Room: Track 2
Purpose: Dual energy (DE) fluoroscopy has been shown to improve markerless lung tumor tracking. Prior work used kV switching, which is prone to motion between DE projections. In this work, we evaluate the performance of lung tumor tracking during a rotational acquisition using a prototype dual layer flat panel detector (DL FPD), which obtains DE projections in a single exposure.
Methods: Tumor tracking experiments were conducted on a benchtop system using a DL FPD. The prototype detector contains two amorphous silicon panels with additional filtration between them. To quantify performance, we used an anthropomorphic chest phantom with a 1.2 cm diameter polyethylene “tumor” connected to a motion controller that simulated respiratory motion. The tumor was centered at the rotation axis, and fluoroscopic images were acquired while the phantom was rotated to study projection angle dependence. The system geometry (1.5x magnification), x-ray energy (120 kV), frame rate (15 fps), and respiratory cycles (6) were typical for on-board kV imaging for radiotherapy. The tumor was tracked in the projections using template matching, and tracking accuracy of single energy (SE) and DE was compared to the known motion. An additional qualitative study was performed, where a ventilated pig lung with a synthetic tumor was placed in the chest phantom and motion was induced using a clinical ventilator. Fluoroscopic images were acquired during rotation.
Results: DE using a DL FPD yielded better accuracy than SE, with a reduction of root-mean-square error (RMSE) by 29% and an improvement of tracking success rate by 6%. Qualitatively, DE greatly improved tumor visibility. However, both SE and DE tracking performance is reduced when the tumor overlapped with substantial soft tissue.
Conclusion: The work demonstrates the feasibility of using a dual-layer FPD for lung tumor tracking. This technique can be beneficial for real-time tumor tracking during clinical VMAT treatments.