Chapter 7

1. Chapter 7 Molecular and Cellular Radiation Biology

2. Radiation Biology • Branch of biology concerned with the effects of ionizing radiation on living systems • Includes • Sequence of events occurring after the absorption of energy from ionizing radiation • Action of the living system to make up for the consequences of this energy assimilation • Injury to the living system that may be produced

3. Reason for Learning Cell Structure, Composition, and Function and the Adverse Effects Caused by Ionizing Radiation • The human body is composed of large numbers of various types of cells, most of which may be damaged by radiation. • Potentially harmful effects of ionizing radiation on living systems occur primarily at the cellular level. • A basic knowledge of aspects of molecular and cellular radiation biology is relevant to the subject of radiation protection. • This information provides a foundation for radiation effects on organ systems.

4. Ionizing Radiation • Damages living systems by ionizing the atoms comprising the molecular structure of these systems • Biologic damage begins with the ionization produced by various types of radiation such as: • Ionized atoms will not bond properly in molecules.

5. Radiation Energy Transfer Determinants • Charge, mass, and energy vary among the different types of radiation. • These attributes determine the extent to which different radiation modalities transfer energy into biologic tissue. • Three important concepts help us to understand the way ionizing radiation causes injury and how the effects may vary in biologic tissue.

6. Linear Energy Transfer (LET) • The average energy deposited per unit length of track by ionizing radiation as it passes through and interacts with a medium along its path • Is described in units of keV/μm • Is a very important factor in assessing potential tissue and organ damage from exposure to ionizing radiation • Radiation categories according to LET

7. General Categories of LET Radiation

8. Relative Biologic Effectiveness (RBE) • Describes the relative capabilities of radiation with differing LETs to produce a particular biologic reaction • RBE of the type of radiation being used is the ratio of the dose of a reference radiation (conventionally 250-kVp x-rays) to the dose of radiation of the type in question that is necessary to produce the same biologic reaction in a given experiment. The reaction is produced by a dose of the test radiation delivered under the same conditions.

9. RBE (Cont.)

10. Oxygen Enhancement Ratio (OER) • The ratio of the radiation dose required to cause a particular biologic response of cells or organisms in any oxygen-deprived environment to the radiation dose required to cause an identical response under normal oxygenated conditions • In general, x-rays and gamma rays have an OER of about 3.0 when radiation dose is high. • OER may be less (approximately 2.0) when radiation doses are below 2 Gyt.

11. OER (Cont.)

12. Molecular Effects of Irradiation • In living systems, biologic damage resulting from exposure to ionizing radiation may be observed on three levels • Any visible radiation-induced injuries of living systems at the cellular or organic level always begin with damage at the molecular level. • Molecular damage results in the formation of structurally changed molecules that may impair cellular functioning.

13. Effects of Irradiation on Somatic and Genetic Cells • Because exposure to ionizing radiation can alter a human cell’s constituent molecules, such exposure may disturb the cell’s chemical balance and ultimately the way it operates. The affected cell can then no longer perform its normal task. • If a sufficient quantity of somatic cells are affected, entire body processes may be disrupted. • If radiation damages the germ cells, the damage may be passed on to future generations in the form of

14. Classification of Ionizing Radiation Interaction • There are two classifications of ionizing radiation interaction on a cell. • Essentially all effects of irradiation in living cells result from indirect action, because the human body is composed of 80% water and less than 1% DNA.

15. Classification of Ionizing Radiation Interaction (Cont.) Figure 07-03.  The action of radiation on the cell can be direct or indirect. It is direct when ionizing particles interact with a vital biologic macromolecule such as DNA. The action is indirect when ionizing particles interact with a water molecule, thus resulting in the creation of ions and reactive free radicals that eventually produce toxic substances that can create biologic damage. (Radiobiology and Radiation Protection: Mosby’s Radiographic Instructional Series, St. Louis, 1999, Mosby)

16. Radiolysis of Water • Ionization of water molecules • Production of free radicals • Production of undesirable chemical reactions and biologic damage • Production of cell-damaging substances • Organic free radical formation Figure 07-04. Radiolysis of Water. The final result of the interaction of radiation with water is the formation of an ion pair (H+ and OH–) and two free radicals (H* and OH*).

17. Effects of Ionizing Radiation on DNA • Single-strand break • Ionizing radiation interacts with DNA macromolecule, transfers energy, and ruptures one of the molecule’s chemical bonds possibly severing one of the sugar-phosphate chain side rails (called a point mutation) Figure 07-06. A single-strand break in the ladderlike DNA molecular structure.

18. Effects of Ionizing Radiation on DNA (Cont.) • Double-strand break • Further exposure of the affected DNA macromolecule to ionizing radiation can lead to additional breaks in the sugar-phosphate molecular chain(s). • Breaks may also be repaired but are not repaired as easily as single-strand breaks. • If repair does not take place, further separation may occur in the DNA chains, threatening the life of the cell.

