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Home , Absorbed dose, Radiation damage, Radiobiology

8/29/2012

Outline

Introduction to • Ionizing Radiation Biology • Development of radiobiological damage • cycle • Cell survival curves

Survey of Clinical Radiation Oncology • Tissue response and fractionation

Lecture 2

Indications for Radiation biology • Radiation therapy may be used to treat almost every type • Radiation biology is the study of the action of of solid tumor, including of the brain, breast, on living organisms cervix, larynx, lung, pancreas, prostate, skin, spine, stomach, uterus, or soft tissue sarcomas • The action is very complex, involving physics, chemistry, and biology • Radiation can also be used to treat leukemia and lymphoma (cancers of the blood-forming cells and – Different types of ionizing radiation lymphatic system, respectively) – Energy absorption at the atomic and molecular level leads to biological damage • Radiation dose to each site depends on a number of – Repair of damage in living organisms factors: the type of and whether there are tissues • Basic principles are used in radiation therapy with and organs nearby that may be damaged by radiation the objective to treat cancer with minimal damage • Palliative radiation therapy also can be given to help to the normal tissues reduce symptoms such as pain from cancer that has spread to the bones or other parts of the body

Development of radiobiological damage Characteristic time scales

• The physical event of absorption occurs over about 10-15 seconds • The biologic lifetime of the free is on the order of 10-10 - 10-9 seconds (10-5 seconds in the presence of air) • The expression of may take up to days to months • The expression of carcinogenesis may take years or generations

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Development of radiobiological damage Absorption of radiation

Incident radiation • Biological systems are very sensitive to radiation • Absorption of 4 Gy in water produces the rise in Radiation absorption temperature ~10-3 oC (~67 cal in 70-kg person) Excitation and • Whole body dose of 4 Gy given to is lethal in 50% of cases (LD50) Free radical formation • The potency of radiation is in its concentration Brakeage of chemical bonds and the damage done to the genetic material of each cell Biological effects

Types of ionizing Types of ionizing radiations • Electromagnetic radiations • If radiation is absorbed in biologic material, and excitations occur in a pattern that depends on the – X-rays and Gamma-rays type of radiation involved • Particulate radiations • Depending on how far the primary ionization events – , , a-particles, heavy charged are separated in space, radiation is characterized as particles sparsely ionizing (x-rays) or densely ionizing (a- – particles) • All charged particles: directly ionizing radiation • Heavier particles with larger charge produce higher ionization density • X and g-rays, as well as neutrons – indirectly • For a given particle type, the density of ionization ionizing radiation decreases as the energy (and velocity) goes up

Linear Energy Transfer

• Linear energy transfer (LET) is the energy transferred per unit length of the track • The special unit usually used for this quantity is keV/mm of unit density material • It is an average quantity, typically track averaged

• The method of averaging makes little difference for x- LET  dE / dl rays or for mono-energetic charged particles, but the track average and energy average are different for neutrons

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Relative Biological Effectiveness Relative Biological Effectiveness • The amount or quantity of radiation is expressed in The RBE terms of the increases as • Equal doses of different types of radiation do not, the dose is decreased however, produce equal biologic effects: 1 Gy of neutrons produces a greater biologic effect than 1 Gy of x-rays due to the difference in the pattern of energy deposition at the microscopic level • The relative biologic effectiveness (RBE) of some test radiation (r) compared with 250 kV x-rays is • Because the x-ray and survival curves have different shapes defined by the ratio D250/Dr where D250 and Dr are, the resultant RBE depends on the level of biologic damage chosen respectively, the doses of x-rays and the test radiation • The RBE for a fractionated regimen with neutrons is greater than for required for equal biological effect a single exposure (because the RBE is larger for smaller doses)

Biological effect Classification of • Radiation damage to mammalian cells can • The biological effect is expressed in cell operationally be divided into three categories: killing, or cell transformation (carcinogenesis – Lethal damage, which is irreversible and irreparable and ) and, by definition, leads irrevocably to cell death; • The primary target of radiation is DNA – Potentially lethal damage (PLD), the component of molecule, suffering breaks in chemical bonds radiation damage that can be modified by post- irradiation environmental conditions; and • Depending on the extent of the damage, it can – Sublethal damage (SLD), which under normal be repaired through several repair mechanisms circumstances can be repaired in hours unless in place in a living organism additional sublethal damage is added (e.g., from a second dose of radiation)

The structure of DNA The structure of DNA • DNA molecule has many deoxyribo-nucleotides (bases) linked in a chain- like arrangement • Bases are held by bonds and are paired complimentary: – adenine with thymine – cytosine with guanine • Each half constitutes a template for reconstruction • The molecule is composed of two anti- parallel helices, twisted like a ladder of the other half • During cell division each strand is self- replicated resulting in identical molecules

