Agreement between Surface Imaging and Cone-Beam CT for Daily Setup of Pelvic Radiotherapy Patients
Abstract
Purpose
To evaluate the clinical agreement between surface-guided radiotherapy (SGRT) and cone-beam CT (CBCT) for daily setup verification in pelvic radiotherapy patients.
Methods
Data from 120 treatment fractions of 30 pelvic cancer patients treated with RapidArc on a Varian TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA) equipped with CBCT and surface imaging (AlignRT, Vision RT, London, UK) were analyzed. CBCT and AlignRT isocenter matching accuracy was maintained within 1 mm. Initial patient positioning was performed using skin markers and room lasers, followed by refinement with SGRT. CBCT-based setup verification was performed immediately afterward, with surface images acquired concurrently. Registrations were performed for both CBCT-to-planning CT and SGRT-to-planning CT. Translational and rotational deviations between SGRT and CBCT were calculated for each fraction, and three-dimensional (3D) translational and rotational magnitudes were derived. Agreement between modalities was assessed using Bland–Altman analysis.
Results
Mean translational differences between SGRT and CBCT were 0.52 ± 2.06 mm (vertical), 1.14 ± 2.48 mm (longitudinal), and −0.24 ± 2.65 mm (lateral). Mean rotational differences were −0.63 ± 1.71° (pitch), −0.15 ± 1.84° (roll), and −0.56 ± 1.40° (yaw). The mean 3D translational difference was 4.03 ± 1.69 mm, with 95% limits of agreement (LoA) of 0.72–7.34 mm. The mean 3D rotational magnitude was 2.79 ± 1.12°, with 95% LoA of 0.60–4.99°. Clinically acceptable agreement (≤5 mm and ≤3°) was observed in 85% of fractions, while approximately 5% exceeded 7 mm or 5°.
Conclusion
SGRT demonstrates good initial setup agreement with CBCT for the majority of pelvic radiotherapy fractions. However, the observed range of discrepancies, particularly the potential for occasional large deviations, underscores that SGRT should be used as a complementary tool. CBCT remains necessary for definitive verification to ensure accurate targeting of deep-seated pelvic structures.