Evaluation and modification of a differential SAR dose calculation algorithm.

摘要

"George's RAdiotherapy Treatment desIgn System" (GRATIS), a treatment planning systems designed at the University of North Carolina by George Sherouse and his colleagues, offers a flexible and fast algorithm for the calculation of dose in 3D conformal radiotherapy. Its calculation routine is a generalization of the CBFAM algorithm created by John Cunningham, which calculates the dose value at a point in phantom by separately evaluating the components of dose that are due to primary and scattered radiation. The calculation algorithm, although fairly general in design, currently makes a number of simplifying assumptions to reduce calculation time, and the corresponding tradeoff is a potential loss of accuracy.;We have attempted to remove these simplifications and test the limits of accuracy of the algorithm for symmetric and asymmetric collimator configurations. Isodoses have been obtained of GRATIS, on a Varian 2300CD Linae, and on the "Alberta Treatment Planning system" (ATP) so that the performance of GRATIS can be evaluated. A modification to the algorithm has been investigated that involves a redefinition of the "pencils" used in the "scattered integration". This modification makes the integration more consistent with the measured data which is used by the algorithm and increases its accuracy. Limitations of this type of calculation have also been demonstrated for off axis cases where the energy spectrum of the accelerator is significantly different from the central axis spectrum.