Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

TABLE 25-1. DETERMINANTS OF BIOLOGIC DAMAGE FROM IONIZINGRADIATIONQualityQuantityDose rateExposure conditionsMolecularCellular VS. multicellularorganismPlant vs. animal VS. humanTimeMacroscopic vs.microscopicStructural vs. functional changesmolecular or cellular level mayor may not result in adverse clinical effects inhumans. Furthermore, although some responses to radiation exposure appearinstantaneously, others take weeks, years, or even decades to appear.Biologic effects of radiation exposure can be classified as either stochastic or deterministic.A stochastic effect is one in which the probability of the effect, rather thanits severity, increases with dose. Radiation-induced cancer and genetic effects arestochastic in nature. For example, the probability of radiation-induced leukemia issubstantially greater after an exposure to 1 Gy (l00 rad) than to 0.01 Gy (l rad),but there will be no difference in the severity of the disease if it occurs. Stochasticeffects are believed not to have a dose threshold, because injury to a few cells or evena single cell could theoretically result in production of the disease. Therefore evenminor exposures may carry some, albeit small, increased risk. It is the basic assumptionthat risks increase with dose and there is no threshold dose below which risks ceaseto exist that is the basis of modern radiation protection programs, the goal of whichis to keep exposures as low as reasonably achievable (ALARA). Stochastic effects areregarded as the principal health risk from low-dose radiation, including exposuresin the diagnostic radiology and nuclear medicine department.If a radiation exposure is very high, the predominant biologic effect is cellkilling that results in degenerative changes in the exposed tissue. In this case, theseverity of the injury, rather than its probability of occurrence, increases with dose.These so-called deterministic effects differ from stochastic effects in that theyrequire much higher doses to produce an effect. There is also a threshold dose belowwhich the effect is not seen. Cataracts, erythema, fibrosis, and hematopoietic damageare some of the deterministic effects that can result from large radiation exposures.Many of these effects are discussed in the sections entitled "Response ofOrgan Systems to Radiation" and "Acute Radiation Syndrome." Deterministiceffects can be caused by serious radiation accidents and can be observed in healthytissue that is unavoidably irradiated during radiation therapy. However, with theexception of some lengthy, fluoroscopically guided interventional procedures (seeChapter 23), they are unlikely to occur as a result of diagnostic imaging proceduresor routine occupational exposure.Energy is deposited randomly and rapidly (in less than 10- 10 sec) by ionizing radiationvia excitation, ionization, and thermal heating. One of the fundamental