PHOTON BUILDUP FACTORS FOR SOME TISSUES AND PHANTOM MATERIALS FOR PENETRATION DEPTHS UP TO 100 MFP

Abstract

The use of photons in diagnostic and therapeutic medicine requires the accurate determination of photon irradiation dose in human tissues. Photon buildup factors represent corrections to photon interaction when the Beer-Lambert conditions are not met. Equivalent atomic numbers, energy absorption and exposure buildup factors of 10 human tissues (adipose tissue, blood, collar bone, brain, breast, eye lens, lungs, ovary, testis and skeletal muscles) and 4 equivalent materials (water, nylon, mylar and polyvinylchloride) were evaluated using the geometric progression (G-P) fitting method. The buildup factors were evaluated for 25 standard photon energies (0.015-15 MeV) and for penetration depth up 100 mfp (mean free path). The magnitudes equivalent atomic numbers and buildup factors for the studied materials varied in similar pattern with energy and penetration depth. The variations in the equivalent atomic number and the buildup factors with photon energy were attributed to elemental compositions of the individual tissue/ equivalent material and the dominance of different photon interaction in the energy spectrum considered. The behaviour of the equivalent atomic number for the equivalent materials suggests that each of them can be used to represent different human tissues for phantom construction and dose evaluation.