Effect of scattered megavoltage x‐rays on markerless tumor tracking using dual energy kilovoltage imaging

PURPOSE: To determine the effect of megavoltage (MV) scatter on the accuracy of markerless tumor tracking (MTT) for lung tumors using dual energy (DE) imaging and to consider a post‐processing technique to mitigate the effects of MV scatter on DE‐MTT. METHODS: A Varian TrueBeam linac was used to acquire a series of interleaved 60/120 kVp images of a motion phantom with simulated tumors (10 and 15 mm diameter). Two sets of consecutive high/low energy projections were acquired, with and without MV beam delivery. The MV field sizes (FS) ranged from 2 × 2 cm(2)–6 × 6 cm(2) in steps of 1 × 1 cm(2). Weighted logarithmic subtraction was performed on sequential images to produce soft‐tissue images for kV only (DE(kV)) and kV with MV beam on (DE(kV+MV)). Wavelet and fast Fourier transformation filtering (wavelet‐FFT) was used to remove stripe noise introduced by MV scatter in the DE images ([Formula: see text]). A template‐based matching algorithm was then used to track the target on DE(kV,) DE(kV+MV), and [Formula: see text] images. Tracking accuracy was evaluated using the tracking success rate (TSR) and mean absolute error (MAE). RESULTS: For the 10 and 15 mm targets, the TSR for DE(kV) images was 98.7% and 100%, and MAE was 0.53 and 0.42 mm, respectively. For the 10 mm target, the TSR, including the effects of MV scatter, ranged from 86.5% (2 × 2 cm(2)) to 69.4% (6 × 6 cm(2)), while the MAE ranged from 2.05 mm to 4.04 mm. The application of wavelet‐FFT algorithm to remove stripe noise ([Formula: see text]) resulted in TSR values of 96.9% (2 × 2 cm(2)) to 93.4% (6 × 6 cm(2)) and subsequent MAE values were 0.89 mm to 1.37 mm. Similar trends were observed for the 15 mm target. CONCLUSION: MV scatter significantly impacts the tracking accuracy of lung tumors using DE images. Wavelet‐FFT filtering can improve the accuracy of DE‐MTT during treatment.