LATERAL ELECTRON-TRANSPORT IN FFT PHOTON DOSE CALCULATIONS

Previously published FFT dose calculations and their verifications hav e been performed for field sizes greater than 10 cm x 10 cm with photo n energies up to 18 MV [Zhu and Boyer, Phys. Med. Biol. 35, 351-368 (1 990)]. With a smaller field size at megavoltage energies, lateral elec tronic disequilibrium exists in low density inhomogeneities which is n ot handled properly by the FFT calculations. In this paper, we extend FFT dose calculations to address this problem without significantly in creasing the computational time. We employ lateral scaling of the fiel d size at each depth along with an inverse scaling of the resultant do se distribution. The concept of effective density is introduced for th e lateral and inverse scaling. We demonstrate that electron transport can be adequately captured in the presence of inhomogeneities, thus im proving 3D FFT dose calculations in inhomogeneous media. (C) 1997 Amer ican Association of Physicists in Medicine.