The Effect of Scan Angle on Target Localization Accuracy Using Digital Tomosynthesis

Abstract

Purpose/Objective(s): To examine the effect of scan angle on target localization accuracy using on-board digital tomosynthesis (DTS) for breath-hold liver treatment. Material/Methods: 20 CBCT-localized fractions accumulated from liver radiation therapy treatments were studied retrospectively. To remove motion artifacts, image sets were acquired under breath-hold. The effect of scan angle, a key parameter for DTS, on target localization accuracy was evaluated for 10, 20 and 40 degree scan angles in both coronal and sagittal views. Assuming CBCT localization is the gold standard, the accuracy of the DTS technique is evaluated by the relative difference compared to the CBCT positioning. Localization accuracy was compared using two registration methods: bony anatomy based and soft tissue based alignment. Bony matching was performed by aligning the vertebral structures and ribs. Soft tissue matching was performed by aligning the liver boundaries, kidneys, and, when visible, the target. Results: The coronal and sagittal DTS had similar performance in aligning the target. Overall, the means and standard deviations for 40, 20 and 10 degree DTS were 0.2 +0.8 cm (40-cor.), 0.2 +0.8 cm (40-sag.), 0.1 +0.7cm (20-cor.), 0.2 +0.9 cm (20-sag.), 0.2 +0.9 cm (10-cor.), and 0.1 +0.7 cm (10-sag.). Figure 1. shows examples of DTS images at 10, 20, and 40 degree. Figure 2. plots the localization differences for the 10 and 40 degree DTS results in the lateral, vertical and longitudinal directions. Conclusion: The effect of scan angle on bony anatomy is minimal, and 10 degree DTS in either coronal or sagittal view is sufficient for bone based alignment. The visibility of liver boundaries was comparable with corresponding CBCT images on all DTS scans. However, detailed soft tissue contrast suffers when scan angle decreases from 40° to 10°. For BH liver treatment, the entire liver is clearly visible and can be used for soft tissue alignment. Partially supported by a Varian Medical Systems Research Grant