Chapter 14: Basic Radiobiology Set of 88 slides based on the chapter authored by N. Suntharalingam, E.B. Podgorsak, J.H. Hendry of the IAEA publication: Radiation Oncology Physics: A Handbook for Teachers and Students Objective: To familiarize the student with the basic principles of radiobiology. Slide set prepared in 2006 by E.B. Podgorsak (Montreal, McGill University) Comments to S. Vatnitsky: dosimetry@iaea.org IAEA International Atomic Energy Agency CHAPTER 14. TABLE OF CONTENTS 14.1. Introduction 14.2. Classification of radiations in radiobiology 14.3. Cell cycle and cell death 14.4. Irradiation of cells 14.5. Type of radiation damage 14.6. Cell survival curves 14.7. Dose response curves 14.8. Measurement of radiation damage in tissue 14.9. Normal and tumour cells: Therapeutic ratio 14.10. Oxygen effect 14.11. Relative biological effectiveness 14.12. Dose rate and fractionation 14.13. Radioprotectors and radiosensitizers IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.
14.1 INTRODUCTION � Radiobiology is a branch of science which combines the basic principles of physics and biology and is concerned with the action of ionizing radiation on biological tissues and living organisms. � The study of basic radiobiological mechanisms deals with biological effects produced by energy absorption in small volumes corresponding to single cells or parts of cells. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.1 Slide 1 14.1 INTRODUCTION � All living entities are made up of protoplasm, which consists if inorganic and organic compounds dissolved or suspended in water. � The smallest unit of protoplasm capable of independent existence is the cell, the basic microscopic unit of all living organisms. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.1 Slide 2
14.1 INTRODUCTION � A group of cells that together perform one or more functions is referred to as tissue. � A group of tissues that together perform one or more functions is called an organ. � A group of organs that perform one or more functions is an organ system or an organism. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.1 Slide 3 14.1 INTRODUCTION � Cells contain: • Inorganic compounds (water and minerals) • Organic compounds (proteins, carbohydrates, nucleic acids, lipids) � The two main constituents of a cell are the cytoplasm and the nucleus: • Cytoplasm supports all metabolic functions within a cell. • Nucleus contains the genetic information (DNA). IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.1 Slide 4
14.1 INTRODUCTION � Human cells are either somatic cells or germ cells. The germ cells are either a sperm or an egg, all other human cells are called the somatic cells. � Cells propagate through division: • Division of somatic cells is called mitosis and results in two genetically identical daughter cells. • Division of germ cells is called meiosis and involves two fissions of the nucleus giving rise to four sex cells, each possessing half the number of chromosomes of the original germ cell. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.1 Slide 5 14.1 INTRODUCTION � When a somatic cell divides, two cells are produced, each carrying a chromosome complement identical to that of the original cell. � The new cells themselves may undergo further division, and the process continues producing a large number of progeny. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.1 Slide 6
14.1 INTRODUCTION � A chromosome is a microscopic, threadlike part of a cell that carries hereditary information in the form of genes. � Every species has a characteristic number of chromosomes; humans have 23 pairs (22 pairs are non-sex chromosomes and 1 pair is sex chromosome). � A gene is a unit of heredity that occupies a fixed position on a chromosome. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.1 Slide 7 14.1 INTRODUCTION � Somatic cells are classified as: • Stem cells, which exists to self-perpetuate and produce cells for a differentiated cell population. • Transit cells, which are cells in movement to another population. • Mature cells, which are fully differentiated and do not exhibit mitotic activity. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.1 Slide 8
14.2 CLASSIFICATION OF RADIATIONS IN RADIOBIOLOGY � Radiation is classified into two main categories: • Non-ionizing radiation (cannot ionize matter) • Ionizing radiation (can ionize matter) � Ionizing radiation contains two major categories • Directly ionizing radiation (charged particles) electrons, protons, alpha particles, heavy ions • Indirectly ionizing radiation (neutral particles) photons (x rays, gamma rays), neutrons IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.2 Slide 1 14.2 CLASSIFICATION OF RADIATIONS IN RADIOBIOLOGY � In radiobiology and radiation protection the linear energy transfer (LET) is used for defining the quality of an ionizing radiation beam. � In contrast to the stopping power, which focuses attention on the energy loss by a charged particle moving through a medium, the LET focuses attention on the linear rate of energy absorption by the absorbing medium as the charged particle traverses the medium. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.2 Slide 2
14.2 CLASSIFICATION OF RADIATIONS IN RADIOBIOLOGY � The ICRU defines the LET as follows: “LET of charged particles in a medium is the quotient dE / d � where dE is the average energy locally imparted to the medium by a charged particle of d � specified energy in traversing a distance of .” IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.2 Slide 3 14.2 CLASSIFICATION OF RADIATIONS IN RADIOBIOLOGY � In contrast to the stopping power, which has a typical unit μ m of MeV/cm, the unit reserved for the LET is keV/ . � The energy average is obtained by dividing the particle track into equal energy increments and averaging the length of track over which these energy increments are deposited. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.2 Slide 4
14.2 CLASSIFICATION OF RADIATIONS IN RADIOBIOLOGY � Typical LET values for commonly used radiations are: μ m Radiation LET (keV/ ) • 250 kVp X rays 2 � • Cobalt-60 rays 0.3 • 3 MeV X rays 0.3 • 1 MeV electrons 0.25 � LET values for other, less common radiations are: μ m Radiation LET (keV/ ) • 14 MeV neutrons 12 • Heavy charged particles 100 - 200 • 1 keV electrons 12.3 • 10 keV electrons 2.3 IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.2 Slide 5 14.3 CELL CYCLE AND CELL DEATH � The cell proliferation cycle is defined by two time periods: • Mitosis M, where division takes place. • The period of DNA synthesis S. � The S and M portions of the cell cycle are separated by two periods (gaps) G 1 and G 2 when, respectively • DNA has not yet been synthesized. • Has been synthesized but other metabolic processes are taking place. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.3 Slide 1
14.3 CELL CYCLE AND CELL DEATH � The time between successive divisions (mitoses) is called the cell cycle time. � The cell cycle time for mammalian cells is of the order of 10 - 20 hours: • The S phase is usually in the range of 6 - 8 hours. • The M phase is less than 1 hour. • G 2 is in the range of 2 - 4 hours. • G 1 is in the range of 1 - 8 hours. The stages of the mitotic cell cycle M = mitosis S = DNA synthesis G 1 and G 2 = gaps IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.3 Slide 2 14.3 CELL CYCLE AND CELL DEATH � The cell cycle time for stem cells in certain tissues is up to 10 days. � In general, cells are most radio-sensitive in the M and G 2 phases, and most radio-resistant in the late S phase. � The cell cycle time of malignant cells is shorter than that of some normal tissue cells, but during regeneration after injury normal cells can proliferate faster. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.3 Slide 3
14.3 CELL CYCLE AND CELL DEATH � Cell death of non-proliferating (static) cells is defined as the loss of a specific function. � Cell death for stem cells and other cells capable of many divisions is defined as the loss of reproductive integrity (reproductive death). IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.3 Slide 4 14.4 IRRADIATION OF CELLS � When cells are exposed to ionizing radiation: • First, the standard physical effects between radiation and the atoms or molecules of the cells occur. • The possible biological damage to cell functions follows. � The biological effects of radiation result mainly from damage to the DNA; however, there are also other sites within the cell that, when damaged, may lead to cell death. IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 14.4 Slide 1
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