Dosimetry characteristics of the Helical Tomotherapy Unit that integrates imaging,

AbstractID: 7696 Title: Commissioning Dosimetry for a UW Clinical Tomotherapy Unit
Dosimetry characteristics of the Helical Tomotherapy Unit that integrates imaging,
planning, delivery, verification and adaptive treatment delivery are evaluated. Beam
alignment from the MLC system and the energy-fluence quantification are described in
this study.
A variety of alignment tests were performed. MLC positioning will be described in
details. Kodak XV film was used to collect data and perform the analysis. Double
exposure with, all odd leaves open for first exposure and all even leaves open for second
exposure was performed. Transverse profiles across the image had sixty-four peaks and
valleys due to the tongue-and-groove effect (T&G). These effects are accentuated with
the MLC misalignment, since the projection of the common region of overlap between
adjacent leaves increases. Therefore, by comparing one side of the transverse profile
with the opposite side, the level of MLC-source alignment can be quantified. Since the
tomotherapy dose calculation code is convolution based the energy fluence output must
be known. This is achieved by iterative modification of an assumed energy-fluence until
calculated and measured dose distributions agree sufficiently well.
The proximity of the MLC to the source and smaller leaf opening compared to
conventional MLC makes the alignment requirements more stringent. MLC alignment is
sensitive to a 0.1 mm adjustment. Furthermore, it was found that the dose image
recorded on the tomotherapy exit detector was of sufficient resolution to adequately
reproduce the T&G image. This also allows for an automated MLC-alignment qualityassurance.
This work was partially supported by National Institute of Health Grant # NIH/NCI: