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Retrospective Multi-Clinic Analysis of the Relationship Between Dose Gradient and PTV Volume for Lung SBRT Patients Treated with a Coplanar VMAT Technique

I Dragojevic , D Hoffman , J Hoisak , R Manger*, UC San Diego, La Jolla, CA


(Tuesday, 7/31/2018) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 7

Purpose: To determine the relationship between the dose gradient and PTV volume for peripheral lung SBRT plans using a coplanar VMAT technique.

Methods: The Varian Eclipse Scripting API was used to data-mine our institution’s multi-clinic planning database and extract several plan quality metrics from coplanar VMAT peripheral lung SBRT plans, planned in accordance with the guidelines of RTOG 0915. The relationship between the Paddick Gradient Index (GI) and PTV volume was observed and compared against RTOG 0915 constraints. The relationship between PTV volume and the Eclipse Gradient Measure (GM), the average distance between the prescription isodose line and 50% isodose line, was investigated. A least squares fit was used to determine a functional form of this relationship.

Results: Searching over the date range Jan 2016 – March 2018, a total of 138 SBRT plans were identified. The PTV volumes ranged from 3.22 to 180.01 cc. For PTV volumes less than 70 cc, the average GI was in the “no deviation� category. For PTV volumes greater than 70 cc, the average GI exhibited a “minor deviation,� as defined by RTOG 0915. The least squares fit relating GM to PTV volume (applicable from 3.22 to 85.62 cc) yielded the function, GM = 0.59 V�.² (R² = 0.7), where GM is in cm and V is the PTV volume in cc. The data were normally distributed about this line with a relative standard deviation of about 10%.

Conclusion: There is a functional relationship between GM and PTV volume for lung SBRT patients planned with a coplanar VMAT technique. The GI specification in RTOG 0915 may be a challenging constraint to meet with no deviation for PTV volumes greater than 70 cc. The PTV-dose gradient relationship should be a helpful quality metric for evaluating peripheral lung SBRT plans treated with a coplanar VMAT technique.


Not Applicable / None Entered.


TH- External beam- photons: extracranial stereotactic/SBRT

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