Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 25-10. Phases of the cell'sreproductive cycle. The timebetween cell divisions is called interphase.Interphase includes theperiod after mitosis but before DNAsynthesis (G1), which is the most variableof the phases; followed by 5phase, during which DNA synthesisoccurs; followed by RNA synthesis inG2, all leading up to mitosis (Mphase), the events of which are differentiatedin prophase, metaphase,anaphase, and telophase. (Adaptedfrom Bushong sc. Radiologic sciencefor technologists, 3rd ed. St. Louis:CV Mosby, 1984.)reproductive cycle. Experiments indicate that, in general, cells are most sensitiveto radiation during mitosis (M phase) and RNA synthesis (G2), less sensitive duringthe preparatory period for DNA synthesis (G1), and least sensitive duringDNA synthesis (5 phase).The response of an organ system to radiation depend not only on the dose, doserate, and LET of the radiation but also on the relative radiosensitivities of thecells that comprise both the functional parenchyma and the supportive stroma.In this case, the response is measured in terms of morphologic and functionalchanges of the organ system as a whole rather than simply changes in cell survivalkinetics.The response of an organ system after irradiation occurs over a period of timewhose onset and period of expression are inversely proportional to the dose. Thehigher the dose, the shorter the interval before the physiologic manifestations of thedamage become apparent (latent period) and the shorter the period of expressionduring which the full extent of the radiation-induced damage is evidenced. Thereare practical threshold doses below which no significant changes are apparent. Inmost cases, the pathology induced by radiation is undistinguishable from naturallyoccurring pathology.Regenerationand RepairHealing of tissue damage produced by radiation occurs by means of cellular regenerationand repair (Fig. 25-11). Regeneration refers to replacement of the damagedcells in the organ by cells of the same type, thus replacing the lost functional capacity.Repair refers to the replacement of the damaged cells by fibrotic scar tissue, in