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DNA strand breaks Chromosomes • Single-strand breaks are of little • DNA molecules carry the biologic consequence because they genetic information are repaired readily using the • Chromosome is an opposite strand as a template organized structure of DNA • Double-strand breaks are believed and DNA-bound proteins to be the most important lesions (serve to package the DNA produced in chromosomes by and control its functions) radiation; the interaction of two • Chromosomes are located double-strand breaks may result in mostly in cell nucleus (some cell killing, carcinogenesis, or amount is in mitochondria)

Chromosome aberrations Radiation-induced aberrations • Damage to DNA may result in lethal damage to the cell • The cell will attempt to repair the damage • Efforts to repair the damage may result in chromosome aberrations • Depending on the types of aberration they can be lethal or non-lethal, but result in mutations which can be perpetuated in subsequent cellular divisions Lethal aberrations include dicentrics (A), rings (B), and anaphase bridges (C)

Radiation-induced aberrations Mutations • If occur in the germ cells (sperm and ova) they A: Symmetric translocation: can be passed on as genetic abnormalities in radiation produces breaks in offspring two different pre-replication chromosomes. The broken • If they occur in the somatic cells (the cells that pieces are exchanged make up an organism) they can lead to the between the two development of including cancer - this is chromosomes, and the called carcinogenesis “sticky” ends rejoin. • There are genes called oncogenes that affect B: Deletion: radiation produces cancer incidence two breaks in the same arm • If an inhibitory oncogene is lost due to a deletion Symmetric translocations and small of the same chromosome the patient is at higher risk for cancer formation deletions are nonlethal

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Direct and indirect actions • In direct action, a secondary Free radicals resulting from absorption • A free radical is an atom or molecule carrying an unpaired of an x-ray interacts with orbital electron in the outer shell. This state is associated the DNA to produce an effect with a high degree of chemical reactivity • In indirect action, the secondary • Since 80% of a cell is composed of water, as a result of the electron interacts with, for interaction with a photon or a charged particle, the water example, a water molecule to molecule may become ionized: produce a (OH-),   which in turn produces the H2O  H2O  e damage to the DNA + -10 • H2O is an ion radical with a lifetime of ~10 s; it decays • The DNA helix has a diameter of to form highly reactive hydroxyl free radical OH  ~ 2 nm; free radicals produced in H O  H O  H O  OH  a cylinder with a diameter ~ 4 nm 2 2 3 can affect the DNA • About 2/3 of the x-ray damage to DNA in mammalian -3 • Indirect action is dominant for cells is caused by the hydroxyl radical (lifetime of ~10 s) sparsely ionizing radiation (x-rays)

The cell cycle Variation of • M - mitosis, identifiable by with cell age in the mitotic cycle microscopy and the most constant time (~ 1 hr) • Cells are most sensitive at or close to M (mitosis) • S - DNA synthesis phase • G2 phase is usually as sensitive as M phase • G1 - the first gap in activity, • Resistance is usually greatest in the latter part between mitosis and the S of S phase due to repairs that are more likely to phase (most variable length) occur after the DNA has replicated • G2 - the second gap in activity, between S phase • If G1 phase has an appreciable length, a resistant and the next mitosis period is evident early in G1, followed by a • If the cells stop progressing sensitive period toward the end of G1 through the cycle (if they

are arrested) they are in G0

Cell cycle times Molecular checkpoint genes

• Fast-growing cells in culture and some • Cell-cycle progression is controlled by a family of cells in self-renewing tissues have total molecular checkpoint genes cell-cycle time length of 10 hours • Their function is to ensure the • Stem cells in resting normal tissue, such as correct order of cell-cycle events • The genes involved in radiation skin, have a cell-cycle time of 10 days effects halt cells in G2, so that an • It usually is found that the malignant cells inventory of chromosome damage can be taken, and repair have the shorter cycle time than the initiated and completed, before normal-tissue cells the mitosis is attempted • Cells that lack checkpoint genes are sensitive to radiation-induced cell killing, and carcinogenesis

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Mechanisms of cell death after Molecular checkpoint genes irradiation • The main target of radiation is cell‟s DNA; single breaks are often reparable, double breaks lethal • Mitotic death – cells die attempting to divide, primarily due to asymmetric chromosome aberrations; most common mechanism • – programmed cell death; characterized by a predefined sequence of events resulting in • DNA damage repair involves activation of a group of cell separation in apoptotic bodies highly interrelated signaling pathways; two major groups: • Bystander effect – cells directly affected by sensors and effectors radiation release cytotoxic molecules inducing • As many as ~700 proteins may be involved death in neighboring cells

