2180 - Reproducibility of the Cervical Spine Setup Using Surface-Guided Radiotherapy (SGRT) for Head-and-Neck Patients with Daily CBCT Confirmation

Purpose/Objective(s): To establish and apply a surface-guided radiotherapy (SGRT) procedure to set up the cervical spine reproducibly using a region of interest (ROI) containing two rigid areas to achieve the alignment of the deformable neck, verified with daily cone-beam computed tomography (CBCT), for treating head-and-neck (HN) cancer with nodal involvement.

Materials/Methods: With an ROI containing both facial and upper-check surfaces, updated from two ROIs covering these two areas used in our previous publication, an SGRT setup procedure was applied to set up HN patients using the motion tracking system inBore on Ethos with daily CBCT setups. The ROI may include part of the neck if it is visible without skinfolds. After initial surface alignment (<3mm/°) to the planning CT reference, the color-coded Deformation View (green within ±3mm, red above, and blue below the reference) was applied to improve surface matching and guide patient adjustment to achieve a green ROI with near-zero 6 degrees of freedom (6DOF) shifts. Then, CBCT was acquired for the final patient setup and used to assess surface and spine canal alignment. Five HN patients participated in this study and three SGRT fractions each were evaluated. Besides SGRT data analysis, the CBCT was registered to planning CT, the anterior surface and spine canal were contoured, and the mean distance to agreement (MDA) and similarity DICE index were calculated. The SGRT setup with a regular standard (<3mm/°) and a high standard (<1.5mm/° in any direction and a >85% green ROI in deformation) were compared, and the c-spine canal matching was performed for 15 fractions in the 5 patients.

Results: Among 15 datasets, 11 fractions in 4 patients met the high standard, showing good alignment for the anterior surface (DICE=0.97±0.01, MDA=0.86±0.21mm) and the c-spine canal (0.82±0.06, MDA=1.10±0.35mm). Under the regular SGRT threshold with uncorrected deformation in the remaining 4 fractions in 2 patients, the c-spine alignment became poor (DICE=0.50±0.12 and MDA=2.94±0.77mm). The local body rotational correction using surface guidance for both head and chest within the ROI is critical to minimize neck deformation for internal organ alignment.

Conclusion: It is feasible to apply SGRT with the high surface-matching standard (near zero 6DOF shifts and minimal deformation) to reproduce the c-spine canal position with a mean DICE greater than 0.8. This suggests an accurate SGRT setup with a unique ROI can lead to good c-spine alignment. This ongoing clinical study will produce more patient data, leading to more detailed and definitive SGRT procedures.