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Are Planning Target Volumes Derived From 4DCT Scans Adequate When Treating Lung SBRT with Static Tracking Methods?


C McGuinness

C McGuinness1*, M Descovich2, (1) ,,,(2) UCSF, San Francisco, CA

MO-F-WAB-8 Monday 4:30PM - 6:00PM Room: Wabash Ballroom

Purpose:
A unique capability of the CyberKnife system is dynamic target tracking. However, at our institution 68% of lung cancer patients are determined ineligible for this approach. Rather, their tumors are tracked statically using the vertebral column for alignment. The internal target volume (ITV) is delineated on the 4DCT scan and an additional margin is added to account for setup uncertainty. We seek appropriate ITV to PTV expansion needed for patients treated in this manner.

Methods:
Data from 12 patients (37 fractions) were included in this study. Each patient was treated with dynamic tracking and underwent 4DCT scanning at the time of simulation. The GTV and ITV were contoured and additional margins were added to the ITV to create the PTV. 3D target positions over the course of treatment (1-1.5 hours) were extracted from the log files. Based on these positions, a set of adjusted GTV contours was created and used to calculate the fraction of GTV extending outside of the PTV over the course of treatment (data were integrated over time for each sampling position).

Results:
Of the 37 fractions included, 35 (95%) show less than 5% of the GTV outside the PTV for a margin of 3.6 mm, and less than 2% of the GTV outside the PTV for a margin of 5 mm.

Conclusion:
The results of this study provide guidelines for selecting appropriate ITV-PTV margins when treating lung patients on CyberKnife using spine alignment. We found that a 5 mm ITV to PTV margin is adequate to assure >98% of tumor coverage for 95% of the patients. However, this margin represents a worse case scenario. It does not reflect the fact that target motion due to patient drift is corrected for by spinal alignment. A method for separating the two sources of motion will be investigated.

Funding Support, Disclosures, and Conflict of Interest: separate research contract from Accuray Inc.

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