Cell survival curves Cell survival curves • A cell survival curve describes the relationship between the radiation dose and the proportion of cells that survive • They usually are presented in the form with dose plotted on a linear scale and surviving fraction on a logarithmic scale • Straight-line dependence means that the surviving fraction is an • Survival curve for HeLa cells in culture exposed to x-rays exponential function of dose • All mammalian cells, normal or malignant, regardless of their • At higher doses the curve bends species of origin, exhibit similar x-ray survival curves

Cell survival curve: Cell survival curve parameters • D – initial slope (the dose multi-target model Multi-target model 1 D / D n required to reduce the fraction • Multi-target single hit model: assume the cell has n S 1 (1 e 0 ) of surviving cells to 37% of its targets to be „hit‟ for the cell to not survive previous value); D0 – final slope • Probability of each „hit‟ not being successful is e-D/D0 • Dq – quasi-threshold, the dose at which the straight portion of the • Probability of each „hit‟ being successful is 1-e-D/D0 survival curve, extrapolated • Probability of all n targets within a cell to be „hit‟ is backward, cuts the dose axis D / D0 n drawn through a survival (1 e ) fraction of unity • The probability of survival of cell containing n • n – extrapolation number targets: N • Radiosensitive cells are S  1 (1 eD/ D0 )n characterized by curves with N0 steep slope D0 and/or small shoulder (low n)

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Survival curves and LQ model Cell survival curve parameters • Linear-quadratic (LQ) Multi-target model D / D0 n model assumes there are S 1 (1 e ) two components to cell for D  D killing, only two 0 adjustable parameters D / D S  ne 0 • No final straight portion that is observed • Setting D=0, find n – the experimentally number of targets • An adequate • To find Dq, set S=1 • Relationship between n representation of the data up to doses used as daily and D : Dq q N 2 S   eaDD fractions in clinical 1 neDq / D0 , D  D lnn radiotherapy q 0 N0

Survival curves and LQ model a/ ratios 2 S  eaDD • If the dose-response relationship is adequately represented by LQ-model: 2 Two breaks due to aDD the same event S ~ e • The dose at which aD=D2, or D= a/ • The a/ ratios can be inferred from multi-fraction experiments • The value of the ratio tends to be – larger (~10 Gy) for tumors and early-responding tissues Two breaks due to – lower (~2 Gy) for late-responding tissues two separate events

Repair of sub-lethal damage Fractionation

• In the presence of repair protracted exposure (LDR) • The main objective mechanisms sublesions may of fractionation is be eliminated before the sparing of the normal next hit arrives - dose rate becomes relevant tissues • As the dose rate decreases • Using LQ model can acute exposure (HDR) the quadratic term (D2) potentially find an becomes smaller equivalent treatment • At very low dose rates only the linear term, aD, remains

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The four Rs of Equivalent treatment • Fractionation of the radiation dose typically • To find biologically equivalent treatments, one would produces better tumor control for a given level of use the following expression: normal-tissue toxicity than a single large dose

2 t 2 t1 • Radiobiological basis for fractionations (4 Rs): a D  qD   a1D1  1q1D1  Tpot Tpot,1 – Repair of sublethal damage in normal tissues • The dose-rate function quantifying sublesion damage – Reassortment of cells within the cell cycle move repair that occurs between events q(t) tumor cells to more sensitive phase – Repopulation of normal tissue cells; however too long • Tpot – potential doubling time characterizes cell kinetics treatment time can lead to proliferation • This equation must be applied separately to early- – Reoxygenation of tumor cells as tumor shrinks (e.g., tumor) and late-responding tissues • Prolongation of treatment spares early reactions

Early and late responding tissues Early effects • Early (acute) effects result from death of a large • Rapidly dividing self-renewing tissues number cells and occur within a few days or weeks respond early to the effects of radiation; of irradiation in tissues with a rapid rate of turnover. examples: skin, intestinal epithelium, bone- • Examples: the epidermal layer of the skin, marrow gastrointestinal epithelium, and hematopoietic • Late-responding tissues: spinal cord, lung, system kidney • The time of on•set of early reactions correlates with • Early or late radiation response reflects the relatively short span of the mature different cell turnover rates functional cells • Acute damage is repaired rapidly and may be completely reversible

Late effects Dose rate effect • Late effects occur predominantly in slow- • LDR – 0.4-2 Gy/h proliferating tissues, and appear after a delay of • HDR – over 12 Gy/h (20 months or years from irradiation cGy/min) • Examples: tissues of the lung, kidney, heart, liver, • EBRT – 100-500 cGy/min and • Dose rate effect results • The time of on•set of late reactions correlates with primary from repair of sublethal damage; the relatively long life span of the mature repopulation may play a role functional cells for treatment times >1-2 days • Late damage may improve but is never completely • Most normal tissues show repaired considerable sparing as the dose rate reduces Kogel, et al., Basic clinical radiobiology

