The helical tomotherapy thread effect

Abstract

Inherent to helical tomotherapy is a dose variation pattern that manifests as a "ripple" ͑peak-totrough relative to the average͒. This ripple is the result of helical beam junctioning, completely unique to helical tomotherapy. Pitch is defined as in helical CT, the couch travel distance for a complete gantry rotation relative to the axial beam width at the axis of rotation. Without scattering or beam divergence, an analytical posing of the problem as a simple integral predicts minima near a pitch of 1 / n where n is an integer. A convolution-superposition dose calculator ͑TomoTherapy, Inc.͒ included all the physics needed to explore the ripple magnitude versus pitch and beam width. The results of the dose calculator and some benchmark measurements demonstrate that the ripple has sharp minima near p = 0.86͑1/n͒. The 0.86 factor is empirical and caused by a beam junctioning of the off-axis dose profiles which differ from the axial profiles as well as a long scatter tail of the profiles at depth. For very strong intensity modulation, the 0.86 factor may vary. The authors propose choosing particular minima pitches or using a second delivery that starts 180 deg off-phase from the first to reduce these ripples: "Double threading." For current typical pitches and beam widths, however, this effect is small and not clinically important for most situations. Certain extremely large field or high pitch cases, however, may benefit from mitigation of this effect.

A novel method to correct for pitch and yaw patient setup errors in helical tomotherapy

“…Film processor artifacts are visible on the −y end of some films. The horizontal stripes in the dose difference maps are due to interplay of the helical delivery pattern with the beam divergence, as described by Kissick et al 13 and Mackie et al Figure 9 shows DTA maps in regions where the local dose gradient is greater than 30% of the maximum dose per cm. ͑The maximum dose in the film with the perfect setup was 1.91 Gy.͒ The first and second moments of these DTA distributions are also listed.…”

A planning study for palliative spine treatment using StatRT and megavoltage CT simulation

“…Using recommended pitch factors (0.86/n) helps in minimizing the them. ( 24 ) Nevertheless plans with a ${\mathrm{normalD}}_{1\%}$ value smaller than 110% and homogeneity index smaller than 17 are still classified “good” or “outstanding”, based on the RTOG criteria. ( 25 ) Further study is needed to understand the clinical significance of the heterogeneous dose distribution with the thread effects.…”

Clinical challenges in the implementation of a tomotherapy service for head and neck cancer patients in a regional UK radiotherapy centre

“…Field width is the width of the fan beam defined by the two jaws of the primary collimator [19]; the machine is commissioned for three field widths: 1 cm, 2.5 cm and 5 cm. Pitch is the couch travel distance relative to the axial beam width during a complete gantry rotation [20], and modulation factor is defined as the maximum divided by the average leafopening time for all non-zero leaf openings [19]. With experience, an appropriate combination of these parameters can be selected for a particular patient in order to achieve a highly conformal plan that can be delivered with a reasonable treatment time.…”