Radioactivity effects

Combinatorial Y and TACE presents an interesting opportunity to assess the cumulative effect of these modalities in effecting tumor kill. Intraarterial infusion of Y in an aerobic environment (non-stasis) could be followed by TACE after the radioactive effect diminishes to sub-therapeutic levels (approximately 2 weeks post Y treatment). TACE administration with cytotoxic tumor exposure in a hypoxic environment would then address any viable (radioresistant) cells that remained. 

Analysis of radioactive effects of normal and abnormal operation... 

Equations 13.31 and 13.32 are only valid if the radioactive element in the tracer has a half-life that is considerably longer than the time needed to conduct the analysis. If this is not the case, then the decrease in activity is due both to the effect of dilution and the natural decrease in the isotope s activity. Some common radioactive isotopes for use in isotope dilution are listed in Table 13.1. 

Additional ionization is effected by including radioactive substances or plasma or glow discharges in the evaporation chamber or by electrical charging of the nebulizer. Such techniques are also discussed in Chapters 8 and 11. 

These chemical effects become important in medicine because living systems operate mostly through the reactions of enzymes, which catalyze all sorts of metabolic reactions but are very sensitive to small changes in their environment. Such sensitivity can lead to preferential absorption of some deleterious isotopes in place of the more normal, beneficial ones. One example in metabolic systems can be found in the incorporation of a radioactive strontium isotope in place of calcium. 

There should be specific, saturable binding to the receptor, accompanied by pharmacological characteristics appropriate to the functional effects, demonstrable using a radioactive, eg, tritium or iodine-125, ligand to label the receptor. Radioligand binding assays (1,6) have become a significant means by which to identify and characterize receptors and enzymes (see Immunoassays Radioactive tracers). Isolation of the receptor or expression of the receptor in another cell, eg, an oocyte can be used to confirm the existence of a discrete entity. 

The analysis of steady-state and transient reactor behavior requires the calculation of reaction rates of neutrons with various materials. If the number density of neutrons at a point is n and their characteristic speed is v, a flux effective area of a nucleus as a cross section O, and a target atom number density N, a macroscopic cross section E = Na can be defined, and the reaction rate per unit volume is R = 0S. This relation may be appHed to the processes of neutron scattering, absorption, and fission in balance equations lea ding to predictions of or to the determination of flux distribution. The consumption of nuclear fuels is governed by time-dependent differential equations analogous to those of Bateman for radioactive decay chains. The rate of change in number of atoms N owing to absorption is as follows ... 

In the startup of a reactor, it is necessary to have a source of neutrons other than those from fission. Otherwise, it might be possible for the critical condition to be reached without any visual or audible signal. Two types of sources are used to supply neutrons. The first, appHcable when fuel is fresh, is califomium-252 [13981-174-Jwhich undergoes fission spontaneously, emitting on average three neutrons, and has a half-life of 2.6 yr. The second, which is effective during operation, is a capsule of antimony and beryUium. Antimony-123 [14119-16-5] is continually made radioactive by neutron... 

Performance assessments are predictions of radioactivity releases, the rate of transfer of contaminants through various media, and the potential for hazard to the pubHc. These are based on a combination of experimental data obtained in the process called site characterization and detaded computations about radionuchdes and their effects. The progressive attack on the metal or ceramic waste container, the diffusion of water into the waste form, the leaching of the radioactive compounds, diffusion out, and washing away of radionuchdes are all considered. 

Radiation Dosimetry. Radioactive materials cause damage to tissue by the deposition of energy via their radioactive emissions. Thus, when they are internally deposited, all emissions are important. When external, only those emissions that are capable of penetrating the outer layer of skin pose an exposure threat. The biological effects of radiation exposure and dose are generally credited to the formation of free radicals in tissue as a result of the ionization produced (17). 

Nonetheless, these methods only estimate organ-averaged radiation dose. Any process which results in high concentrations of radioactivity in organs outside the MIRD tables or in very small volumes within an organ can result in significant error. In addition, the kinetic behavior of materials in the body can have a dramatic effect on radiation dose and models of material transport are constandy refined. Thus radiation dosimetry remains an area of significant research activity. 

Medical Uses. A significant usage of chelation is in the reduction of metal ion concentrations to such a level that the properties may be considered to be negligible, as in the treatment of lead poisoning. However, the nuclear properties of metals may retain then full effect under these conditions, eg, in nuclear magnetic resonance or radiation imaging and in localizing radioactivity. 

Tritium is readily detectable because of its radioactivity. Under certain conditions concentrations as low as 370 )-lBq/mL (10 //Ci/mL) can be detected. Most detection devices and many analytical techniques exploit the ioni2ing effect of the tritium P-decay as a principle of operation (62,63). 

In NAA the sample is made radioactive by subjecting it to a high dose (days) of thermal neutrons in a reactor. The process is effective for about two-thirds of the elements in the periodic table. The sample is then removed in a lead-shielded container. The radioisotopes formed decay by B emission, y-ray emission, or X-ray emission. The y-ray or X-ray energies are measured by EDS (see Chapter 3) in spe-... 

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