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 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 ( ${\mathrm{DE}}_{\mathrm{kV}+\mathrm{MV}}^{\mathrm{Corr}}$ ). A template-based matching algorithm was then used to track the target on DE_kV, DE_kV+MV , and ${\mathrm{DE}}_{\mathrm{kV}+\mathrm{MV}}^{\mathrm{Corr}}$ 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 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 ( ${\mathrm{DE}}_{\mathrm{kV}+\mathrm{MV}}^{\mathrm{Corr}}$ ) 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.

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.