On the making of sharp longitudinal dose profiles with helical tomotherapy.
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.
Kissick
Michael W
MW
Department of Medical Physics, University of Wisconsin-Madison, Madison, WI 53706, USA.
Flynn
Ryan T
RT
Westerly
David C
DC
Mackie
Thomas Rockwell
TR
Hoban
Peter W
PW
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