title: On the making of sharp longitudinal dose profiles with helical tomotherapy.
abstract: Since the beam width on the helical tomotherapy machine produced by TomoTherapy Inc., is typically a few centimeters in the longitudinal direction (into the bore), the optimizer must choose to have a relatively high intensity local to the inside edge of a tumor or planning treatment volume (PTV) when avoiding an immediately adjacent organ at risk (OAR), either superior or inferior. By using a standalone version of the TomoTherapy dose calculator, a realistic beam is applied to idealized deconvolution schemes including the MATLAB Optimizer Toolbox for a simple one-dimensional PTV with adjacent OARs. The results are compared to a clinical example on the TomoTherapy planning station. It is learned that a Gibbs phenomenon type of oscillation in the dose within the tumor under these special circumstances is not unique to TomoTherapy, but is related to the attempt to form a sharp dose gradient-sharper than the beam profile with typical optimization constraints set to achieve a uniform dose as close as possible to the prescription. The clinical implication is that the Gibbs-induced cold spots force the dose to increase in the PTV if a typical PTV dose-volume constraint is used. It is recommended that the dose prescription be smoothed prior to optimization or the dosimetric goals for an OAR adjacent to the PTV are such that a sharp dose falloff is not demanded, especially if the user reduces the requirements that such an OAR be of both high importance and immediately adjacent to the PTV edge.
authors.affiliation: Department of Medical Physics, University of Wisconsin-Madison, Madison, WI 53706, USA.
authors.forename: Michael W
authors.initials: MW
authors.lastname: Kissick
authors.forename: Ryan T
authors.initials: RT
authors.lastname: Flynn
authors.forename: David C
authors.initials: DC
authors.lastname: Westerly
authors.forename: Thomas Rockwell
authors.initials: TR
authors.lastname: Mackie
authors.forename: Peter W
authors.initials: PW
authors.lastname: Hoban
mesh_terms: D000465:Algorithms; D004867:Equipment Design; D006801:Humans; D015233:Models, Statistical; D009369:Neoplasms / Q000532:radiotherapy*; D009991:Oscillometry; D010315:Particle Accelerators; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated; D036542:Tomography, Spiral Computed / Q000379:methods*; D014057:Tomography, X-Ray Computed