Dose enhancement by a thin foil of high-Z material: A Monte Carlo study

The purpose of this work is to study the dose enhancement by a thin foil (t hickness of 0.2-4 mm) of high-Z material in a water phantom, irradiated by high-energy photon beams. EGS4 Monte Carlo technique was used. Perturbation s on the beam spectra due to the presence of the foils, and dose enhancemen t dependence of photon-beam quality, beam incident angle, atomic number (Z) , the thickness and size of the foil, and the depth of the foil situated in the phantom were studied. Analysis of photon and secondary-electron spectr a indicates that the dose enhancement near an inhomogeneity interface is pr imarily due to secondary electrons. A calculation for I-mm-thick planar lea d foil in a water phantom shows that the dose enhancements at 0.25, 1, 2 an d 3 mm away from the foil in the backward region were 58%, 37%, 24% and 17% , respectively, for a 15 MV beam. Calculations for a variety of planar foil s and photon beams show that dose enhancement: (a) increases with Z; (b) de creases with decreasing foil thickness when the foils are thinner than a ce rtain value (1 mm for lead foil for 15 MV); (c) decreases with decreasing i ncident photon-beam energies; (d) changes slightly for beam incident angles less than 45 degrees and more prominently for larger angles; (e) increases with size of foil; and (f) is almost independent of the depth at which the foil is situated when the foil is placed beyond the range of secondary ele ctrons. The dose enhancement calculation is also performed for a cylindrica lly shaped lead foil irradiated by a four-field-box. The dose enhancement o f 34%/13% was obtained at 0.25/2 mm away from the cylindrical outer interfa ce for a 15 MV four-field-box. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)92107-0].