High LET irradiation

High-LET Advantage. Thirdly, high-LET irradiation is delivered at the level of the SOBP where the PTV (thus the cancer cell population) is located. Heavy-ion beams are thus specifically effective against some tumor types as discussed for fast neutrons (see Secs. 4.1.1 and 4.1.3). 

Baumstark-Khan, C., Heilmann, J., Rink, H. (2003). Induction and repair of DNA strand breaks in bovine lens epithelial cells after high LET irradiation. Adv. Space Res. 31 1583-91. 

A comparison of the nature of the DNA-radicals produced with y-radiation relative to those produced by ion-beams shows that some radical species which are found in low amounts using /-radiation (low LET) increase by an order of magnitude in absolute yield with ion-beam (high LET) irradiation. For example, work using oxygen ion-beams found evidence for a phosphoryl radical (R0P02 , Scheme 1) in DNA. This radical, formed by dissociative electron attachment (Sec. 3), is clear evidence for a prompt strand break.In addition, formation of sugar radicals via exited states was proposed (Sec. 2.7). 

Roots R., Hoi ley W., Chatterjee A., Irizarry M., Kraft G., The formation of strand breaks in DNA after high-LET irradiations a comparison of data from in vitro and cellular systems, Int. J. Radiat. Biol., 1990, 58,55-69. 

Low-LET irradiation (y-iay, electron) High-LET irradiation (heavy ions)... 

Repair Capacity. Finally, when fractionated treatment is applied, the high-LET radiation at the level of the SOBP (i.e., PTV) partly prevents or reduces cell repair. In contrast, the normal tissues irradiated at the level of the initial plateau are exposed to... 

Figure 28 Schematic presentation of the relative situation of the different types of radiations used in therapy. Two criteria are considered the physical selectivity and the LET (or radiobiological properties). For the low-LET radiations, the physical selectivity was improved from the historical 200-kV x-rays to cobalt-60 gamma rays and the modern linacs. Even with the linacs today, significant improvement is continuously achieved (IMRT, etc.). Among the low-LET radiation, the proton beams have the best physical characteristics, but one of the issues is the proportion of patients who will benefit from proton irradiation. A similar scale can be drawn for high-LET radiation the heavy-ion beams have a physical selectivity similar to protons. Selection between low- and high-LET radiation is a biological/medical problem it depends on the tumor characteristics, and reliable criteria still need to be established (see text). (From Ref 54.)...

The main detectable DNA lesions, as damaged bases are concerned, were also found in irradiated cells. Table 12.7 compiles data that were obtained with y (low LET) and carbon ion (high LET) radiations. 

The 100 eV-yield of crosslinking, G(X), of polystyrene is much higher at high LET than G(X) at low LET for gamma irradiation 0 [8, 9]. Similar results have been obtained by fast neutron irradiation [10]. Generally G(X) of polystyrene is reported to increase with increasing LET with the exception of one paper [11]. 

Early on, the differences in the products from radiolysis of water were noticed. It was found, as mentioned above, that the radiolysis of pure water seemed to lead to almost no damage. If there were impurities in the water, radiation damage would occur. However, irradiation by high LET radiation would clearly lead to the formation of hydrogen. Further experiments showed that if one irradiated a sealed sample with high-LET radiation, and then the sample was irradiated with a low-LET source, the gas formed by the radiation would then disappear and it would appear as if there were no long-term decomposition. These data were part of the reason that A. O. Allen proposed the theory of radiation chemistry where back reactions occurred. ... 

At low LET radiation like gamma and X-rays, this is easily achieved and the water behaves as if it was completely inert under irradiation. At high LET (alpha rays) the ratio of the molecular products over the free radicals increases and there are comparatively too few H to Oj atoms. The recombination of and 0 is thus incomplete, the chain reaction between Hj and H Oj cannot start and water decomposition is observed. 

Figure 4 Water decomposition yield, G(-HO), as a function ofthe flux ratio alpha/y irradiation (mixture of high and low LET particles). Increasing this ratio means increasing the LET value. The observed decomposition threshold Is shifted to the right (higher LET) and disappears betwen. 30 °C and 200 °C. The scale corresponds also to the concentration In °B. The difference between circle and triangle symbols Is the Initial enrichment In °B In the chemicals. Adding °B that absorbs neutrons under neutron/gamma Irradiation produces alpha rays having high LET (200 eV/nm) [6],...

Swift heavy ion beams with extremely high LETs are used to degrade strongly polymer chains within tracks over a range of a few tens of micrometers. After irradiation, the top surface of bulk thermoplastics such as polyethyleneterephtalate, polycarbonate, polyvinylidene fluoride or polyimide materials as well as thin films made of the same material are subsequently etched by a wet chemical treatment that reveals pores of which the shape, surface density and dimensions can be controlled by choosing appropriate conditions. High aspect ratio cylindrical or conical traces of diameter ranging from a few nanometers to some... 

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