19. Effects of Ionizing Radiation on DNA (Cont.) Figure 07-07. A widely spaced double-strand break in the ladderlike DNA molecular structure.

20. Effects of Ionizing Radiation on DNA (Cont.) • Double-strand break in same • Rung of DNA • Result is a cleaved or broken chromosome with each new portion containing an unequal amount of genetic material • If damaged chromosome divides, each new daughter cell will receive an incorrect amount of genetic material culminating in the death or impaired functioning of the new daughter cell.

21. Effects of Ionizing Radiation on DNA (Cont.) Figure 07-08. A double-strand break in same rung of the DNA ladderlike molecular structure (A) causes complete chromosome breakage, resulting in a cleaved or broken chromosome (B).

22. Effects of Ionizing Radiation on DNA (Cont.) • Mutation • In general, interaction of high-energy radiation with a DNA molecule causes either a loss of or change in a nitrogenous base on the DNA chain. • Direct consequence of this damage is an alteration of the base sequence, a mutation. • May not be reversible and may cause acute consequences for the cell • If cell remains viable, incorrect genetic information will be transferred to one of the two daughter cells when the cell divides.

23. Effects of Ionizing Radiation on DNA (Cont.) Figure 07-09. Alteration of the nitrogen base sequence on the DNA chain caused by the action of high-energy radiation directly on a DNA molecule.

24. Effects of Ionizing Radiation on DNA (Cont.) • Covalent cross-links • Chemical unions created between atoms by the single sharing of one or more pairs of electrons • Initiated by high-energy radiation • Following irradiation, some molecules can fragment or change into small, spurlike molecules that become very interactive (“sticky”) when they themselves are exposed to radiation, causing these molecules to attach to other macromolecules or to other segments of the same macromolecule chain. • Can occur in many different patterns.

25. Effects of Ionizing Radiation on DNA (Cont.) Figure 07-10. Interstrandcovalent cross-link produced by high-energy radiation acting directly on a DNA molecule.

26. Effects of Ionizing Radiation on Chromosomes • Large-scale structural changes in a chromosome produced by ionizing radiation may be as grave for the cell as are radiation-induced changes in DNA. • Radiation-induced chromosome breaks in both somatic and reproductive cells • Chromosomal fragments • Chromosome anomalies • Chromosome aberrations • Chromatid aberrations • Structural changes in biologic tissue caused by ionizing radiation

27. Consequences to the Cell from Structural Changes in Biologic Tissue • Restitution Figure 07-11. The process of restitution, whereby the breaks rejoin in the original configuration with no visible damage. A, The chromatid break occurs because of a photon interaction. B, The fragment is fully separated from the rest of the chromatid. This same type of damage could occur to a chromosome if S phase had already occurred. C, The broken fragment has reattached in its original location through the action of repair enzymes.

28. Consequences to the Cell from Structural Changes in Biologic Tissue (Cont.) • Deletion Figure 07-12. The process of deletion, in which part of a chromosome is lost at the next cell division, thus creating an acentric fragment. A, The chromatid break results from a photon interaction. B, The fragment is fully separated from the rest of the chromatid. C, After the next DNA synthesis phase of the cell cycle (labeled S), the remainder of the chromosome has been replicated normally but with fragments missing from the two arms of the chromosome. The replicated fragment is acentric, a section of genetic material without a centromere.

29. Consequences to the Cell from Structural Changes in Biologic Tissue (Cont.) • Broken-end rearrangement producing grossly misshapen chromosomes Figure 07-13. The process of broken-end rearrangement may result in grossly misshapen chromatids. A, Two chromatid breaks occur in a single chromatid as a result of the interactions of two photons. B, The fragments from opposite ends unite before the DNA synthesis phase. C, The ends of the chromatid that are still attached to the centromere also unite and form a “ring” chromatid. D, Chromatid breaks occur in two different chromatids. E, The fragments are fully separated from the rest of their respective chromatids. F, The ends of the chromatids and the ends of the fragments have joined before DNA synthesis, thus forming a dicentric (two centromeres) and an acentric (no centromere) fragment. G, After DNA synthesis (labeled S), the chromatid is elongated but cannot split in two. The two centromeres are “bridged.” This type of chromatid damage leads to reproductive death of the cell (i.e., it cannot replicate or divide into two cells).

30. Consequences to the Cell from Structural Changes in Biologic Tissue (Cont.) • Broken-end rearrangement without visible damage to the chromatids Figure 07-14. The process of broken-end rearrangement may result in no visible damage to the chromatid, although the chromatid’s genetic material has been rearranged—a result that will drastically alter its function within the cell and probably lead to cell death or failure to replicate. This same type of damage could occur to a chromosome if S phase had already occurred. In this case, the cell may divide, but the genetic material in the daughter cells is compromised, and those cells may not function properly.

