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Online Adaptive Stereotactic Body Radiotherapy for Lung Lesions


E Ahunbay

E Ahunbay*, F Liu, M Bedi, E Gore, X Li, Medical College of Wisconsin, Milwaukee, WI

SU-D-WAB-3 Sunday 2:05PM - 3:00PM Room: Wabash Ballroom

Purpose:
During the delivery of stereotactic body treatments (SBRT) for lung cancer with single or multiple targets, the targets may experience considerable inter-fractional variations, such as shape and volume change and/or independent motion between multiple targets, which cannot be fully addressed by the IGRT repositioning. Here we devised a method to quickly generate a dedicated plan of the day immediately before each SBRT fraction.

Methods:
The main idea for the proposed method is to quickly modify the original plan based on the anatomy of the day and deliver the new plan with respiration gating without repositioning the patient. An auto-segmentation tool (ABAS, Elekta) is used to quickly delineate targets and organs-at-risk (OAR) with manual editing, if necessary, based on the respiration-gated CT acquired using a CT-on-rails (CTVision, Siemens) with the patient in the treatment position. Appropriate PTV margin (e.g., 3 mm) is used to account for residual uncertainties. Each beam aperture is adjusted to conform to each PTV with the same beam orientation as the original plan. Automatically-generated ring structures are used during beam weight optimization to enhance dose conformity, where only beam (segment) weights (MU), not the shapes, are optimized. The method was tested on representative cases with two targets.

Results:
The proposed online replanning method can deliver conformal dose distribution optimized for each fraction during SBRT and can be completed within 10 minutes. The structure delineation was generally less than 3 minutes due to the small target size and abundant contrast. The percentage PTV volume covered by prescription dose and percentage lung volume covered by 20 Gy were improved from 86% and 22% for repositioning to 95% and 8% for the online replanning, respectively.

Conclusion:
Online adaptive replanning is dosimetrically advantageous and practically feasible for lung SBRT with possibility of using single isocenter for multiple targets.

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