Chapter 34 Principles of Radiobiology In 1906, two French scientists, Bergonie and Tribondeau, theorized and observed that radiosensitivity was a function of the metabolic state of the tissue being irradiated. There observations became the Law of Bergonie & Tribondeau

Law of Bergonie & Tribondeau Stem cells are radiosensitive; mature cells are radio-resistant. Younger tissues and organs are radiosensitive. Tissues with high metabolic activity are radiosenstive. High proliferation rate for cells and high growth rate for tissues result in increased radiosensitivity.

Law of Bergonie & Tribondeau Basically it states that radiosensitivity of living tissue varies with maturity and metabolism. In diagnostic imaging the law serves to remind us that a fetus is considerably more sensitive to radiation exposure than a child or mature adult.

Physical factors Affecting Radiosensitivity When tissue is irradiated, the response of the tissue is determined principally by the amount of energy deposited per unit of mass: the dose in Rads (Gy). Even under controlled conditions, the response to like exposure may be different.

Physical factors Affecting Radiosensitivity Physical property factors Linear energy transfer (LET) Relative Biological Effectiveness (RBE) Fractionation and Protraction

Linear Energy Transfer The rate which energy is transferred from ionization to soft tissue is the (LET). It is another method of expressing radiation quality and determining the value of the tissue weight factor used in radiation protection. It is expressed in the units of kiloelectron volts of energy transferred per micrometer in soft tissue.

Linear Energy Transfer The ability of ionizing radiation to produce a biologic response increases as the LET of the radiation increases. The LET of diagnostic x-rays is approximately 3 keV/µm.

Relative Biologic Effectiveness As the LET of the radiation increases, the ability to produce biologic damage also increases. This quantification is referred to as the Relative Biologic Effects (RBE). The RBE of diagnostic x-ray is 1. Radiations with a lower LET will have a RBE of less than 1. Radiations with a higher LET will have a RBE greater than 1.

The LET & RBE of Various Radiations Type of Radiation LET RBE 25 MV x-rays 0.2 0.8 60Co gamma rays 0.3 0.9 1MeV electrons Diagnostic X-ray 3.0 1.0 10 MeV protons 4.0 5.0 Fast Neutrons 50.0 10 5 MeV Alpha Particles 100.0 20

LET & RBE Graph As the LET increases, the RBE also increases but a maximum level is reached followed by a reduction due to overkill.

Fractionation & Protraction If the dose is administered over a long time rather than quickly, the effects of that dose will be less. If the time of irradiation is lengthened, a higher dose is required to produce the same effect. Dose protraction and fractionation cause less effect allowing time for intracellular repair and tissue recovery.

Protraction If we give an exposure of 600 rads at 300 rads/minute, the effects will be less than if the same exposure is given at 600 rads/ minute. This us called protraction.

Fractionation If that 600 rads is given at 150 rad per day over 4 days, the effects would be less than 600 rads given over 1 day. This is called fractionation.

Biologic Factors Affecting Radiosensitity Oxygen Effect: Tissue is more sensitive to radiation if the tissue is oxygenated or aerobic state than when irradiated in the anoxic ( w/o oxygen) or hypoxic state (low oxygen) state. The characteristic of tissue is described numerically as the Oxygen Enhancement Ratio. (OER)

Biologic Factors Affecting Radiosensitity Oxygen Enhancement Effect for diagnostic x-ray is full oxygenation. The OER is LET dependent. The OER for highest for low LET radiation having a maximum value of approximately 3, decreasing to 1 for high LET radiation.

Oxygen Enhancement Ratio The OER is high for low LET radiation and decreases as the LET increases.

Biologic Factors Affecting Radiosensitity Age affects the biologic structure’s radiosensitivity. Humans are most sensitive before birth. Sensitivity decreases until maturity. In old age, sensitivity increases again.

Biologic Factors Affecting Radiosensitity Recovery: If the dose of radiation is sufficient to kill the cell before its next division, interphase death will occur. If the dose is sub lethal, the cell may recover from the damage. Some cell types are more capable of recovery.

