Highly energetic heavy ions (HZE), of the type found in galactic cosmic rays, deposit energy in the form of ionizations along the tracks they take as they travel through a cell. These ionizations are tightly spaced along these tracks and this spacing makes them highly damaging to cellular structures. Some of the particles energy is also imparted to electrons (delta rays) which can travel great distances and produce damage, albeit less severe, in cells far removed from the original particle track. The ranges of these electrons are proportional to the energy of the original HZE particle. While these delta rays by themselves are not likely to produce much in the way of measurable damage, investigators have evidence that these electrons can add damage to that forming along independent HZE particle tracks that will eventually be expressed in the form of chromosome exchanges of greater complexity.
Chromosome exchanges result when radiation breaks a chromosome by severing the DNA of which it is composed. The cell has means to repair the chromosome and can splice the DNA together. On some occasions, if one chromosome break is close to another break, the cell might make a mistake and join the broken ends to inappropriate partners. Two or more breaks can be involved in the process; the greater the number of breaks, the more complicated the resulting chromosome exchange will be. Investigators plan to test the hypothesis that these exchange complexity increases will be related to the primary particle energy and resultant maximum delta ray range. Higher particle energies will result in longer delta ray maximum ranges allowing more primary particle tracks to be close enough to irradiate a cell with delta electrons for any specific fluence (particle tracks per unit area; equivalent to dose).
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Investigators irradiated human skin cells with low and high energy beams matching linear energy transfer (LET), energy deposited per unit track length and fluences so that the only variable will be particle energy. A dose response curve for chromosome exchanges irradiated with low particle fluences was constructed. At the same time they looked for breaks that fail to rejoin. One experiment was conducted at a low dose rate which prevented damage from delta rays from being added to chromosome exchanges forming along primary particle tracks by spreading damage formation out in time and allowing some breaks to be repaired or exchanges to be completely formed before other breaks occurring later can be added to them. Investigators also grew populations from single surviving cells allowing them to measure the transmissibility of chromosome exchanges a number of cell generations removed from the irradiation. Chromosome aberrations are tightly linked to cancer development. With this study they hoped to gain a better understanding of the processes underlying chromosome exchange formation following irradiation with HZE particles similar to those astronauts might encounter in space. Investigators aimed to determine the shape of the exchange dose response curve for these particles at low doses and at low dose rate, processes involving the mis-repair of radiation induced damage that could lead to carcinogenesis. At the same time they looked at terminal deletions and incomplete exchanges resulting from the cell’s failure to repair damage. These lesions are likely to kill the cell and eliminate it from a population that might develop into cancer. Finally, they looked for exchanges found only in a fraction of the cells making up a clonal population indicating genomic instability, the process which drives carcinogenesis.
For curvature to be observed in the dose response, the delta rays from one primary particle would have to produce chromosome breaks that were close enough to be involved in an exchange with other breaks produced by a separate and independent primary particle, a process termed track interaction. The investigators believed that the range of the delta rays would be of utmost importance in this process. As each of the different particle beams deposits the same amount of energy as it travels through the cell, those with shorter tracks deposit more energy along those tracks. Therefore, the lower energy particles produce a more concentrated distribution of energy induced damage closer to the primary particle track (path it takes through the cell), while the higher energy ions spread this same amount of damage over a longer distance. While the delta rays travel longer distances and can potentially encounter more independent tracks the concentration of that damage is less making it a more remote chance that a track interaction event could occur. The calcium ion beam is taken to represent an optimal energy where the delta rays have sufficiently long range to pass near independent tracks and produce damage with adequate density to allow track interaction events to occur with measurable frequency.
No datasets exist for this study. A final report was archived.
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