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Using Respiratory Motion as a Degree of Freedom Through Weighted Phase-Correlated 4D Planning


H Tachibana

H Tachibana*, A Sawant, UT Southwestern Medical Center, Dallas, TX

TU-E-141-1 Tuesday 2:00PM - 3:50PM Room: 141

Purpose:
We investigate a novel 4D treatment planning strategy that exploits respiratory motion as an additional degree of freedom. The key idea is to deliver more (less) fluence when the tumor target is out-of-line (in-line) with an organ-at-risk (OAR). We have developed our weighted-phase-correlated (WPC) 4D treatment planning paradigm on a commercial platform (Eclipse, Varian) and designed it for an "open-gate" delivery, where beam is on throughout the respiratory cycle.

Methods:
4DCT data were collected from twenty lung SBRT patients who exhibited >= 5mm tumor motion despite the use of abdominal compression. For each patient, (i) f x θ 3D-conformal plans were created and corresponding 3D dose distributions were calculated, where θ = number of respiratory phases (10) and f = number of fields (7 - 12) (ii) For each beam, the dose from nine phases was registered to the reference phase (end-exhale) using a GPU-based B-spline deformable registration, NiftyReg. (iv) using dynamic-penalized-likelihood optimization, fluence weights were optimized over all dose distributions. This key step ensured that for each motion-induced state of the anatomy, an optimal fluence would be delivered. (v) A summed fluence map was imported back into Eclipse and an MLC leaf sequence was generated. In order to ensure deliverability, a maximum leaf velocity constraint (v <= 3.5 cm/s) was imposed. Each WPC-4D plan was compared with the corresponding, clinically-delivered, ITV-based plan.

Results:
In all 20 patients, WPC-4D plans gave PTV conformity comparable to ITV-based plans. The percent dose-sparing (maximum, average) achieved using WPC-4D compared to ITV-based planning: Spinal Cord Dmax=(66,20); Esophagus Dmax=(94,29), Heart Dmax=(84,30); Heart Dmean=(82,35); Lung V20=(27,7); Lung Dmean=(27,13).

Conclusion:
We have developed a novel 4D-planning solution that exploits rather than trying to mitigate respiratory motion. Initial results indicate that this approach achieves significant dose-sparing in OARs compared to conventional motion-management (abdominal compression) + ITV-based planning.

Funding Support, Disclosures, and Conflict of Interest: This research was partially supported by Varian Medical Systems, Palo Alto, CA

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