the radiobiology of radiation therapy
DESCRIPTION
The Radiobiology of Radiation Therapy. Type of Injuries. Nuclear DNA is major target Cellular membrane damage – minor Nuclear membrane damage – minor Cellular organelle injury – minor Mitochondrial DNA ??. Mechanism. Two mechanisms of injury Direct Ionization of the DNA, ≈ 15% - PowerPoint PPT PresentationTRANSCRIPT
The Radiobiology of Radiation The Radiobiology of Radiation TherapyTherapy
Type of InjuriesType of Injuries
Nuclear DNA is major targetNuclear DNA is major target Cellular membrane damage – minorCellular membrane damage – minor Nuclear membrane damage – minorNuclear membrane damage – minor Cellular organelle injury – minorCellular organelle injury – minor
• Mitochondrial DNA ??Mitochondrial DNA ??
MechanismMechanism Two mechanisms of injuryTwo mechanisms of injury
• Direct Ionization of the DNA, ≈ 15%Direct Ionization of the DNA, ≈ 15%• Indirect Ionization of the DNA, ≈ 85% Indirect Ionization of the DNA, ≈ 85%
DNA damaged by free radicals formed in the DNA damaged by free radicals formed in the micro-environment of the DNAmicro-environment of the DNA
Water is most important sourceWater is most important source Oxygen is important in fixating injuryOxygen is important in fixating injury Sulfhydryl compounds promote repairSulfhydryl compounds promote repair
Types of DNA InjuryTypes of DNA Injury Base pair injuryBase pair injury Base pair deletionBase pair deletion Base pair cross linkageBase pair cross linkage Single strand break in backboneSingle strand break in backbone Double strand break in backboneDouble strand break in backbone Gene suppression or activationGene suppression or activation
Base Pair InjuryBase Pair Injury Damage to one of the pairs of Damage to one of the pairs of
nitrogenous bases in the DNA nitrogenous bases in the DNA sequence. sequence.
Easily repaired by cellular repair Easily repaired by cellular repair mechanisms. mechanisms.
Repair is error freeRepair is error free
Base Pair DeletionBase Pair Deletion Complete destruction of a pair of the Complete destruction of a pair of the
nitrogenous bases in the sequencenitrogenous bases in the sequence Rapidly repaired by cellular repair Rapidly repaired by cellular repair
mechanismsmechanisms Not necessarily error free repair.Not necessarily error free repair.
Base Pair Crosslinkage InjuryBase Pair Crosslinkage Injury Abnormal pairing of the nitrogenous Abnormal pairing of the nitrogenous
bases. bases. May effect conformation of DNA May effect conformation of DNA Repaired efficiently Repaired efficiently
Single Strand BreakSingle Strand Break Result of ionization of the sugar-Result of ionization of the sugar-
phospate rail of the DNA moleculephospate rail of the DNA molecule Most is easily repaired unless base Most is easily repaired unless base
pairs are also lostpairs are also lost Repair is rapid and accurate but Repair is rapid and accurate but
some is not repairable.some is not repairable.
Double Strand BreakDouble Strand Break Breakage of both strands of the DNA Breakage of both strands of the DNA
backbone in close proximity to each backbone in close proximity to each other.other.
Difficult to repair Difficult to repair Repair is quite prone to errors.Repair is quite prone to errors. High dose and High LET event. High dose and High LET event.
Gene Suppression or ActivationGene Suppression or Activation Radiation injury may result in Radiation injury may result in
upregulation of some genes.upregulation of some genes.• Tumor Promoter genesTumor Promoter genes• Tumor Suppressor genesTumor Suppressor genes
Radiation injury may result in down Radiation injury may result in down regulation of the same genesregulation of the same genes
Down regulation of genes controlling Down regulation of genes controlling intracellular repair. intracellular repair.
Cell Survival CurvesCell Survival Curves Cell survival curve expressed on a Cell survival curve expressed on a
log/linear plot. log/linear plot. Developed through many years of Developed through many years of
experimentationexperimentation Different curves are derived for Different curves are derived for
different types of radiation.different types of radiation.
