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Injury to DNA is the primary mechanism by which ionizing radiation kills cells (4,83). Most DNA damage is repaired, but lethal double-strand breaks are thought to persist in the form of locally multiply damaged sites (300) of about 15 to 20 nucleotides in size that cause micronuclei formation, chromosome aberrations, and cell death through loss of the reproductive integrity of the cell's genome (38,66,112,158,235,236). However, many biologic factors affect the relationship between the amount of physical energy deposited, the extent of DNA damage that is caused, the number of cells that are killed, and the severity of the tissue response.
The energy initially deposited by ionizing radiation is largely converted into the generation of free radicals. Since cells are made up largely of water (about 85%), most of the damage to biologic molecules caused by x-rays (perhaps 65% or more) is mediated through free radicals formed by activated water, and in particular, hydroxyl radicals, and most DNA damage is therefore indirect. A chain of physicochemical reactions is initiated that is heavily influenced by the intracellular milieu, which influences the persistence of free radicals and the other chemical species that are formed, and the damage that results. Perhaps the most important molecular presence is that of oxygen (5), although other electron-affinic molecules will also play a role. Oxygen can participate in the free radical-generation chain, fix free radical damage, and limit chemical repair. Conversely, sulfhydryl molecules, which vary in natural abundance, scavenge free radicals and may limit the extent of damage.
In contrast to sparsely ionizing x-rays, densely ionizing high linear energy transfer (LET) radiations (e.g., neutrons, α-particles) deposit their energy so intensely along their tracks that lethality relies more on direct ionization to cause damage in DNA and other molecules than on indirect action through ionization of water. The outcome of the interaction of the physicochemical events initiated by ionizing radiation with the biologic system therefore varies with the nature of the radiation and the intracellular milieu, in addition to obvious factors such as dose and dose rate.
Further complexities are that cells sense and respond to radiation damage and this mechanism varies, depending on their biochemical and genetic make-up. Several molecules have been identified by which cells sense damage to DNA (the DNA damage response), and to other intracellular structures and molecules, including mitochondria, membrane lipids, and certain growth factor receptors. Recognition of damage leads to activation of signal transduction pathways aimed at making a coherent and appropriate response to injury. The internal molecular signaling network that exists within a cell as well as the external signals they are receiving (e.g., from hypoxia, cytokines, cell-cell contact, and the extracellular matrix) influence the nature of the response. The resultant radiation-induced pathways can promote cell death or survival, cell cycle arrest or progression, and DNA repair or instability. In other words, the way the cell “perceives” radiation damage plays an important role in determining the final response. Since tumorigenesis requires mutations in molecular pathways that govern cell death, cell cycle, and DNA repair, it follows that genetic alterations associated with cancer frequently affect the response to radiation therapy. It should be noted that similar pathways are often activated in response to stresses other than radiation, including chemotherapy, hyperthermia, oxidative agents, and inflammation. However, radiation differs from these in that it causes a relatively large number of large lesions in DNA that are not only frequently lethal but drive a predominance of pathways triggered by DNA double-strand break formation.
Finally, in addition to molecular and cellular factors that determine intrinsic cellular radiosensitivity, tissue-related and clinical features of radiation exposure add several additional layers of complexity. For example, the number of cells in a tissue capable of regenerating function will be important, as will the way the regenerative potential is distributed as functional subunits within the tissue. Tissue responses may not relate directly to radiation's cytotoxic effects. For example, in tumors, although local control requires elimination of tumor clonogens, in some circumstances vascular damage could be important, especially when irradiation is combined with biologics or chemotherapeutic drugs. Also, irradiation modifies the tumor-host relationship, including interactions with infiltrating cells, such as macrophages and lymphocytes, which have been shown to be able to both promote and inhibit tumor growth. Effects have been described, mainly in vitro, in which the irradiated cell affects the viability or mutability of surrounding “bystander” cells. Such bystander effects may involve more than one mechanism but are presumably, in vivo in normal tissues, a “danger” signaling mechanism for responses to irradiation aimed at tissue healing. Further, some side effects of radiation therapy on
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normal tissues (13) may result from the release of cytokines and other biologic factors or may be associated with remodeling of the normal tissues, rather than cell death per se. For example, late radiation-induced normal tissue fibrosis depends to an extent on cell depletion by radiation, but is more an attempt at a healing response that can be modified by numerous factors, including health status, presence of infection, surgery, injury, and so on.
So, while the initial deposition of energy and subsequent radiochemical events are complete within thousandths of a second following irradiation, a chain of biologic events is initiated that induce programmed cell death or survival, tissue repair and remodeling, all of which depend on the intercellular signaling network, all of which are influenced by systemic and local physiologic conditions. Given the complexity of the biologic condition, it is impossible to predict biologic or clinical outcomes from the amount of physical energy deposited. In other words, biological dose differs from physical dose.
Remarkably, while cells and tissues may respond differently to the same physical dose of radiation, any given tissue appears to respond in a fairly predictable way. The reason for this apparent constancy is that tissue responses are governed largely by cell turnover and the regenerative reserve in the tissue that is generally similar between individuals. Therefore, normal mucosal reactions occur at the same time interval after the start of irradiation and have similar dose-response relationships in most patients. While tumor responses may be more variable than those in normal tissues, certain histologic types of tumors are regarded as more curable by irradiation, and others are not. Reproducible differences in biologic response between tissues are, in fact, exploited for therapeutic benefit. A good example is the use of standard dose fractionation in conventional radiation therapy. This protocol was derived empirically but actually exploits differences in the biologic response between tumor and normal tissues to the same physical dose of radiation. The radiobiologic rationale for the use of dose fractionation in standard radiation therapy has been encapsulated in the 4 “Rs” (reoxygenation, redistribution, repair, and repopulation) (311). This chapter aims to explain how radiobiologic concepts derived from studies dealing with responses within and between normal tissues and tumors are relevant in clinical radiotherapy.