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The Law of Bergonie and Radiosensitivity of cancer cells Tribondeau • Highly radiosensitive cancer cells are rapidly • Formulated in 1906: “X-rays are more killed by modest doses of radiation. These include effective on cells which have a greater leukemia, most lymphomas, some sarcomas reproductive activity” (connective tissue cancers), and germ cell tumors • Now refined to the more rapidly responding • The majority of epithelial cancers (carcinomas) a population of cells the more have only moderate radiosensitivity radioresponsive they are • Some types of cancer, such as renal cell cancer and melanoma, are notably radioresistant, with much higher doses required to produce a radical cure than may be safe in clinical practice

Radiosensitivity of cancer cells Cell radiosensitivity

• There is a number of factors Summary of D 0 that influence cell radiation values for cells sensitivity even in vitro of human origin (position in cell cycle, genetic (in vitro studies) abnormalities, environment) • The mechanism of cell death can be dominated by apoptosis or mitosis; most • Cells from human tumors have a wide range of cells are in-between radiation sensitivities • Cells in mitosis show the • In general, squamous cell carcinoma cells are more same sensitivity resistant than sarcoma cells

Dose-response relationships Genetic control of radiosensitivity

• A number of genes is involved in determining • Curves are typically radiosensitivity of mammalian cells sigmoid (S) -shaped for • In many cases this sensitivity has been related both tumor and normal greatly reduced ability to repair double-strand cells DNA breaks • Therapeutic ratio (index): • Some of the inherited human syndromes are tumor response for a associated with high radiosensitivity fixed level of a normal – telangiectasia (AT), Down's syndrome, etc. tissue damage

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Chemical factors influencing Therapeutic ratio cell response • The time factor is often employed to • Radioprotectors – chemicals that reduce the manipulate the TR biological effects of radiation (hyperfractionation for • Mechanisms of action: sparing of late- responding normal – Scavenging of free radicals tissues) – Facilitation of direct chemical repair at sites of DNA • Addition of a drug, a damage - Tissue max chemotherapy agent, or • Amifostine (trade name Ethyol) is the only FDA tolerance a radio-sensitizer may approved drug for use in radiation therapy, e.g. improve the TR head and neck cancer treatments

Chemical factors influencing effect cell response • Oxygen makes the damage produced by free • Radiosensitizers are employed to increase the radicals permanent; the damage can be repaired in the absence of oxygen lethal effects of radiation in tumor cells • Oxygen enhancement ratio OER=3 can be • Have to show the differential affect between achieved for x-rays; OER=1.6 for neutrons; only 1 tumor and normal cells for a-particles • Two types have found practical use in radiotherapy: Only 3 mm Hg, or about 0.5% of oxygen is – Halogenated pyrimidines: more drug is incorporated required to achieve a in fast-proliferating tumor cells relative radiosensitivity – Hypoxic cell sensitizers: hypoxic cells occur only in halfway between anoxia tumors and full oxygenation

Tissue response to radiation Tumor oxygenation damage • Oxygen can diffuse • Cells of normal tissues are not independent at only about 70 mm • For an tissue to function properly its organization from the blood vessel and the number of cells have to be at a certain level • Solid tumors often • Typically there is no effect after small doses outgrow their blood supply and become • The response to radiation damage is governed by: hypoxic – The inherent cellular radiosensitivity and position in the • Cells not receiving cell cycle at the time of radiation oxygen and nutrients – The kinetics of the tissue become necrotic – The way cells are organized in that tissue

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The volume effect in radiotherapy References • Generally, the total dose that can be tolerated • E.J. Hall., A. Giaccia, Radiobiology for the Radiologist depends on the volume of irradiated tissue • H.E. Johns, J.R. Cunningham, The physics of • It also depend on the structural organization of the tissue in terms of the spatial arrangement of • Kogel, et al., Basic clinical radiobiology functional sub-units, FSU‟s (the nephron in the • L.M. Coia, D.J. Moylan, Introduction to clinical radiation kidney, the lobule in the liver): oncology – If FSUs are arranged in a series, elimination of any unit is • Physics, AAPM Summer School, July critical to the organ function (example: spinal cord) 2005, Editors: B.R. Thomadsen, Mark Rivard, and Wayne – If FSUs are arranged in parallel, elimination of a single Butler unit is not critical to the organ function (kidney and lung) • National Cancer Institute, http://www.cancer.gov • Canadian Nuclear Association, http://www.cna.ca

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