Radiation enhancement

The defects generated in ion—soHd interactions influence the kinetic processes that occur both inside and outside the cascade volume. At times long after the cascade lifetime (t > 10 s), the remaining vacancy—interstitial pairs can contribute to atomic diffusion processes. This process, commonly called radiation enhanced diffusion (RED), can be described by rate equations and an analytical approach (27). Within the cascade itself, under conditions of high defect densities, local energy depositions exceed 1 eV/atom and local kinetic processes can be described on the basis of ahquid-like diffusion formalism (28,29). 

The limiting temperature for graphite use in fusion systems is defined by tliermal sublimation (--1500-2000°C). However, a process which is very similar to thermal sublimation (in cause and in effect) appears to define the current temperature limit. This phenomenon, which is known as radiation enhanced sublimation (RES), is not clearly understood but dominates above a temperature of about 1000°C and increases exponentially with increasing temperatme. 

W. Eckstein and V. Philipps, Physical Sputtering and Radiation-Enhanced Sublimation. In Physical Processes of the Interaction of Fusion Plasmas with Solids, W.O. Hofer and J. Roth, Eds., 1996, Academic Press, pp. 93-134. 

Petit J-C, Langevin Y, Dran J-C (1985a) Radiation-enhanced release of uranium from accessory minerals in crystalline rocks. Geochim Cosmochim Acta 49 871-876 Petit J-C, Langevin Y, Dran J-C (1985b) " U/ U disequilibrium in natnre theoretical reassessment of the varions proposed models. Brill Mineral 108 745-753... 

The depth resolution (i.e. the ability to discriminate between atoms in adjacent thin layers) is limited by the primary beam causing redistribution of target atoms prior to their emission as ions, and to segregation and radiation-enhanced diffusion processes. The local topography can also lead to a loss of depth resolution with sputter depth. 

BOCCALANDRO, H.E., MAZZA, C.A., MAZZELLA, M.A., CASAL, J.J., BALLARE, C.L., Ultraviolet B radiation enhances a phytochrome-b-mediated photomorphogenic response in Arabidopsis, Plant Physiol., 2001, 2, 780-788. 

At any specified angle the wavelengths that are whole multiples of the value d sin 6 will be enhanced and are known as first-, second- and third-order spectra. The unwanted wavelengths can be eliminated by the use of simple cut-off filters as previously described for interference filters. If a particular grating, for instance, has 1000 slits per millimetre and is viewed at an angle of 8.6°, the wavelengths of radiation enhanced will be 150 nm, 300 nm, 450 nm, 600 nm, etc. 

Keeping the above principles in mind, Herscher et al. gives an excellent overview of the characteristics of an idealized radiation modifier (35). In the case of the radiation protector more dose can be delivered to the tumor and in the case of a sensitizer more effective dose can be delivered. In theory the ideal radiation enhancer will have selective systemic activity against malignant but not against nonmalignant cells, will reach the tumor in adequate concentrations to affect radiation, and will have been studied such that the ideal timing with relation to radiation treatment delivery will be defined. Such a compound will increase the effects of radiation in one of several ways (see Fig. 2) ... 

Of course other methods of radiation enhancement may take place with reoxygenation of hypoxic tumor cells that may be increased with the use of chemotherapies like paclitaxel or gemcitabine (41,42), improved drug access to the tumor cells after radiotherapy, or a lowering of the threshold for radiation-induced apoptosis as has been described with the use of gemcitabine (43). 

Fig. 6. FdUrd-induced radiosensitization is greater under treatment conditions that produce cytotoxicity. Data from Miller and Kinsella et al. and the data from Bruso et al. and Lawrence et al., the average radiation enhancement ratio is shown for a variety of FdUrd treatment conditions.

Kirichenko AV, Travis EL, Rich TA. Radiation enhancement by 9-aminocamptothecin. Evidence for improved therapeutic ratio with a multiple dose schedule. Ann NY Acad Sci 1996 803 312-314. 

Kirichenko AV, Rich TA. Radiation enhancement by 9-aminocamptothecin the effect of fractionation and timing of administration. Int J Radiat Oncol Biol Phys 1999 44(3) 659-664. 

Mose S, Karapetian M, Juling-Pohlit L, et al. Radiation enhancement of gemcitabine in two human squamous cell carcinoma cell lines. Anticancer Res 2000 20(1A) 401 405. 

Ionizing radiation induces von Willebrand factor expression (75), and promotes platelet aggregation (76). Radiation enhances platelet adhesion to the extracellular... 

Verheij M, Dewit LG, Bloomgaard MN, et al. Ionizing radiation enhances platelet adhesion to the extracellular matrix of human endothelial cells by an increase in the release of von Willebrand factor. RadiatRes 1994 137 202-207. 

Radiation Enhanced Surface Activity of an Ordnance Explosive, a-HMX , InUApplRad -Isotope 19, 495 (1968) 181) Y.A. Zakharov... 

---