Biologic Factors Affecting Radiosensitity Recovery: At the whole body level, recovery is assisted by repopulation by the surviving cells. If the tissue or organ receives a sufficient dose, it will respond by shrinking in size. This is called atrophy. Atrophy happens because cells die, disintegrate and are carried away as waste.

Recovery Recovery = Intracellular Repair+ Repopulation Some chemical agents can modify radiation response. Radiosensitizers enhance the effects Radioprotectors reduce the effects

Radiosensitizers Agents that enhance the effects of radiation are radiosensitizers. Example include: Halogenated pyrimidines that become incorporated in the cell DNA and effectively double the effect of the radiation. Vitamin K Must be present at the time of irradiation.

Radioprotectors Radio protector agents exist but have not found any human application. They must be given in toxic levels to be effective. The protective agent can be worse than the radiation.

Hormesis There is a growing body of radiobiologic evidence that suggest that a little bit of radiation is good for you. Studies have shown that animals live longer lives when they receive low radiation doses.

Hormesis The prevailing explanation is that a little radiation stimulates hormonal and immune responses to toxic environmental agents. Regardless of Hormesis, we still practice ALARA as a known safe response to radiation safety.

Radiation Dose-Response Relationships Radiobiology is a relatively new science. Interest increased in the 1940’s with the advent of the atomic age. The object of nearly all of the research is the establishment of radiation dose-response relationships.

Radiation Dose-Response Relationships Radiation Dose-Response Relationships have two important functions. Designing therapeutic treatment routines for patients with cancer. Provide information on the effects of low dose irradiation.

Radiation Dose Response Relationship Characteristics Every exposure has two characteristics. It is either: Linear Threshold or Non-Threshold Non-Linear

Linear Dose-Response Relationship Radiation-induced cancer and genetic effects follow a linear, nonthreshold dose response relationship. Any exposure above zero is expected to cause some response. Exposure can also be a linear threshold type where the dose axis is greater than zero.

Linear Dose-Response Relationship Linear dose graphs have a straight line graph starting at zero for nonthreshold exposures or at a point greater than zero for threshold exposures.

Non-Linear Dose Response All other radiation dose response relationships are defined as non-linear. If the dose response curve starts at zero, it has a nonthreshold. The shape of the curve will determine the rate of response.

Non-Linear Dose Response Radiation death and skin effects of high dose fluoroscopy follow a sigmoid-type dose relationship. At exposure levels below where the graph threshold, no effect had been identified. The point where the curve stops bending up is the inflection point.

Non-linear Dose Response Above the inflection point, the incremental dose increase becomes less effective. Dose response graphs are used to discuss the type and degree of radiation injury.

Non-Linear Dose Response In diagnostic exposures it is most exclusively concerned with late effects and therefore with linear non-threshold dose response relationships. The principle interest in diagnostic responses to very low level exposures.

Linear Non-threshold Dose Response Since this cannot be done directly, the dose response is extrapolated from known high dose exposures using the linear response graphs.

Exposure to Diagnostic X-ray Diagnostic x-ray is usually and primarily concerned with the late effects of radiation exposure. The existence of radiation hormesis is highly controversial. Regardless of it’s existence, no human response has been observed following doses less than 10 rad (100 mGy).

Chapter 35 Molecular & Cellular Radiobiology When macromolecules are irradiated in vitro( outside the body) it take a considerable amount of radiation to produce a measurable effect. Irradiation in vivo (inside the body) in a living cell in solution, macromolecules are considerably more radiosensitive in their natural state.

Results of irradiation of macro-molecules. There are three major results of irradiation of macro-molecules in solution: A-Main chain Scission B- Cross linking C- Point lesions

Main Chain Scission Main-chain scission is the breakage of the backbone of the long chain macro-molecules. This results in many smaller molecules which still may be macro-molecules.

Main Chain Scission This not only changes the size of the molecule but the viscosity of the solution also increases. Measurement of the viscosity determines the degree of main chain scission.

Cross Linking B- Cross Linking Some macro-molecules have small spur like side structures extending off the main chain. Other produce the spurs as a result of irradiation.

