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.