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Home , Ionizing radiation, Radiation damage, Radiation therapy, Radiobiology, Radiosensitivity

Outline Introduction to • Terminology Biology • Development of radiobiological damage • cycle • Cell survival curves Survey of Clinical Radiation • Radiobiological damage: oxygenation, fractionation, and 4 R’s of Lecture 2 • Cell and tissue

Radiation biology Types of ionizing • Radiation biology is the study of the action of • Electromagnetic radiations on living organisms – X-rays and Gamma-rays • The action is very complex, involving physics, chemistry, and biology • Particulate radiations – Different types of ionizing radiation – , , a-particles, heavy charged – absorption at the atomic and molecular level particles to biological damage – – Repair of damage in living organisms • All charged particles: directly ionizing radiation • Basic principles are used in radiation with the objective to treat with minimal damage • X and g-rays, as well as neutrons – indirectly to the normal tissues ionizing radiation

Types of ionizing radiations • If radiation is absorbed in biologic material, and excitations occur in a pattern that depends on the • Linear energy transfer (LET) is the energy type of radiation involved transferred per unit length of the track • Depending on how far the primary events • The special unit usually used for this quantity is are separated in space, radiation is characterized as sparsely ionizing (x-rays) or densely ionizing (a- keV/mm of unit density material particles) • It is an average quantity, typically track averaged • Heavier particles with larger charge produce higher ionization density • For a given particle type, the density of ionization LET  dE/ dl decreases as the energy (and velocity) goes up

1 Linear Energy Transfer Relative Biological Effectiveness • Equal doses of different types of radiation do not 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 defined D RBE  250kV • The method of averaging makes little difference for x- Dr

rays or for mono-energetic charged particles, but the track • D250kV and Dr are the doses of x-rays and the test average and energy average are different for neutrons radiation required for equal biological effect

Relative Biological Effectiveness Development of radiobiological damage

• Because the x-ray and survival curves have different shapes the resultant RBE depends on the level of biologic damage chosen • The RBE for a fractionated regimen with neutrons is greater than for a single exposure (because the RBE is larger for smaller doses)

Development of radiobiological Characteristic time scales damage

• The physical event of absorption occurs over Incident radiation about 10-15 seconds • The biologic lifetime of the free is on Radiation absorption the order of 10-10 - 10-9 seconds (10-5 seconds in the presence of air) Excitation and ionization • The expression of may take up to Free radical formation days to months • The expression of may take Brakeage of chemical bonds years or generations Biological effects

2 Absorption of radiation Biological effect

• Biological systems are very sensitive to radiation • The biological effect is expressed in cell • Absorption of 4 Gy in water produces the rise in killing, or cell transformation (carcinogenesis temperature ~10-3 oC (~67 cal in 70-kg person) and ) • Whole body dose of 4 Gy given to is • The primary target of radiation is DNA lethal in 50% of cases (LD50) , suffering breaks in chemical bonds • The potency of radiation is in its concentration • Depending on the extent of the damage, it can be repaired through several repair mechanisms and the damage done to the genetic material of in place in a living organism each cell

The structure of DNA DNA as a target • DNA molecule has many deoxyribo-nucleotides (bases) • Single-strand breaks are of little linked in a chain-like arrangement biologic consequence because they • Bases are held by bonds are repaired readily using the and are paired complimentary opposite strand as a template (adenine with thymine; cytosine • Double-strand breaks are believed with guanine) to be the most important lesions • Each half is a template for produced in chromosomes by reconstruction of the other half radiation; the interaction of two • During cell division each strand is self-replicated double-strand breaks may result in resulting in identical cell killing, carcinogenesis, or

Direct and indirect actions • In direct action, a secondary Free radicals resulting from absorption • A free radical is an 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 , 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 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)

3 Chromosomes Chromosome aberrations • DNA molecules carry the • Damage to DNA may result in lethal damage genetic information or repair efforts modulated by specific • Chromosome is an enzymes may result in mutations which can be organized structure of DNA perpetuated in subsequent cellular divisions and DNA-bound proteins (serve to package the DNA • Mutations are mostly characterized by and control its functions) deletions (where part of the genetic message is lost) or translocations where a segment of a • Chromosomes are located chromosome is lost from its proper location mostly in cell nucleus (some and recombines with another chromosome amount is in mitochondria)

