Method comparison of automated matching software-assisted cone-beam CT and stereoscopic kilovoltage x-ray positional verification image-guided radiation therapy for head and neck cancer: a prospective analysis

Abstract

We sought to characterize interchangeability and agreement between cone-beam computed tomography (CBCT) and digital stereoscopic kV x-ray (KVX) acquisition, two methods of isocenter positional verification currently used for IGRT of head and neck cancers (HNC). A cohort of 33 patients were near-simultaneously imaged by in-room KVX and CBCT. KVX and CBCT shifts were suggested using manufacturer software for the lateral (X), vertical (Y) and longitudinal (Z) dimensions. Intra-method repeatability, systematic and random error components were calculated for each imaging modality, as were recipe-based PTV expansion margins. Inter-method agreement in each axis was compared using limits of agreement (LOA) methodology, concordance analysis and orthogonal regression. 100 daily positional assessments were performed before daily therapy in 33 patients with head and neck cancer. Systematic error was greater for CBCT in all axes, with larger random error components in the Y- and Z-axis. Repeatability ranged from 9 to 14 mm for all axes, with CBCT showing greater repeatability in 2/3 axes. LOA showed paired shifts to agree 95% of the time within +/-11.3 mm in the X-axis, +/-9.4 mm in the Y-axis and +/-5.5 mm in the Z-axis. Concordance ranged from 'mediocre' to 'satisfactory'. Proportional bias was noted between paired X- and Z-axis measures, with a constant bias component in the Z-axis. Our data suggest non-negligible differences in software-derived CBCT and KVX image-guided directional shifts using formal method comparison statistics.

Figure 1

(a), (b) Screenshot of automated shift output. Screen capture of image/numeric output of automated shift correction using KVX (2D–2D registration (a) and CBCT (3D–3D registration (b), using tissue equivalent bone-in phantom (Honeybaked Ham, Norcross, GA).

Figure 2

(a)–(c) Altman–Bland plots. Altman–Bland plots of method agreement by cardinal axis between CBCT and KVX, in mm, with 95% density ellipse in red. Mean difference, SD of difference and 95% limits of agreement (1.96 × SD) are labeled on (a).

Figure 3

(a)–(c) Deming regression plots. Variance-weighted orthogonal regression fit (dark line) and line of identity (dashed line) for each axis between CBCT and KVX, in mm. The 95% distribution is shown in dashed ellipses. Deming orthogonal regression coefficients are shown as captions.