The Effect of Registration Volume Extent on Residual Errors Assessed Using Cone-Beam Computed Tomography in Radiation Treatment of Head and Neck Cancer

Abstract Purpose The objective of this study was to investigate the effect of the varying extent of cone-beam computed tomography (CBCT) registration volumes (RVs) on setup errors for head and neck (H&N) radiotherapy. Methods and Materials Daily CBCT images for 31 patients receiving H&N intensity-modulated radiotherapy (IMRT) were reviewed. Registrations using anatomically defined RVs with a fixed superior border at base of sella and varying inferior extent were used retrospectively to evaluate patient setup. The inferior extent was defined as the number of cervical bodies included, from none (C0) to six (C6). The frequency of residual displacements at four landmarks (clivus, vertebral bodies C5-C6, manubrium-sterni, and anterior body of mandible) was assessed. Results Expansion of the RVs inferiorly reduced the occurrence of residual displacements for the C5-C6 vertebral bodies (from 57% to 93% of fractions with residual displacements ≤ 3 mm) and increased the rate of simultaneous positioning of C5-C6 and clivus (from 41% to 76%). Maximum residual displacements for mandible (48%–64% ≤ 3 mm) and manubrium (73%–81% ≤ 3 mm) varied somewhat by the inferior extent of the RV. Residual displacements for clivus were small (88%–96% ≤ 3 mm) in all cases. Random and systematic errors were clinically acceptable for a 5-mm planning margin around the clinical targets. Conclusions In conclusion, expansion of the RV inferiorly to include C6 will improve the positioning of structures in the C5-C6 region (adjacent nodal zones 3 and 4) without compromising clival positioning. Insufficient inferior extent of the RV reduces reliability of low neck positioning. Substantial variability can occur for structures not included in the RV. Based on these data, we use the C6 RV except in cases with planning concerns outside this volume.