31. Target Theory: Master, or Key, Molecule • A master, or key, molecule that maintains normal cell function is believed to be present in every cell. Figure 07-15. A master, or key, molecule that maintains normal cell function is believed to be present in every cell. This molecule is vital to the survival of the cell and is presumed to be DNA. (Radiobiology and Radiation Protection: Mosby’s Radiographic Instructional Series, St. Louis, 1999, Mosby)

32. Target Theory • Master, or key, molecule is necessary for the survival of the cell. • Target theory may be used to explain cell death and nonfatal cell abnormalities caused by exposure to radiation. Figure 07-16. The target theory holds that the cell will die after exposure to ionizing radiation only if the master, or key, molecule (DNA) is inactivated in the process. A, An x-ray photon passes through the cell without interacting with the master molecule, which is located in the cell nucleus; no measurable effect results. B, An x-ray photon enters the nucleus and interacts with and inactivates the master molecule; the cell dies as a result.

33. Cellular Effects of Irradiation • Ionizing radiation can adversely affect the cell. • Damage to the cell’s nucleus reveals itself in one of the following ways • Instant death • Reproductive death • Apoptosis, or programmed cell death (interphase death) • Mitotic, or genetic, death

34. Survival Curves for Mammalian Cells • Cells vary in their radiosensitivity. • Cell survival curve is a classic method of displaying the sensitivity of a particular type of cell to radiation. • Curve is constructed from data obtained by a series of experiments. Figure 07-17. Cell survival curves for the same cell line irradiated with both low- and high-linear energy transfer (LET) radiation. With low-LET radiation, a “shoulder” to the curve at lower doses indicates the cell’s ability to repair some damage at low doses. High-LET radiation typically has no shoulder, thus indicating that little or no repair takes place.

35. Cell Radiosensitivity • Cell maturity and specialization • The human body is composed of different types of cells and tissues, which vary in their degree of radiosensitivity.

36. Cell Radiosensitivity (Cont.) • Amount of radiation energy transferred to biologic tissue • Plays a major role in determining the amount of biologic response • As LET increases, the ability of the radiation to cause biologic effects also generally increases until it reaches a maximal value. • LET can influence cell radiosensitivity.

37. Cell Radiosensitivity (Cont.) • Oxygen enhancement effects • Oxygen enhances the effects of ionizing radiation on biologic tissue by increasing tissue radiosensitivity. • During diagnostic imaging procedures, fully oxygenated human tissues are exposed to x-radiation or gamma radiation. • In radiotherapy, when radiation is used to treat certain types of cancerous tumors, high-pressure (hyperbaric) oxygen has sometimes been used in conjunction with it to increase tumor radiosensitivity.

38. Cell Radiosensitivity (Cont.) • Law of Bergoiné and Tribondeau • Observed the effects of ionizing radiation on testicular germ cells of rabbits they had exposed to x-rays • Established that radiosensitivity was a function of the metabolic state of the cell receiving the exposure • States that the radiosensitivity of cells is directly proportional to their reproductive activity and inversely proportional to their degree of differentiation

39. Cell Radiosensitivity (Cont.) • Law of Bergoiné and Tribondeau (Cont.) • Law was originally applied only to germ cells; it is actually true for all types of cells in the human body. • The most pronounced radiation effects occur in cells having the least maturity and specialization or differentiation, the greatest reproductive activity, and the longest mitotic phases.

40. Cell Radiosensitivity (Cont.) • Effects of ionizing radiation on human cells • Equal doses of ionizing radiation produce different degrees of damage in different kinds of human cells because of differences in cell radiosensitivity

41. Cell Radiosensitivity (Cont.) • Effects of ionizing radiation on human cells (Cont.) • Blood cells • Hematologic depression • Depletion of immature blood cells • Repopulation after a period of recovery • Effects on stem cells of the hematopoietic system • Whole-body doses in excess of 5 gyt • Effects of ionizing radiation on lymphocytes • Effects of ionizing radiation on neutrophils

42. Cell Radiosensitivity (Cont.) • Effects of ionizing radiation on human cells (Cont.) • Blood cells (Cont.) • Effects of ionizing radiation on granulocytes • Effects of ionizing radiation on thrombocytes (platelets) • Radiation exposure during diagnostic imaging procedures • Monitoring of patients undergoing radiation therapy treatment • Occupational radiation exposure monitoring

43. Cell Radiosensitivity (Cont.) • Effects of ionizing radiation on human cells (Cont.) • Epithelial tissue • Muscle tissue • Nervous tissue • Nerve tissue in the human adult • Nerve tissue in the embryo-fetus • Reproductive cells • Spermatogonia • Ova Figure 07-18. A Nerve Cell (Neuron). Nerve cells relay messages to and from the brain. A message enters a nerve cell through its dendrites, passes through the cell body, and exits the cell through the axon, which transmits the message across a synapse, the communication area leading to the next nerve cell in the chain.