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Implementation of a Mobile CT for Adaptive Planning for a Compact Proton Therapy System

J Oliver

J Oliver*, O Zeidan , S Meeks , A Shah , J Pukala , P Kelly , N Ramakrishna , T Willoughby , UF Health Cancer Center - Orlando Health, Orlando, FL


MO-RPM-GePD-J(A)-4 (Monday, July 31, 2017) 3:45 PM - 4:15 PM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: To commission the AIRO mobile in-room CT (Mobius, BrainLab) for adaptive proton therapy on a compact system (Mevion Medical Systems).

Methods: Verification scans of a Gammex electron density plug phantom and several anthropomorphic phantoms (head, thorax, and pelvis) were acquired in the proton treatment room using the AIRO. Treatment planning was performed on scans from a Philips Brilliance BigBore using Pinnacle. AIRO specific CT-stopping power ratio (SPR) curves were created using two methods: (1) commonly used stoichiometric method using Gammex phantom, (2) direct mapping of AIRO CT numbers to clinical SPR curve for corresponding ROIs in AIRO CT and Phillips CT for Gammex and anthropomorphic phantoms. To test both SPR curves, the Pinnacle adaptive workflow was used to copy the proton treatment plan from the Philips CT to the AIRO CT. Dose was re-calculated on AIRO scans using both SPR curves. All plans were exaggerated by traversing several cm of heterogeneity.

Results: Gamma analysis between treatment plans and AIRO verification plans demonstrated better dose agreement for the mapping SPR method compared to the stoichiometric method. Average passing rates for all sites for stoichiometric and mapping methods, respectively were 99.3% vs 99.7% (3%, 3mm), 97.6% vs 98.3% (2%, 2mm), and 73.9% vs 84.8% (1%, 1mm). The relatively high passing rate for the mapping method indicates that calculated proton dose on AIRO image sets is sensitive enough to monitor dosimetric changes that warrant plan adaptation.

Conclusion: The mapped CT-SPR curve methodology provides better overall dosimetric agreement than conventional stoichiometric for dose calculations on the AIRO mobile CT. This approach provides promising solutions to adaptive planning by allowing efficient and convenient in-room CT scanning. Our work also demonstrates the clinical value of a mobile CT in single room compact proton systems that cannot accommodate integrated CT units due to space restrictions.

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