Cell survival, neutrons vrs. Cell survival, neutrons vrs. xraysxrays
Single Hit KillingSingle Hit Killing Lethal damage to DNA by single Lethal damage to DNA by single
photon. photon. Mostly due to double strand breaksMostly due to double strand breaks May be due to pro apoptotic gene May be due to pro apoptotic gene
activationactivation Represented by the initial straight Represented by the initial straight
portion of the photon survival curveportion of the photon survival curve
Multi-hit KillingMulti-hit Killing Lethal injury to the DNA following Lethal injury to the DNA following
multiple hits of the DNA by photon multiple hits of the DNA by photon radiationradiation
Coincident single strand breaks Coincident single strand breaks result in a double strand breakresult in a double strand break
Activation of pro apoptotic genesActivation of pro apoptotic genes Increases with doseIncreases with dose Represented by steep part of curveRepresented by steep part of curve
Survival Curve ShoulderSurvival Curve Shoulder Represents the transition zone Represents the transition zone
between single and multiple hit between single and multiple hit killingkilling
The shoulder is representative of the The shoulder is representative of the repair capability of the cell repair capability of the cell populationpopulation
Wider in slowly dividing cellsWider in slowly dividing cells Narrower in rapidly dividing cellsNarrower in rapidly dividing cells
Alpha/Beta RatioAlpha/Beta Ratio Really is determined by a dose pointReally is determined by a dose point Point on survival curve where single Point on survival curve where single
and multi-hit killing are equaland multi-hit killing are equal Larger in cell lines with a wider repair Larger in cell lines with a wider repair
shoulder. shoulder.
Alpha/Beta RatioAlpha/Beta Ratio
LET and Effect on SurvivalLET and Effect on Survival LET = Linear Energy TransferLET = Linear Energy Transfer
• Measured in keV/micronMeasured in keV/micron• Characteristic of particulate radiation Characteristic of particulate radiation
High LET radiation increase killing High LET radiation increase killing per unit energy deposited. per unit energy deposited. • Results in severe repair deficiencies Results in severe repair deficiencies
Effectively removes the repair Effectively removes the repair shouldershoulder
LET and Effect on SurvivalLET and Effect on Survival High LET radiation is densely ionizingHigh LET radiation is densely ionizing Averages >1 ionization event within Averages >1 ionization event within
the span of a DNA molecule.the span of a DNA molecule. High ionization density increases High ionization density increases
probability of double strand breaks. probability of double strand breaks. Reaches a maximum effect at about Reaches a maximum effect at about
100 keV/micron.100 keV/micron.
LET and Effect on SurvivalLET and Effect on Survival Photons have an average LET of Photons have an average LET of
about 1. about 1. <1 ionization event within the <1 ionization event within the
diameter of a DNA Molecule. diameter of a DNA Molecule. Single strand breaks predominateSingle strand breaks predominate Repair is permittedRepair is permitted
LET and Effect on SurvivalLET and Effect on Survival
Cell Cycle and Radiation InjuryCell Cycle and Radiation Injury M phase – mitosis very sensitive to M phase – mitosis very sensitive to
radiation injuryradiation injury G1 phase – resting phase, G1 phase – resting phase,
moderately resistantmoderately resistant S phase – DNA synthesis, S phase – DNA synthesis,
moderately resistant to radiationmoderately resistant to radiation G2 resting phase – sensitiveG2 resting phase – sensitive G0 non cycling cells – moderate G0 non cycling cells – moderate
resistance resistance
Cell Cycle and Radiation InjuryCell Cycle and Radiation Injury MitosisMitosis
• Chromosomes are condensedChromosomes are condensed DNA is closely packed – bigger targetDNA is closely packed – bigger target
• Repair mechanisms are shut downRepair mechanisms are shut down• Very compressed time scale = 1 hr.Very compressed time scale = 1 hr.• Any DNA injury is fixed in placeAny DNA injury is fixed in place• Cell may loose large segments of DNA Cell may loose large segments of DNA
Fragments excluded from nucleusFragments excluded from nucleus
Cell Cycle and Radiation InjuryCell Cycle and Radiation Injury S phaseS phase
• Phase of DNA synthesisPhase of DNA synthesis• Most radiation resistant phaseMost radiation resistant phase• Cellular repair mechanisms are activeCellular repair mechanisms are active
Increases repair of radiation damageIncreases repair of radiation damage• Lasts about 5 hours. Lasts about 5 hours.
