactic/SBRT

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