Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

paragraphs, including dose rate, LET, and the presence of oxygen. Inherentradiosensitivity includes those biologic factors that are characteristics of the cellsthemselves, such as the mitotic rate, the degree of differentiation, and the stage ofthe cell cycle.The rate at which a dose oflow-LET radiation is delivered has been shown to affectthe degree of biologic damage for a number of biologic end points including chromosomalaberrations, reproductive failure, and death. In general, high dose rates aremore effective at producing biologic damage than low dose rates. The primaryexplanation for this effect is the diminished potential for repair of radiation damageat high dose rates. Cells have a greater opportunity to repair sublethal damageat low dose rates than at higher dose rates, reducing the amount of damage andincreasing the survival fraction. Figure 25-6 shows an example of the dose rate effecton cell survival. Note that the broader shoulder and higher Do associated with lowdose-rateexposure indicate its diminished effectiveness compared with the samedose delivered at a higher dose rate. This dose-rate effect is not seen with high-LETradiation primarily because the dense ionization tracks typically produce enoughhits in critical targets to overwhelm the cell's repair mechanism or produce types ofdamage that cannot be repaired. However, for a given dose rate, high-LET radiationis considerably more effective in producing cell damage than low-LET radiation(Fig. 25-7).ClC 1.0.S;.~::::JUJ.!!!Q) 0.10-0c0;:;(,)0.01cu •..U.0.0012 4 6 8 10 14 18Dose (Gy)FIGURE 25-6. Cell survival curves illustrating the effect of dose rate. Lethality isreduced because repair of sublethal damage is enhanced when a given dose of radiationis delivered at a low versus a high dose rate.