Cross Linking These structures can behave as though they had a sticky substance on end. They can attach to other macro-molecules or another segment of the same molecule. Also increases viscosity of the solution.

Point Lesions Radiation interaction can also result in disruption of a single chemical bond. Such point lesions are not detectable but can result in minor modifications to the cell that can cause it to malfunction.

Point Lesions Radiation interaction can also result in disruption of a single chemical bond. Such point lesions are not detectable but can result in minor modifications to the cell that can cause it to malfunction.

Point Lesions At low radiation doses, point lesions are considered to be the cellular radiation damage resulting in the late effects observed at the whole body level.

Irradiation of Macro-molecules Laboratory experiments have shown that all these types of radiation effects on macro-molecules are reversible through intracellular repair and recovery. Radiation damage may result in cell death or late effects. DNA is the most radiosensitive molecule. It forms chromosomes and controls cell and human growth and development.

Normal & Radiation Damaged Chromosomes Terminal deletion Dicentrics Formation Ring formation

Radiation Responses of DNA Types of damage. One side rail severed. Both side rails severed Cross linking Rung breakage All are reversible

Point mutation A change or a loss of a base is called a point mutation. It destroys the triplet code and may not be reversible. It is a molecular lesion of the DNA. One of the daughter cells will receive incorrect genetic code.

Principle effects of DNA irradiation The principle effects are: Cell death Malignant disease Genetic damage The latter two conform to the linear, nonthreshold dose response relationship.

Radiolysis of Water Because the human body is an aqueous solution containing 80% water molecules, irradiation of water represents the principle radiation interaction with the body. When water is irradiated, it dissociates into other molecular products. This is referred to as radiolysis of water.

Radiolysis of Water Following ionization, a number of reactions can happen. The ion pair can rejoin into a stable water molecule. If they don’t rejoin, the negative ion (electron) can join with another water molecule.

Radiolysis of Water This results in a third type of ion. H2O + e-→HOH- HOH+ and HOH- are relatively unstable and can dissociate into still smaller molecules.

Radiolysis of Water The final result of the radiolysis of water is the formation of an ion pair, H+ and OH- and two free radicals. The ion pairs can recombine and therefore no biologic damage would occur.

Free Radicals A free radical is an uncharged molecule containing a single unpaired electron in the outer shell. They are highly reactive and unstable with a lifetime of less than 1 ms. They are capable of diffusion through the cell and interaction at a distant site.

Free Radicals They contain excess energy that can be transferred to other molecules to disrupt bonds and produce point lesions. OH* free radicals can join with similar molecules to form hydrogen peroxide. Hydrogen Peroxide is poisonous to the cell and therefore acts as a toxic agent.

Free Radicals H* free radicals can react with O2 to form hydroperoxyl. Hydroperoxyl and hydrogen peroxide are considered to be the principle damaging products following radiolysis of water. Hydroperoxyl free radicals can form Hydrogen peroxide.

Direct & Indirect Effect When biologic material is irradiated in vivo, the harmful effects occur because of damage to a particularly sensitive molecule such as DNA. If the initial ionization event occurs on the target molecule, the effect of radiation is direct. If it occurs on a distant, non critical molecule, which then transfers the energy of ionization to the target molecule, it is an indirect effect.

Direct & Indirect Effect The human body is 80% water and less than 1% DNA, the principle effect of radiation on humans is indirect. When oxygen is present, as in living tissue, the indirect effect is amplified because of the additional types of free radicals that are formed.

Target Theory The cell contains many molecules, most of which exist in overabundance. Radiation damage to such molecules would probably not result in noticeable injury to the cell. Some molecules in the cell are considered particularly necessary for normal cell function. They are rare and radiation damage could severely effect the cell function.

Target Theory This concept of a key molecule is the basis for the target theory. If the target molecule is deactivated by radiation, the cell may die. When radiation does interact with the target molecule, it is called a hit.

Target Theory Radiation interact with other than the target molecule can result in a hit. It is not possible to distinguish between direct and indirect hit. When a hit occurs through indirect effect, the size of the target appears to be much larger because of mobility of free radicals.

