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

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.

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 cm² -6 × 6 cm² in steps of 1 × 1 cm² . 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 ( $$ ). A template-based matching algorithm was then used to track the target on DE_kV, DE_kV+MV , and $$ images. Tracking accuracy was evaluated using the tracking success rate (TSR) and mean absolute error (MAE).

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 cm² ) to 69.4% (6 × 6 cm² ), while the MAE ranged from 2.05 mm to 4.04 mm. The application of wavelet-FFT algorithm to remove stripe noise ( $$ ) resulted in TSR values of 96.9% (2 × 2 cm² ) to 93.4% (6 × 6 cm² ) and subsequent MAE values were 0.89 mm to 1.37 mm. Similar trends were observed for the 15 mm target.

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.