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BEST IN PHYSICS (JOINT IMAGING-THERAPY): Functional-Guidance for Lung Radiation Therapy Planning: Does Ventilation Imaging Correlate with Gas Exchange?

L Rankine

L Rankine1*, Z Wang2 , B Driehuys2 , L B Marks3 , C Kelsey4 , S Das1 , (1) The University of North Carolina at Chapel Hill, Chapel Hill, NC, (2) Duke University, Durham, NC, (3) The University of North Carolina School of Medicine, Chapel Hill, NC, (4) Duke University Medical Center, Durham, NC


TH-AB-FS4-11 (Thursday, August 3, 2017) 7:30 AM - 9:30 AM Room: Four Seasons 4

Purpose: Interest in functionally-guided Radiation Therapy planning is rising due to the ability to conveniently derive lung ventilation maps from 4-Dimensional Computed Tomography. An assumption of this method is that lung ventilation is an accurate surrogate for true regional lung function, i.e., gas exchange across alveolar barriers into capillary Red Blood Cells (RBCs). In this work, we used a novel Magnetic Resonance Imaging (MRI) technique to measure and compare spatial distributions of lung ventilation and gas exchange in humans.

Methods: 15 IRB-approved human subjects (13 healthy volunteers, 1 RT patient and 1 emphysema patient) underwent breath-hold MRI using hyperpolarized ¹²⁹Xe gas. The solubility and chemical shift of ¹²⁹Xe permits the acquisition of 3D images of the relative ¹²⁹Xe quantities in the airspaces, alveolar barrier tissue, and RBCs (representing true gas exchange). We compared distributions of ¹²⁹Xe in airspaces (ventilation) and in RBCs (end-to-end function) on a per-voxel basis by calculating Pearson correlation coefficients (ρ). Further, high-signal regions of interest (ROIs), like those used for functionally-guided RT (>67ᵗʰ- and >90ᵗʰ-percentiles), were created independently from ventilation and RBC images and compared using the Dice Similarity Coefficient (DSC).

Results: The average correlation of ventilation and RBC in the healthy volunteer group was ρ=0.41±0.12. The RT patient’s coefficients, ρ=0.49 (pre-RT) and ρ=0.37 (6-weeks post-RT), were within 1σ of the healthy average, whereas the emphysema patient, ρ=0.24, was >1σ below average. Examining subjects by lung region, inferior lung, ρ=0.48, was significantly more correlated than superior lung, ρ=0.36 (p<0.03), as was posterior lung, ρ=0.53, vs. anterior lung, ρ=0.39 (p<0.02). High-signal ROIs from ventilation and RBC demonstrated poor spatial agreement: DSC=0.50±0.06 and 0.26±0.12 for the >67ᵗʰ- and >90ᵗʰ-percentile ROIs, respectively.

Conclusion: Ventilation may not be an effective surrogate for true regional lung function. New approaches to define regional lung function may improve the efficacy of functionally-guided RT.

Funding Support, Disclosures, and Conflict of Interest: This work was supported in part by grants R01HL105643 and P41 EB015897. Bastiaan Driehuys reports a leadership/advisory role and financial investments in Polarean, Inc.

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