REFINEMENTS OF THE FINITE-SIZE PENCIL BEAM MODEL OF 3-DIMENSIONAL PHOTON DOSE CALCULATION

Modern three-dimensional (3-D) photon dose calculation algorithms need to be fast and accurate if they are to be practical for treatment opt imization. Refinements to a previously proposed finite-size pencil bea m (FSPB) method are presented in order to fulfill these needs. Specifi cally, a fast Fourier transform (FFT) convolution technique is used to speed calculation of the FSPB; the fluence spectrum is modeled, and t he effects of finite source size, a Gaussian x-ray source intensity pr ofile and partial transmission through a multileaf collimator (MLC) le af are approximated. The use of FFT techniques in the calculation of s mall diverging fields involves approximations that are investigated fo r a 6 MV beam and shown to introduce errors that vary with energy but do not exceed 0.7% on the central axis. Dose distributions calculated by FSPB superposition are in excellent agreement with those calculated by full field FFT convolution. Two key advances over the original imp lementation of the FSPB model are demonstrated: the fast calculation o f the FSPB facilitates development, and the incorporation of realistic beam parameters enables accurate modeling of clinical beams. (C) 1997 American Association of Physicists in Medicine.