Introduction to Radiation ChemistryIntroduction to Radiation Chemistry

Mike Robbins, PhDMike Robbins, PhDRadiation Biology SectionRadiation Biology Section

Department of Radiation OncologyDepartment of Radiation OncologyWake Forest University School of MedicineWake Forest University School of Medicine

Ionizing Radiation Can Dissipate Its Ionizing Radiation Can Dissipate Its Energy By Two MethodsEnergy By Two Methods

EXCITATIONEXCITATION

IONIZATIONIONIZATION

ExcitationExcitation

Amount of energy absorbed raises an electron in Amount of energy absorbed raises an electron in an atom/molecule to a higher energy level an atom/molecule to a higher energy level without ejection of the electron. without ejection of the electron.

Occurs following exposure to nonOccurs following exposure to non--ionizing ionizing radiation e.g., UVradiation e.g., UV

IonizationIonization

Occurs when the absorbed radiation has Occurs when the absorbed radiation has enough energy to eject one or more orbital enough energy to eject one or more orbital electrons from the atom/molecule.electrons from the atom/molecule.

Radiation with such energy called ionizing Radiation with such energy called ionizing radiation, e.g., X rays, radiation, e.g., X rays, γγ raysrays

Examples: X rays Examples: X rays ((extranuclearextranuclear) and ) and γγ rays rays ((intranuclearintranuclear))

Can view as either a wave Can view as either a wave of electrical and of electrical and mechanical energy or as mechanical energy or as photons (packets of photons (packets of energy) energy)

Electromagnetic SpectrumElectromagnetic Spectrum

Electromagnetic RadiationElectromagnetic Radiation

Irradiating biological material leads to unequal Irradiating biological material leads to unequal distribution of energy in tissues and cellsdistribution of energy in tissues and cells

With ionizing radiation photons contain With ionizing radiation photons contain sufficient energy to break chemical bonds sufficient energy to break chemical bonds leading to biological effectsleading to biological effects

Ionization and Radical FormationIonization and Radical Formation

Ionization of water leads to generation of an ion pairIonization of water leads to generation of an ion pair

HH22O + radiation O + radiation →→ HH22OO++•• + e+ e--

HH22OO++•• is an ion radical: ion is an atom/molecule that is is an ion radical: ion is an atom/molecule that is electrically charged since has lost an electron.electrically charged since has lost an electron.

Free radical is atom/molecule that possesses one or more Free radical is atom/molecule that possesses one or more unpaired electrons; highly reactiveunpaired electrons; highly reactive

Ionization and Radical FormationIonization and Radical Formation

HH22O O ++•• →→ HH++ + HO+ HO••

ee-- + H+ H22O O →→ ee--aqaq

A hydrogen free radical can also be produced, together with someA hydrogen free radical can also be produced, together with somehydrogen peroxide:hydrogen peroxide:

ee--aqaq + H+ H++ →→ HH••

HOHO•• + HO+ HO•• →→ HH22OO22

Ionization and Radical FormationIonization and Radical Formation

Radiolysis of water:Radiolysis of water:

HH22O + radiation O + radiation →→ ee--aqaq + HO+ HO•• +H+H•• + H+ H22OO22

G values G values 2.63 2.72 0.55 0.682.63 2.72 0.55 0.68

G value: measured yield of molecules produced by absorption of G value: measured yield of molecules produced by absorption of 100 eV X rays. For low LET, highest yields are 100 eV X rays. For low LET, highest yields are ee--aqaq and HOand HO••..

Direct and Indirect Effects of RadiationDirect and Indirect Effects of Radiation

Direct effect: target molecule itself reacts Direct effect: target molecule itself reacts directly with radiationdirectly with radiation

RH RH →→ RHRH++

RHRH++ →→ RR•• + H+ H++

Direct and Indirect Effects of RadiationDirect and Indirect Effects of Radiation

Indirect effect: ionizing radiation generates free radicals Indirect effect: ionizing radiation generates free radicals from the radiolysis of waterfrom the radiolysis of water

These can indirectly form radicals with the target These can indirectly form radicals with the target moleculemolecule

RH + HORH + HO•• →→ RR•• + H+ H22OO

RH + HRH + H++ →→ RR•• + H+ H22

““FixationFixation”” of Biological Injuryof Biological Injury

In the presence of In the presence of oxygen, an organic oxygen, an organic peroxyl radical is peroxyl radical is formed. This cannot formed. This cannot easily be repaired, and easily be repaired, and so acts to so acts to ““fixfix”” the the biological injurybiological injury

RR•• + O+ O22 →→ ROOROO••

TARGET THEORYTARGET THEORYFor bacteria, viruses and For bacteria, viruses and somesome mammalian cell lines, semimammalian cell lines, semi--log log plot of SF vs. dose gives a straight lineplot of SF vs. dose gives a straight line

Unit increase in dose produces a corresponding fractional Unit increase in dose produces a corresponding fractional decrease in survivaldecrease in survival

Shape of curve explained in terms of Target Theory: certain Shape of curve explained in terms of Target Theory: certain critical sites in cells must be hit if the cell is to be killedcritical sites in cells must be hit if the cell is to be killed

Single hit in a single target would give an exponential survivalSingle hit in a single target would give an exponential survivalcurvecurve

SingleSingle--target Single Hit Modeltarget Single Hit Modelprobability of survival probability of survival pp ==

pp (0 hits) = e(0 hits) = e--D/DoD/Do

D = dose applied; DD = dose applied; D00 = dose that = dose that gives an average of 1 hit/targetgives an average of 1 hit/target

A dose of DA dose of D00 will reduce survival will reduce survival from 1 to 0.37 i.e., efrom 1 to 0.37 i.e., e--11, or from 0.1 , or from 0.1 to 0.037, etc.to 0.037, etc.

