AbstractID: 8992 Title: The "Thread Artifact" of Helical Tomotherapy Inherent to helical tomotherapy is a helical delivery artifact referred to as the “thread artifact.” This artifact manifests as a helical dose modulation increasing from zero in the center up to several percent off-center, depending on the pitch. The magnitude and primary causes of this ripple are explored and determined to be the result mostly of axial beam divergence. Modeling, in addition to a sample comparison to representative data will be presented. The thread artifact magnitude is a complicated function of the pitch: couch motion for a complete gantry rotation relative to the axial beam width at the rotation axis. There are particular pitch values that minimize this artifact, but another method to reduce this ripple is to deliver the dose to the same volume at various initial phases with the same pitch. A discussion related to investigations that use only twodimensional calculations is provided. Such simplified two-dimensional calculations, in extreme cases, could be incorrect by up to ~10% within a slice from just this artifact. Another concern is with QA procedures that do not use full helical 3-D plans. Relatively simple modeling with a first-collision dose (TERMA) ray-tracing scheme does produce a remarkably accurate illustration of this thread artifact. This work is sponsored by University of Wisconsin Radiological Sciences Training Grant, through the National Cancer Institute (NIH 5 T32 CA09206).