Radiation-induced aberrations Radiation-induced aberrations

A: Symmetric translocation: radiation produces breaks in two different pre-replication chromosomes. The broken pieces are exchanged between the two chromosomes, and the “sticky” ends rejoin. B: Deletion: radiation produces two breaks in the same arm of the same chromosome Lethal aberrations include dicentrics (A), rings (B), and Symmetric translocations and small anaphase bridges (C) deletions are nonlethal

The Mutations • M - , identifiable by • If occur in the germ cells (sperm and ova) they microscopy and the can be passed on as genetic abnormalities in most constant time (~ 1 hr) offspring • S - DNA synthesis phase

• If they occur in the somatic cells (the cells that • G1 - the first gap in activity, make up an organism) they can to the between mitosis and the S development of including cancer - this is phase (most variable length)

called carcinogenesis • G2 - the second gap in • There are genes called that affect activity, between S phase cancer incidence and the next mitosis • If an inhibitory is lost due to a deletion • If the cells stop progressing the patient is at higher risk for cancer formation through the cycle (if they are arrested) they are in G0

4 Variation of radiosensitivity Molecular checkpoint genes with cell age in the mitotic cycle • Cell-cycle progression is • Cells are most sensitive at or close to M (mitosis) controlled by a family of molecular checkpoint genes • G2 phase is usually as sensitive as M phase • Their function is to ensure the • Resistance is usually greatest in the latter part correct order of cell-cycle events of S phase due to repairs that are more likely to • The genes involved in radiation occur after the DNA has replicated effects halt cells in G2, so that an inventory of chromosome • If G1 phase has an appreciable length, a resistant damage can be taken, and repair period is evident early in G1, followed by a initiated and completed, before the mitosis is attempted sensitive period toward the end of G1 • Cells that lack checkpoint genes are sensitive to radiation-induced cell killing, and carcinogenesis

Mechanisms of cell death after Cell survival curves • 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 cell separation in apoptotic bodies • A cell survival curve describes the relationship between the • Bystander effect – cells directly affected by radiation dose and the proportion of cells that survive radiation release cytotoxic molecules inducing • Usually presented in the form with dose plotted on a linear scale death in neighboring cells and surviving fraction on a log scale

Cell survival curve parameters Survival curves and LQ model • D – initial slope (the dose Multi-target model 1 required to reduce the fraction 2 D / D n aDD S ~1 (1 e 0 ) of surviving cells to 37% of its S ~ e Two breaks due to previous value); D0 – final slope the same event • Dq – quasi-threshold, the dose at which the straight portion of the survival curve, extrapolated backward, cuts the dose axis drawn through a survival fraction of unity Two breaks due to • n – extrapolation number two separate events • Radiosensitive cells are • Linear-quadratic model assumes there are two characterized by curves with components to cell killing, only two adjustable parameters steep slope D0 and/or small • No final straight portion that is observed experimentally, shoulder (low n) but an adequate representation of the data up to doses used as daily fractions in clinical radiotherapy

5 a/ ratios Repair of sub-lethal damage

• If the dose-response relationship is protracted exposure • In the presence of repair represented by LQ-model: mechanisms sublesions may 2 S ~ eaDD be eliminated before the next hit arrives - dose rate • The dose at which aD=D2, or D= a/ becomes relevant • The a/ ratios can be inferred from acute exposure • As the dose rate decreases multi-fraction experiments the quadratic term (D2) • The value of the ratio tends to be becomes smaller – larger (~10 Gy) for early-responding • At very low dose rates only tissues and tumors the linear term, aD, remains – lower (~2 Gy) for late-responding tissues

Fractionation Equivalent treatment

• Fractionation has a profound • To find biologically equivalent treatments use LQ model: effect on cell survival curves for low LET radiation (some S  exp(aD  D2 ) for high LET) aD  D2  aD  D 2  E • The main objective clinically is 1 1 sparing of the normal tissue by d giving it time to repair E /a  nd(1 ) sublethal damage a  • Typically normal tissue repair • Here d – dose per fraction, n – number of fractions mechanisms are much more • Should be evaluated separately for tumor and normal effective than those of cancer tissues cell’s

Oxygen effect Tumor oxygenation • makes the damage produced by free radicals permanent; the damage can be repaired in • Oxygen can diffuse the absence of oxygen at only about 70 mm • OER=3 can be from the blood vessel achieved for x-rays; OER=1.6 for neutrons; only 1 • Solid tumors often for a-particles outgrow their blood supply and become Only 3 mm Hg, or hypoxic about 0.5% of oxygen is • Cells not receiving required to achieve a oxygen and nutrients relative radiosensitivity become necrotic halfway between anoxia and full oxygenation

