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Are R50% and D2cm Completely Independent Parameters in Lung SBRT Plans?

G Narayanasamy

G Narayanasamy1*, D Desai2 , X Zhang1 , J Fang1 , S Morrill1 , S Maraboyina1 , J Penagaricano1 , (1) University of Arkansas for Medical Sciences, Little Rock, AR, (2) Northside Hospital, Hixson, TN


SU-F-FS1-3 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: Four Seasons 1

Purpose: Stereotactic body radiation therapy (SBRT) of lung tumors provides a steep dose fall-off to restrict the normal tissue toxicity. The lung SBRT metrics for dose fall-off, R50% and D2cm were evaluated retrospectively. This study was aimed at validating the two metrics and questions their independence.

Methods: The retrospective study on lung SBRT includes 64 treatment plans (Eclipse TPS ver 11.0) from Institution-1 including 41 based on dynamic IMRT and 23 RapidArc plans. In addition, 41 RapidArc plans (Eclipse TPS ver 8.9) from Institution-2 were reviewed. The plans included both co-planar and non-coplanar beam arrangement and for all the reviewed plans, Acuros XB dose computation was employed on 4D CT-based treatment planning and patients were treated using 6 or 10 MV beam in a TrueBeam linac with HD-MLCs. Conformity index (CI), R50%, D2cm and the dose fall-off product of R50% and D2cm (DFP) were evaluated as a function of volume of PTV based on a power law.

Results: IIn the Institution-1, large than RTOG 0915 accepted deviations were observed in R50% or D2cm values (20/46 coplanar and 3/18 non-coplanar plans in R50%; 10/46 in coplanar and 5/18 non-coplanar in D2cm). Among the RapidArc plans from Institution-2, 1 coplanar and 1 non-coplanar had a large deviation in R50%, while none were found in D2cm. Excluding 10 outlier plans, the dose fall-off product (DFP) of R50% and D2cm ranged from 165 to 451 with a mean±SD of 277.9±55.8. DFP was found to have a small dependence on PTV with a negative slope of 9% for PTVs ranging from 5 to 150 cc.

Conclusion: This multi-institutional study questions the assumption of independence of R50% and D2cm in lung SBRT plans. Plans with co-planar beam arrangement had larger deviations of R50% and D2cm than their non-coplanar counterparts.

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