SU‐E‐T‐783: The TomoTherapy Thread Effect Revisited

Purpose: The TomoTherapy thread effect is a longitudinal pitch‐dependent dose‐ripple artifact caused by helical radiation delivery. It has been observed and studied before. However, previously empirically identified optimal pitches, 0.86/n, are only for the jaw width 5 cm and off‐axis distance 5 cm. Through theoretical analysis and simulation, we were able to identify optimal pitches for various jaw widths at various off‐axis distances. We further investigated the effect of optimization in reducing this ripple artifact. Methods: As the ripple in the dose profile is essentially a 1D problem, we set up a 1D model that account for the contributing factors, including profile divergence, the inverse square law, attenuation, and the cone effect, to study the thread effect. Based on the 1D model, we analyzed individual and combined factors theoretically and numerically to identify optimal pitches. We further set up optimization to reduce ripples in the 1D cases and simulate 3D cases using TomoTherapy's optimization. Results: The 1D model captures the thread effect found in 3D. At the off‐axis distance of 5 cm, for the jaw width of 5 cm, the derived optimal pitches agree with 0.86/n. However, for other jaw widths or off‐axis distances, the optimal pitches do not follow 0.86/n. For a fixed jaw width, the optimal pitches shift downward as the off‐axis distance increases. Optimization in both 1D and 3D show consistent results, and the ripples are largely suppressed by optimization through intensity modulation. Conclusions: With optimized intensity modulation, the optimal pitches tend to move toward the ones for the profile divergence factor, as optimized modulation is expected to compensate for the intensity‐related factors up to the restriction of modulation factor and optimization iterations. Choosing a good pitch, though further improves the thread effect, may not be as critical, as optimization already bring down the thread effect considerably.