6 The four Rs of radiobiology Tissue response to radiation • Fractionation of the radiation dose typically damage produces better tumor control for a given level of • Cells of normal tissues are not independent normal-tissue toxicity than a single large dose • For an tissue to function properly its organization • Radiobiological basis for fractionations (4 Rs): and the number of cells have to be at a certain level – Repair of sublethal damage in normal tissues • Typically there is no effect after small doses – Reassortment of cells within the cell cycle move • The response to is governed by: tumor cells to more sensitive phase – The inherent cellular radiosensitivity and position in the – Repopulation of normal tissue cells; however too long cell cycle at the time of radiation treatment time can lead to proliferation – The kinetics of the tissue – Reoxygenation of tumor cells as tumor shrinks – The way cells are organized in that tissue • Prolongation of treatment spares early reactions

Response to radiation damage • In tissues with a rapid turnover rate, damage becomes evident Early and late responding tissues quickly • In tissues in which cells divide rarely, radiation damage to • Rapidly dividing self-renewing tissues cells may remain latent for a long period of time and be expressed very slowly respond early to the effects of radiation; • Radiation damage to cells that are already on the path to examples: skin, intestinal , - differentiation (and would not have divided many times marrow anyway) is of little consequence - they appear more radioresistant • Late-responding tissues: spinal cord, , • Stem cells appear more radiosensitive since loss of their kidney reproductive integrity results in loss of their potential • Early or late radiation response reflects descendants different cell turnover rates • At a cell level survival curves may be identical, but tissue radioresponse may be very different

Dose-response relationships Therapeutic ratio

• The time factor is often • Curves are typically employed to sigmoid (S) -shaped for manipulate the TR both tumor and normal (hyperfractionation for cells sparing of late- responding normal • Therapeutic ratio tissues) (index): tumor response • Addition of a drug, a for a fixed level of a - Tissue max agent, or tolerance normal tissue damage a radio-sensitizer may improve the TR

7 The volume effect in radiotherapy Radiosensitivity of specific tissues and organs • Generally, the total dose that can be tolerated • Each organ has established tolerance for whole depends on the volume of irradiated tissue and partial organ irradiation (volume fraction) • However, the spatial arrangement of functional • Organs are classified as: subunits (FSUs) in the tissue is critical – Class I - fatal or severe morbidity (, heart, brain, spinal cord, kidneys, ) – FSUs are arranged in a series. Elimination of any – Class II - moderate to mild morbidity (skin, esophagus, unit is critical to the organ function eye, bladder, rectum) – FSUs are arranged in parallel. Elimination of a – Class III - low morbidity (muscle, cartilage, breasts) single unit is not critical to the organ function

Indications for • Radiation therapy may be used to treat almost every type Radiosensitivity of cancer cells of solid tumor, including of the brain, breast, • Highly radiosensitive cancer cells are rapidly cervix, larynx, lung, pancreas, , skin, spine, stomach, uterus, or soft tissue killed by modest doses of radiation. These include , most , and germ cell tumors • Radiation can also be used to treat leukemia and (cancers of the blood-forming cells and • The majority of epithelial cancers () , respectively) have only moderate radiosensitivity • Radiation dose to each site depends on a number of • Some types of cancer, such as renal cell cancer factors: the type of cancer and whether there are tissues and , are notably radioresistant, with and organs nearby that may be damaged by radiation much higher doses required to produce a radical • Palliative radiation therapy also can be given to help cure than may be safe in clinical practice reduce symptoms such as pain from cancer that has spread to the or other parts of the body

Cell radiosensitivity References

• E.J. Hall., A. Giaccia, Radiobiology for the Radiologist Summary of D0 values for cells • H.E. Johns, J.R. Cunningham, The physics of of human origin • L.M. Coia, D.J. Moylan, Introduction to clinical radiation (in vitro studies) oncology • Ganderson, Tepper (Eds.), Clinical radiation oncology • National Cancer Institute, http://www.cancer.gov • Canadian Nuclear Association, http://www.cna.ca • Cells from human tumors have a wide range of radiation

sensitivities • In general, squamous cell cells are more resistant than cells (but - osteosarcoma is radioresistant)

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