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Dynamic Treatment of Clinical Margins Beyond the PET-Avid Target in Emission Guided Radiation Therapy: A Retrospective Patient Study

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S Mazin

A Nanduri1 , Q Fan2 , J Yang3 , T Yamamoto4 , E Graves3 , B Loo3 , S Mazin1*, (1) RefleXion Medical, Burlingame, CA, (2) University of Florida, Gainesville, FL, (3) Stanford University, Stanford, CA, (4) UC Davis School of Medicine, Sacramento, CA


TH-E-BRF-11 Thursday 1:00PM - 2:50PM Room: Ballroom F

Purpose: Emission guided radiation therapy (EGRT) is a new modality that uses PET emissions for direct real-time tumor tracking. Radiation beamlets are delivered along PET lines of response (LOR's) by a fast rotating PET-Linac closed ring gantry. In this work, we develop a scheme to treat clinical margins defined proximal to the moving PET-avid tumor, while maintaining EGRT's inherent real-time tracking ability.
Methods: The principle of EGRT is to deliver radiation along PET emission paths to concentrate dose in the PET-avid gross tumor volume (GTV). To account for adjacent non-PET avid regions in the clinical volume (CTV) a method was developed that expands the set of radiation beamlet responses to include the effective margin extension from the GTV to the CTV. An LOR detection may now result in multiple beamlet responses: one along the original LOR, and others that are adjacent to it in the direction of margin extension. Evaluation studies were performed on a 4D digital patient as well as a clinical breast cancer patient with moving lung tumors. Emission data were obtained using GATE and a commercial PET scanner. Dose delivery was simulated using VMC++. For the patient study, Philips Pinnacle was used for planning and Mirada RTx was used for deformable dose registration across multiple breathing phases.
Results: Compared with IMRT, the EGRT margin extension method achieved a 25.3% and 9.0% relative increase in dose to 95% of the CTV for the digital and clinical patients, respectively. The corresponding CTV dose increases without margin extension were 9.7% and 1.4%. The organs at risk doses were kept similar or lower for EGRT in both cases, with tumor tracking preserved.
Conclusions: With the capability of accurate treatment of the moving CTV, EGRT has the potential to enable a practical and effective implementation of 4D biologically guided radiation therapy.

Funding Support, Disclosures, and Conflict of Interest: Authors SRM and AN are stockholders of RefleXion Medical

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