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Dosimetric Impact of the Use of Jaw Tracking for Fields Smaller Than 3x3cm2 in Lung SBRT VMAT Plans

E Hubley*, R Scheuermann, D Carlson, C Kennedy, University of Pennsylvania, Philadelphia, PA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The Varian Eclipse treatment planning system (TPS) models output factors (OFs) down to 3x3cm². When jaw tracking (JT) is used, the jaws may collimate fields below this limit. The objective of this work is to determine the dosimetric impact on VMAT plans containing fields =3x3cm².

Methods: OFs were measured for jaw-defined fields using a PTW60019 diamond detector and verified with A16/CC13 ion chambers using the intermediate field method in water. OFs were compared to TPS-calculated OFs to determine maximum deviation between measurement and calculation when at least one jaw-pair opening was <3cm, <2cm, and <1cm. For eight phantom and 12 patient lung-SBRT VMAT plans with PTV volumes 2.3-17cc, deviation factors were assigned to each control point (CP) based on jaw-pair openings. These factors were then weighted based on the fraction of MUs delivered for each CP. The total estimated deviation was obtained by summing over all CPs in the plan. Patient-specific QA (PSQA) was performed using a Sun Nuclear ArcCHECK on the eight phantom plans.

Results: Observed differences between measured and calculated jaw-defined OFs were up to 2.7%, 8%, and 17.5% when one jaw-pair opening was <3cm, <2cm, or <1cm, respectively. Conservative weighting factors used to determine MU-weighted deviations from TPS were 3%, 10%, and 20%. In both patient and phantom plans, larger deviations in planned dose were observed for smaller target volumes (R=0.85). PTV volumes =8cc, =5cc, and =2.3cc were susceptible to deviations from TPS-calculated dose of =1.0%, =2.9%, and =3.1% respectively. No significant (Wilcoxon signed-rank, p>0.05) differences in PSQA were detected using the ARC Check (2%,2mm).

Conclusion: While measured OFs can deviate from TPS-calculated OFs for field sizes below 3x3cm² by up to 17.5%, differences in jaw-defined OFs did not translate into deviations in plan quality =3.1% for targets =2.3cc.

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Collimation, Dosimetry, Treatment Planning


TH- External Beam- Photons: General (most aspects)

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