The Correlation of Regional Lung Ventilation and Gas Transfer to Red Blood Cells: Implications for Functional-Avoidance Radiation Therapy Planning

Publication date: Available online 14 April 2018
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): Leith J. Rankine, Ziyi Wang, Bastiaan Driehuys, Lawrence B. Marks, Christopher Kelsey, Shiva K. Das
Backgroundand Purpose: Interest in functionally-guided radiation therapy (RT) planning has been bolstered by the ability to derive lung ventilation maps from 4-Dimensional Computed Tomography. However, this assumes that regional lung ventilation is an accurate surrogate for true regional lung function, i.e., gas exchange between the airspaces and capillary Red Blood Cells (RBCs). This work uses the emerging technology of hyperpolarized (HP)-¹²⁹Xe Magnetic Resonance Imaging (MRI) to investigate the degree to which lung ventilation and gas exchange are regionally correlated.Material and MethodsHP-¹²⁹Xe MRI studies were performed on 17 IRB-approved human subjects, including 13 healthy volunteers, one emphysema patient, and 3 non-small cell lung cancer (NSCLC) patients imaged prior to and ∼11 weeks following RT. Subjects inhaled 1 liter of HP-¹²⁹Xe mixture, followed by the acquisition of interleaved ventilation and gas exchange images, from which maps of relative HP-¹²⁹Xe distribution were obtained in: 1) the lung airspaces; 2) dissolved interstitially in alveolar barrier tissue; and 3) transferred to the capillary RBCs. The relative spatial distributions of HP-¹²⁹Xe in airspaces (regional ventilation) and RBCs (regional gas transfer) were compared. Further, we investigated the degree to which ventilation and RBC images identified similar functional regions of interest (ROIs) suitable for functionally-guided RT. For the RT patients, both ventilation and RBC functional images were used to calculate differences in the lung dose-function histogram (DFH) and functional effective uniform dose (fEUD).ResultsThe correlation of ventilation and RBC transfer was ρ=0.39±0.15 in healthy volunteers. For the RT patients, this correlation was ρ=0.53±0.02 pre-treatment and ρ=0.39±0.07 post-treatment; for the emphysema patient it was ρ=0.24. Comparing functional ROIs, ventilation and RBC transfer demonstrated poor spatial agreement: DSC=0.50±0.07 and 0.26±0.12 for the highest-33%- and highest-10%-function ROIs in healthy volunteers, and in RT patients (pre-treatment) these were 0.54±0.02 and 0.35±0.06. The average magnitude of the differences between RBC- and ventilation-derived fEUD, fV20Gy, fV10Gy, and f5Gy, were 1.5±1.4 Gy, 4.1%±3.8%, 5.0%±3.8%, and 5.3%±3.9%.ConclusionsVentilation may not be an effective surrogate for true regional lung function for all patients.

Teaser

Functionally-guided radiation therapy (RT) planning using 4-Dimensional Computed Tomography (4DCT)-derived ventilation is gaining momentum. However, an important question remains: is regional lung ventilation a good surrogate for end-to-end lung function, i.e., gas transfer to Red Blood Cells (RBCs). We acquired functional image data for N=17 human subjects, using state-of-the-art hyperpolarized (HP)-¹²⁹Xe Magnetic Resonance Imaging (MRI). We analyzed the correlation of ventilation and RBC transfer, and calculated the similarity of planning optimization structures created from each.


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