ABSTRACT
Inner-shell electrons in so tissue, on the other hand, tend to have binding energies on the order of 1 keV or less that, coupled with the much lower average atomic number of organic materials, means the photoelectric eect is a relatively small contribution to the absorption in so tissue whereas it is the dominant component of gold ionization events at energies
up to hundreds of kiloelectron volts. Taking these eects together, the signicantly increased photoelectric cross-section means high-Z metals absorb substantially more energy per unit mass than do so tissue, translating to a signicant increase in local dose when even a small amount of metal is present in the tumor [2]. For certain photon beam qualities, this also results in a change of photon interaction cross-section within the tumor. is fact has been exploited by the development of phototherapy, whereby an orthovoltage (140-250 kVp) x-ray beam is used for treatment. is low photon beam quality (i.e., low in comparison to the normally used megavoltage in radiation therapy) results in a predominance of the photoelectric absorption interaction in the CM-containing tumor. Because the atomic cross-section for photoelectric absorption exhibits a Z4 to Z4.8 dependence, photon absorption is augmented specically in the tumor volume and the absorbed dose is enhanced relative to surrounding tissues.
