Optimizing Computed Tomography-derived Surface Monitoring for Accurate Cancer Radiotherapy Setup

Surface-guided radiation therapy (SGRT) is being increasingly applied in cancer radiotherapy to support markerless setup, respiratory management, and intrafraction surface monitoring. When computed tomography (CT)-derived digital imaging and communications in medicine (DICOM) surfaces are imported into SGRT systems, the CT threshold used for external-contour generation may introduce systematic online delta values following image-guided setup correction. This phantom-based study investigated practical CT-threshold selection for accurate AlignRT-guided surface monitoring in cancer radiotherapy.

Two independent TrueBeam-AlignRT configurations were considered, one equipped with HD cameras and the other with Horizon cameras, using a cube phantom and an anthropomorphic thoracic phantom. External contours were generated for CT thresholds from -150 to -750 Hounsfield units (HU) in 100-HU increments. Following a cone-beam CT-based setup process and six-degree-of-freedom correction, DICOM surface selection was performed. AlignRT real-time delta values were recorded 2-5 s after surface selection. Additional technical analyses assessed the delta-value dependence on the treatment planning system, field-of-view/in-plane pixel size, and CT acquisition position under restricted conditions.

Variation of the CT threshold had little apparent effect on the lateral and longitudinal delta values; however, the vertical delta values were more threshold-dependent. Lower thresholds, particularly of -550 to -750 HU, generally produced smaller vertical delta values. For the cube phantom and both system configurations, a -650-HU threshold yielded the smallest vertical delta value. For the thoracic phantom, comparable performance was observed for thresholds of -550, -650, and -750 HU; thus, a single universal optimum could not be defined.

CT-threshold selection influences CT-derived DICOM surface generation for AlignRT-guided cancer radiotherapy, particularly in the vertical direction. A threshold of approximately -650 HU may be a practical starting point for similar workflows; however, institution-specific commissioning is required before clinical implementation.