Fetal dose assessment from invasive special procedures by Monte Carlo methods

The assessment of fetal dose from a special procedure in the clinical envir onment is difficult as patient size, fluoroscopic beam motion, and imaging sequences vary significantly from study to study. Fetal dose is particularl y difficult to estimate when the fetus is exposed partially or totally to s catter radiation from images taken in other locations of the mother's body. A method to reliably estimate fetal dose has been developed by using templ ate based input files for the Monte Carlo radiation transport code MCNP. Fe male patient phantoms at 0, 3, 6, and 9 months of pregnancy and source term s for common diagnostic tube potentials are used to rapidly build an input file for MCNP. The phantoms can be easily modified to fit patient shape. Th e geometry and beam location for each type of image acquired (i.e. fluorosc opy, spot filming, etc.) is verified by the use of a 3D visualization code (Sabrina). MCNP is then run to estimate the dose to the embryo/fetus and th e exposure to skin entrance (ESE) for the beam being modeled. The actual ES E for the beam is then measured with ion chambers and the fetal dose is det ermined from the MCNP supplied ratio of ESE to fetal dose. Runs are made fo r each type of imaging and the doses are summed for the total fetal dose. F or most procedures, the method can provide an estimate of the fetal dose wi thin one day of the study. The method can also be used to prospectively mod el a study in order to choose imaging sequences that will minimize fetal do se. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99 )2208-7].