Study to Compare the Effect of Different Registration Methods on Patient Setup Uncertainties in Cone-beam Computed Tomography during Volumetric Modulated Arc Therapy for Breast Cancer Patients.

Purpose: This study compared three different methods used in registering cone-beam computed tomography (CBCT) image set with planning CT image set for determining patient setup uncertainties during volumetric modulated arc therapy (VMAT) for breast cancer patients.

Materials and Methods: Seven breast cancer patients treated with 50 Gy in 25 fractions using VMAT technique were chosen for this study. A total of 105 CBCT scans were acquired by image guidance protocol for patient setup verification. Approved plans' CT images were used as the reference image sets for registration with their corresponding CBCT image sets. Setup errors in mediolateral, craniocaudal, and anteroposterior direction were determined using gray-scale matching between the reference CT images and onboard CBCT images. Patient setup verification was performed using clip-box registration (CBR) method during online imaging. Considering the CBR method as the reference, two more registrations were performed using mask registration (MR) method and dual registration (DR) (CBR + MR) method in the offline mode. For comparison, systematic error (∑), random error (σ), mean displacement vector ( R ), mean setup error ( M ), and registration time ( R t ) were analyzed. Post hoc Tukey's honest significant difference test was performed for multiple comparisons.

Results: Systematic and random errors were less in CBR as compared to MR and DR ( P > 0.05). The mean displacement error and mean setup errors were less in CBR as compared to MR and DR ( P > 0.05). Increased R t was observed in DR as compared to CBR and MR ( P < 0.05). In addition, multiple comparisons did not show any significant difference in patient setup error ( P > 0.05).

Conclusion: For breast VMAT plan delivery, all three registration methods show insignificant variation in patient setup error. One can use any of the three registration methods for patient setup verification.