FIGURE 3-1 Pathways of biological damage produced by exposure to radiation.

An irradiated cell may be able to repair any damage that it has suffered, using one or more of several chemical pathways available in nature. If it repairs itself correctly, then, by definition, it is indistinguishable from a normal cell and goes back to its undisturbed state. If the cell does not repair the damage, it may die. In that case, it is removed from the system. In principle, this is a good thing, because a dead cell cannot become a cancer cell. However, if too many cells of a tissue die, organ function will be compromised. If sensitive cells, for example, cells in the gut, die in large enough numbers, then the gut cannot absorb food or maintain electrolyte balance. This is why, after a dose of radiation killing a large enough number of cells, nausea and vomiting set in. However, if the radiation dose is delivered over a period of time that is long compared with the repair time constant of the cells, the cells can repair and maintain, or delete and replace, a sufficient number of cells for function to be undisturbed. These different radiation time courses are referred to as “early” and “late,” to describe how the body responds to different dose rates.

Cellular repair mechanisms are not always fully successful. In some cases, repair can leave the cell in sufficiently good shape to limp through another few cell divisions, thus maintaining the number of cells for a while. However, the daughter cells will inherit some of the original, incompletely or poorly repaired damage and die off or lead to dying or aberrant cells in subsequent divisions. This unstable state of the cell and its progeny is often called genomic instability, but is not fully understood and is likely to encompass a much broader range of phenomena, including possible responses to molecular signals from nearby damaged cells. In any case, the eventual death of cells is also good for the organism, again since dead cells cannot initiate cancer. However, just as in the case of acute effects, the loss of too many cells will compromise tissue and organ function, possibly leading to serious consequences and even death. While these effects are similar to acute effects, the fact that they do not

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