Coupling Continuous Positive Airway Pressure (CPAP) and MR-guided Radiation Therapy

Abstract

Continuous positive airway pressure (CPAP) is a cost effective and readily available device that increases lung volumes and has shown promise in conventional x-ray-based radiation therapy (RT). However, limited data are available to quantify the impact of CPAP on lung reproducibility due to the use of ionizing radiation during imaging. We propose a novel pilot study combining CPAP with the powerful soft tissue capabilities of MR-guided RT to reduce the amount of radiation exposure to organs at risk, with the overarching goals of quantifying the impact of CPAP on lung stability under free-breathing (FB) and deep inspiration breath-hold (DIBH) conditions and assessing feasibility. An MR-safe configuration was devised by affixing several CPAP breathing circuits and verifying pressure maintenance using a manometer. Six healthy volunteers (median age 38, range: 28-54) underwent MRIs of the thorax (25 second TrueFISP, 1.5×1.5×3 mm3 resolution) using a 0.35T MR-Linac. FB and 2 verbally coached DIBH acquisitions were performed at CPAP of 0, 6, 10, 12, and 15 cm H20. To define a mutual coordinate system between successive datasets, automated rigid registration was performed (translations only) based on bony anatomy to the reference condition (FB, CPAP 0 cm H20). To quantify the linear relationship between lung volume and pressure under FB conditions, R2 was estimated for each subject. To study positioning reproducibility that may depend on increased pressures in the setting of DIBH, a Spearman correlation coefficient was calculated based on the centroid differences in lung volume. A paired t-test was used to compare the difference between pressures of 0, 6, 10, 12, and 15 cm H20. Image quality with and without CPAP under FB conditions were assessed. Surveys about volunteer perceptions of CPAP were administered after initial CPAP tolerability screening and following the imaging session based on a ten-point scale (10 = least tolerable). FB lung volumes increased as CPAP increased (R2 = 0.85 ± 0.13, range: 0.57 to 0.99) with visible reductions in motion artifacts. A significant negative correlation was observed between CPAP and the lung anterior/posterior centroid differences under DIBH, indicating a reduced difference on repeated measures (i.e., increased lung stability) as pressure increased. Paired t-tests showed significantly better reproducibility in lung volumes at pressures of 6, 10, 12, and 15 cm H20, as compared to 0 cm H20. Patient-reported difficulty tolerating CPAP was perceived to be lower after the study session (mean 2.0, range 1-4) than before (mean 2.83, range 1-5). This study confirms that integrating CPAP into MR-guided RT is feasible and well-tolerated. CPAP not only increases lung volumes under FB conditions, but also improves reproducibility of DIBH, offering potential for reducing treatment-related side effects regardless of treatment delivery platform.