Impact of Continuous Positive Airway Pressure (CPAP) on Vocal Cord Separation and its Potential Application for Unilateral Vocal Cord Irradiation

Abstract

<h3>Purpose/Objective(s)</h3> Unilateral vocal cord (VC) irradiation may be indicated in selected cases of early larynx cancer and may offer improved voice quality compared to bilateral VC irradiation or cordectomy. The proximity of the involved VC to the normal VC in the confined thyroid cartilage space and the need to account for motion and set-up errors may limit the dose reduction obtained from unilateral VC irradiation. We hypothesized that use of CPAP, which hyperinflates the upper airway would increase separation between the VC's and reduce the dose to the contralateral VC and larynx. <h3>Materials/Methods</h3> Archived planning CT-datasets that included larynx images from a previous CPAP trial between 2014-2021 for lung SBRT were selected for analysis. Planning CT scans were performed first with free breathing (FB) and then with CPAP. The CPAP protocol included a full-face mask covering both mouth and nose. Air pressure was increased gradually over 30 minutes from 6 cmH₂O to 15 cmH₂O and kept at this level for the duration of the planning CT. The VC's and larynx were contoured using soft tissue windows (50-350 HU). The separation between the VCs on FB and CPAP CT-scans were measured on axial planes at the anterior and posterior thirds. One VC was used for the GTV and then expanded 2 mm and 3 mm for the CTV and PTV respectively. Synthetic plans were generated in selected cases using identical optimization parameters. Doses to the contralateral VC and larynx were compared. Paired Students t-test was used to compare VC separations. <h3>Results</h3> Thirty cases were available for analysis. The distance between VC's increased in 19/30 (63.3%). The 19 cases showing increased VC spacing with CPAP use were used for analysis. Compared to FB, CPAP use increased the mean separation between the anterior third of the VCs by 67.3% (IQR 38-228) from 5.7mm (SD 0.25) to 8.6mm (SD 0.2), (p<.001) and the posterior third of the VCs by 28% (IQR 20-59) from 9.2mm (SD 0.2) to 11mm (SD 0.22), (p<.001). Plans generated in 5 cases showed use of CPAP decreased mean dose to the contralateral VC and larynx from 47 Gy to 38 Gy and 32.6 to 27.7 Gy respectively. The V50's of the contralateral VC and larynx decreased from 48.5% to 23% and 19% to 7.8% respectively. <h3>Conclusion</h3> This hypothesis generating study demonstrated CPAP use increased the distance between the VCs in most patients and when using unilateral radiation, reduced dose to the contralateral VC and larynx. The use of CPAP warrants further study for potential use in unilateral VC radiation.