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4π Non-Coplanar Radiotherapy: From Mathematical Modeling to Clinical Implementation

V Yu

V Yu*, D Nguyen , A Tran , D Ruan , M Cao , T Kaprealian , P Kupelian , D Low , K Sheng , Department of Radiation Oncology, UCLA, Los Angeles, CA


TU-CD-304-5 (Tuesday, July 14, 2015) 10:15 AM - 12:15 PM Room: 304

To develop and clinically implement 4π radiotherapy, an inverse optimization platform that maximally utilizes non-coplanar intensity modulated radiotherapy (IMRT) beams to significantly improve critical organ sparing.

A 3D scanner was used to digitize the human and phantom subject surfaces, which were positioned in the computer assisted design (CAD) model of a TrueBeam machine to create a virtual geometrical model, based on which, the feasible beam space was calculated for different tumor locations. Beamlets were computed for all feasible beams using convolution/superposition. A column generation algorithm was employed to optimize patient specific beam orientations and fluence maps. Optimal routing through all selected beams were calculated by a level set method. The resultant plans were converted to XML files and delivered to phantoms in the TrueBeam developer mode. Finally, 4π plans were recomputed in Eclipse and manually delivered to recurrent GBM patients.

Compared to IMRT utilizing manually selected beams and volumetric modulated arc therapy plans, markedly improved dosimetry was observed using 4π for the brain, head and neck, liver, lung, and prostate patients. The improvements were due to significantly improved conformality and reduced high dose spillage to organs mediolateral to the PTV. The virtual geometrical model was experimentally validated. Safety margins with 99.9% confidence in collision avoidance were included to the model based model accuracy estimates determined via 300 physical machine to phantom distance measurements. Automated delivery in the developer mode was completed in 10 minutes and collision free. Manual 4π treatment on the GBM cases resulted in significant brainstem sparing and took 35-45 minutes including multiple images, which showed submillimeter cranial intrafractional motion.

The mathematical modeling utilized in 4π is accurate to create and guide highly complex non-coplanar IMRT treatments that consistently and significantly outperform human-operator-created plans. Deliverability of such plans is clinically demonstrated.

Funding Support, Disclosures, and Conflict of Interest: This work is funded by Varian Medical Systems and the NSF Graduate Research Fellowship DGE-1144087.

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