OER and Target Theory With low LET radiation and no O2 the probability of a hit is low. When O2 , free radicals are formed the probability is increased.

OER and Target Theory With high LET radiation, the distance between ionizations is so close that the probability of a hit by direct effect is high. When O2 is added, the added sphere of influence does not result in more hits. The maximum hits has already been produced by direct effect.

Single Target, Single Hit Model The single target single hit model applies to biologic targets, such as enzymes, viruses or simple cells like bacteria. The multitarget, single hit applies to more complicated systems such as human cells. Radiation interacts randomly with matter. Poisson distribution is used to determine the probability of a hit.

Poisson Distribution If there were 100 squares and 100 rain drops, 63% of the squares would be hit and 37% would remain dry. If rain fell uniformly, all 100 squares would be wet. If the rain drops were x-rays, some of the squares are hit more than once that would result in wasted radiation since they are already killed by the first hit.

D37 If we used increasing increments of radiation until we reached a level that would kill 63%(37% survival) of the cells, it would be referred to as D37. If we doubled the D37 dose, 14% of the cells would survive. The lethal effects of radiation are determined by observing cell survival, not cell death.

D37 A cell with a low D37 is highly radiosensitive. A cell with a high D37 represents radio-resistance. For these purposes, a hit is not just an ionizing event, but rather ionization that inactivates the target molecule. If there was no wasted hits, uniform not random interactions, D37 is the dose that would kill 100% of the cells.

Multitarget, Single Hit Model Complex cells such as human cells are said to have more than one critical target. As an example if the cell has two target molecules and both had to be hit to deactivate the cell, there would need to be a significant dose to hit both targets since radiation is random.

Multitarget, Single Hit Model At very low radiation doses cell survival is nearly 100%. As the dose increases, less survive as more cells get hits in both targets. At high radiation doses, all cells that survive have one hit. Therefore at even higher doses, the dose response would appear as the single target, single hit model.

Do The Do is called the mean lethal dose and it is a constant related to radiosensitivity of the cell. A large Do indicates radio-resistant cells. A small Do is characteristic of radiosensitive cells.

DQ The DQ is called the threshold dose. This is related to the cells ability to recover from sublethal damage. It is a measure of the capacity to accumulate sublethal and the ability to recover from sublethal damage.

Dose (DQ) for mammalian cell lines Cell Type Mouse skin Human Bone Marrow Human fibroblasts Human lymphocytes Do (rad) DQ (rad) 135 350 137 100 150 160 400 100

Split-Dose Irradiation This graph illustrates cell recovery from a relatively large dose. D0=160rad DQ=110rad The 1st exposure is 470rad. The surviving cells are re-incubated and grow into another large population.

Split-Dose Irradiation The second population is subject to additional incremental doses of radiation. The second graphs has the same shape at first dose response by the threshold dose.

Recovery For full recovery the time between each exposure must be at least as long as the cell generation. Experiments have demonstrated that cells that survive an initial radiation insult exhibits the same characteristics as non-irradiated cells. The surviving cells have fully recovered from a sublethal dose.

Cell-Cycle Effect Human cells replicate by mitosis. The time from one mitosis to the next mitosis is called the Cell-cycle time or generation time. Most cells that are in a state of normal proliferation have generation times of 10 to 20 hours. The G1 or pre DNA synthesis phase is the most time variable of cell phases

Cell-Cycle Effect The change in radiosensitivity as a function of the phase of cell cycle is the age-response function. Cells in mitosis are always most sensitive. The fraction of surviving cells is lowest in this phase. The G1-S phase is the next most sensitive. The late S phase is the most radio-resistant.

LET, RBE and OER Many experiments have been done to measure the effect of various types of radiation and to determine the magnitude of various dose modifiers such as oxygen. The LET (linear energy transfer) determines the magnitude of RBE and OER.

LET, RBE and OER For high LET radiation (alpha particles or neutrons), cell survival follows the single-target, single-hit. Low LET radiation (diagnostic x-ray), cell survival follows the multitarget, single hit model. Oxygen enhancement is maximizes the effects of low LET radiation.

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