Sensitivity to radiation

The ability of various selenium heterocycles to check the loss of orthophosphate caused by irradiation of ATP has been studied by Brucker and Bulka (92). They found that only 2-amino-4,5-dimethyiselenazole shows radioprotective properties, while other 2-aminoselenazoles, selenosemicarbazides, and acetone selenosemicar-bazones possess no such activity but are in addition very sensitive to radiation (93). 

In air, PTFE has a damage threshold of 200—700 Gy (2 x 10 — 7 x 10 rad) and retains 50% of initial tensile strength after a dose of 10" Gy (1 Mrad), 40% of initial tensile strength after a dose of 10 Gy (10 lad), and ultimate elongation of 100% or more for doses up to 2—5 kGy (2 X 10 — 5 X 10 rad). During irradiation, resistivity decreases, whereas the dielectric constant and the dissipation factor increase. After irradiation, these properties tend to return to their preexposure values. Dielectric properties at high frequency are less sensitive to radiation than are properties at low frequency. Radiation has veryHtde effect on dielectric strength (86). 

Substitution of phenyl for methyl groups on the sdoxane iacreases the radiation resistance. Sihcone hydride groups are highly sensitive to radiation... 

The theory of radiation-induced grafting has received extensive treatment. The direct effect of ionizing radiation in material is to produce active radical sites. A material s sensitivity to radiation ionization is reflected in its G value, which represents the number of radicals in a specific type (e.g., peroxy or allyl) produced in the material per 100 eV of energy absorbed. For example, the G value of poly(vinyl chloride) is 10-15, of PE is 6-8, and of polystyrene is 1.5-3. Regarding monomers, the G value of methyl methacrylate is 11.5, of acrylonitrile is 5.6, and of styrene is >0.69. 

Kaplan78 studied the X-ray radiolysis of poly(a>-chloroolefin sulfone)s and found that the chlorine-containing polymers are less sensitive to radiation than the non-chlorine containing species. He found that dehydrochlorination is a significant reaction in the radiolysis of poly(to-chloroolefin sulfone)s, but it is not a necessary one in order to lose... 

Radioactivity also Is used to treat certain diseases. Some cancers respond particularly well to radiation therapy. Radioactivity must be used with care, because exposure to radiation damages healthy cells and eventually causes cancer. The key to radiation therapy is that cancer cells reproduce more rapidly than normal cells, and rapidly reproducing cells are more sensitive to radiation. If concentrated doses of radiation are focused on the malignant cells, a cancer may be destroyed with minimal damage to healthy tissue. Nevertheless, radiation therapy always has unpleasant side effects, including nausea and hair loss. 

There are marked species differences in susceptibility to specific emetogens. For example, dog and man are very sensitive to apomorphine [40], while the rhesus monkey is completely resistant [41]. The ferret exhibits intermediate sensitivity to apomorphine, but there may be some controversy regarding this species [42-44], On the other hand, the ferret shows high sensitivity to radiation-induced emesis [44] followed by dog, man, monkey and cat. The cat in general is more resistant to apomorphine and radiation-induced emesis than dog or man [40, 45],... 

RADIATION-SENSITIVE GROUPS. Although the absorption of radiation energy is dependent only on the electron density of the substrate and therefore occurs spatially at random on a molecular scale, the subsequent chemical changes are not random. Some chemical bonds and groups are particularly sensitive to radiation-induced reactions. They include COOH, C-Hal, -SO2-, NHz, C=C. Spatial specificity of chemical reaction may result from intramolecular or intermolecular migration of energy or of reactive species -free radicals or ions. 

While PMMA is an attractive material because of its resolution characteristics, its sensitivity to radiation induced degradation is low, and its dry-etching characteristics are poor. The e-beam and deep-UV exposure doses for PMMA are —100 nC cm-2 (38) and >1 Jem-2 (40), respectively. The fact that nanometer resolution is readily achieved in this material has, however, prompted many researchers to examine substituted systems in attempts to effect improved performance. 

Variation in different strains of mice to X radiation has recently been studied.46 The percentage depression of growth (below the controls) produced by radiation of different strains was as follows Z, 24 per cent RI, 27 per cent S, 35 per cent E, 59 per cent LWG, 85 per cent, BALB/GW, 161 per cent. Heavier strains tended to be more resistant, and females were more resistant than males. In another study, a 4-fold decrease in head sensitivity (to radiation) was observed, in the following order dba, Marsh, C57 black, and C3H. Sensitivity seems to be related to the adrenal hormones.47... 

In order for 5-FU to render cells sensitive to radiation, a demonstrable degree of cell killing by 5-FU has to occur. In other words, some effectiveness of drug alone must be seen (equivalent quantitatively to killing slightly short of a clinical partial response). 

In most studies where a supraadditive interaction was seen, cells were incubated with the drug prior to irradiation (38). In general the use of moderate concentrations (5-100 nmol/L) of paclitaxel in the culture medium for over 24 h lead to maximum sensitization. Most studies used actively proliferating cells in order to conduct their experiments and most investigators reached the conclusion that maximal radiosensitization occurred when cells were arrested in G2 and M, as this part of the cell cycle is the most sensitive to radiation damage. This assumption does, however, presuppose that the majority of the G2/M arrested cells will not die unless they are exposed to ionizing XRT. Plateau phase, i.e., nonproliferating, cells were found to be sensitized to radiation... 

In an excellent review of the literature on the interaction between radiation and the taxanes, especially looking at the effects of paclitaxel, Milas et al.(38) outline how they came to realize that reoxygenation played such a substantial role in the potentiation of tumor radioresponse. It has been well established for years that tumors contain areas of hypoxic cells that are normally 2.5 to 3 times less sensitive to radiation than normal cells (37). Both radiation and chemotherapies can cause reoxygenation through their preferential killing of those oxygenated cells that are located close to blood vessels. Milas et al. summarized observations that showed ... 

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