D/DD/D00 is the average number of is the average number of hits/targethits/target

MultiMulti--event Modelsevent Models

Mammalian cells tend to show a different Mammalian cells tend to show a different responseresponse

At low doses see a shoulder, only see At low doses see a shoulder, only see exponential response at higher dosesexponential response at higher doses

Various multiVarious multi--event models have been event models have been proposedproposed

MultiMulti--target singletarget single--hit Modelhit ModelProposes 2 or more targets in a cell; each Proposes 2 or more targets in a cell; each

must receive a single hit before the cell must receive a single hit before the cell is killed. is killed.

Model is described by the following Model is described by the following equation:equation:

SF SF = 1= 1--(1(1-- ee--D/D0D/D0))nn

where SF is the surviving fraction after a where SF is the surviving fraction after a dose Ddose D

DD00 is the dose needed to reduce cell SF to is the dose needed to reduce cell SF to 1/e (37%) of its initial value on the 1/e (37%) of its initial value on the exponential portion of the survival exponential portion of the survival curve. Also the reciprocal slope of the curve. Also the reciprocal slope of the survival curve, survival curve,

nn is the extrapolation number. is the extrapolation number.

MultiMulti--target singletarget single--hit Modelhit Model

Has proved useful for describing the response of Has proved useful for describing the response of mammalian cells at high dosesmammalian cells at high doses

However, does not describe survival response at However, does not describe survival response at lower more clinically relevant doseslower more clinically relevant doses

Imply zero slope at very low doses; most data show Imply zero slope at very low doses; most data show finite or nonfinite or non--zero initial slopezero initial slope

Two Compartment ModelTwo Compartment ModelCombines the simple multiCombines the simple multi--target target

model with a singlemodel with a single--target target component. component.

This model fits the majority of This model fits the majority of mammalian cell survival curves, mammalian cell survival curves, implies the possibility of both implies the possibility of both single and multisingle and multi--hit events. hit events.

f f = e= e--D/D1D/D1 [1[1--(1(1-- ee--D/D2D/D2)])]nn

where Dwhere D11 and Dand D22 refer to the initial refer to the initial and final slopesand final slopes..

Two Compartment ModelTwo Compartment Model

Disadvantages:Disadvantages:

Changes in cell survival over the range 0Changes in cell survival over the range 0--DDqq occur occur almost linearlyalmost linearly

Implies no sparing of damage at doses per fraction Implies no sparing of damage at doses per fraction less than ~ 2 Gy less than ~ 2 Gy

LinearLinear--Quadratic ModelQuadratic Model

Gives a better description of the Gives a better description of the radiation response of cells in radiation response of cells in the low dose region (0the low dose region (0--3 Gy)3 Gy)

f f = e= e--((ααD +D +ββD2)D2)

Gives a continuously bending Gives a continuously bending survival curve with no straight survival curve with no straight portion at high radiation doses portion at high radiation doses

Shape or bendiness of the curve is Shape or bendiness of the curve is determined by the determined by the αα//ββ ratio; ratio; represents the dose (Gy) at represents the dose (Gy) at which linear contribution to which linear contribution to cell kill equals quadratic cell kill equals quadratic contribution.contribution.

Lethal, potentially Lethal Damage (LPL) ModelLethal, potentially Lethal Damage (LPL) Model

Ionizing radiation produces 2 kinds of lesions: repairable (poteIonizing radiation produces 2 kinds of lesions: repairable (potentially lethal) ntially lethal) lesions and nonlesions and non--repairable (lethal) lesions. repairable (lethal) lesions.

The nonThe non--repairable lesions produce single hit lethal events; linear comrepairable lesions produce single hit lethal events; linear component ponent of cell kill. of cell kill.

The effect of the repairable lesions depends on the competing prThe effect of the repairable lesions depends on the competing processes of ocesses of repair and binary misrepair; leads to quadratic component.repair and binary misrepair; leads to quadratic component.

At higher doses the probability of binary interaction of potentAt higher doses the probability of binary interaction of potentially lethal ially lethal lesions increases.lesions increases.

DNA Damage is the Critical Event in DNA Damage is the Critical Event in RadiationRadiation--induced Cell deathinduced Cell death

Microirradiation studies indicate that to kill cells by irradiatMicroirradiation studies indicate that to kill cells by irradiation the ion the cytoplasm requires much greater doses than the nucleus; >250 Gy cytoplasm requires much greater doses than the nucleus; >250 Gy compared with ~2 Gycompared with ~2 Gy

Isotopes such as Isotopes such as 33H and H and 125125I that emit short range I that emit short range ββ particles, when particles, when incorporated intercellular DNA, efficiently produce radiation ceincorporated intercellular DNA, efficiently produce radiation cell kill ll kill and DNA damageand DNA damage

The incidence of chromosomal aberrations following irradiation iThe incidence of chromosomal aberrations following irradiation is s closely linked to cell kill.closely linked to cell kill.

Thymidine analogues such as IUdr and Thymidine analogues such as IUdr and BrUdrBrUdr when specifically when specifically incorporated into DNA modify radiosensitivity. Substituted incorporated into DNA modify radiosensitivity. Substituted deoxyuridines, which are not incorporated into DNA, have no suchdeoxyuridines, which are not incorporated into DNA, have no suchaffect on cellular radiosensitivity. affect on cellular radiosensitivity.