Cell Cycle and Radiation InjuryCell Cycle and Radiation Injury G1G1
• Functional part of cell cycleFunctional part of cell cycle• Resistance varies with part of phaseResistance varies with part of phase
Goes down as cell nears the G1-S interfaceGoes down as cell nears the G1-S interface Point in cell cycle where apoptosis occursPoint in cell cycle where apoptosis occurs
• Cell death at this point is referred to as Cell death at this point is referred to as interphase deathinterphase death
• Longest part of cycle. Longest part of cycle. Lasts hours to yearsLasts hours to years
Cell Cycle and Radiation InjuryCell Cycle and Radiation Injury G2G2
• Short rest phase before MShort rest phase before M• Quite radiation sensitive Quite radiation sensitive • Short time allows little for injury repairShort time allows little for injury repair• Radiation injury incurred in S-phase may Radiation injury incurred in S-phase may
be repaired be repaired May result in a mitotic delay in G2May result in a mitotic delay in G2
• Apoptosis-like death may also occurApoptosis-like death may also occur
The Four R’sThe Four R’s RepairRepair Reassortment Reassortment ReoxygenationReoxygenation RepopulationRepopulation
RepairRepair Rapid repair of injuryRapid repair of injury Initiated within seconds of injuryInitiated within seconds of injury Complete by 6 hours after injuryComplete by 6 hours after injury Can be modified by environmental Can be modified by environmental
conditionsconditions• Presence or absence of oxygen or free Presence or absence of oxygen or free
radical scavengers. radical scavengers. Responsible for shoulder of survival Responsible for shoulder of survival
curvecurve
ReassortmentReassortment When cells killed in sensitive phases When cells killed in sensitive phases
it leave a gap in the cell population it leave a gap in the cell population for those phases.for those phases.
Within two cycles cells from less Within two cycles cells from less sensitive parts of cycle replace themsensitive parts of cycle replace them
Some non-cycling cells may be Some non-cycling cells may be recruited into the cycling pool.recruited into the cycling pool.
ReoxygenationReoxygenation Most tumors larger than 1 cm have Most tumors larger than 1 cm have
some hypoxic cells in themsome hypoxic cells in them• Some tumor types have larger %Some tumor types have larger %• May be transient or chronicMay be transient or chronic
Radiation preferentially kills Radiation preferentially kills oxygenated cells oxygenated cells (O(O22 fixation of injury) fixation of injury)
Major contributor to tumor radiation Major contributor to tumor radiation resistance.resistance.
ReoxygenationReoxygenation
ReoxygenationReoxygenation
RepopulationRepopulation Following killing of cells in a Following killing of cells in a
population by any means there is population by any means there is either replacement or repopulation of either replacement or repopulation of the cells killedthe cells killed
Usually there is days to weeks delay Usually there is days to weeks delay before this beginsbefore this begins
Tissues with large clonogenic Tissues with large clonogenic populations are able to do this betterpopulations are able to do this better
RepopulationRepopulation Tends to be a low dose phenomenonTends to be a low dose phenomenon Usually is most important in rapidly Usually is most important in rapidly
cycling cell population.cycling cell population.• This includes tumorsThis includes tumors
Rapid repopulation may reduce level Rapid repopulation may reduce level of repairof repair
Tissue Level Radiation EffectsTissue Level Radiation Effects All mammalian cells equally sensitive All mammalian cells equally sensitive
in cycling populations in cell culturein cycling populations in cell culture However, in tissue the rate of cell However, in tissue the rate of cell
replacement is variablereplacement is variable Some cell populations turn over Some cell populations turn over
every 3-5 days and some never do. every 3-5 days and some never do. • Cell growth fractions and cell death Cell growth fractions and cell death
fractions should be in balance.fractions should be in balance.
Tissue EffectsTissue Effects Radiation response at tissue level is Radiation response at tissue level is
tied to cell deathtied to cell death• Cell death is mostly tied to cell Cell death is mostly tied to cell
reproductionreproduction ApoptosisApoptosis
• Radiation induction of apoptosis pathwaysRadiation induction of apoptosis pathways Mitotic linked deathMitotic linked death
• Reproductive failure due to missing DNAReproductive failure due to missing DNA• Long cell cycle times blunt responseLong cell cycle times blunt response
Tissue EffectsTissue Effects Long cell cycle times promote repair Long cell cycle times promote repair
and slow repopulationand slow repopulation Short cell cycle times promote Short cell cycle times promote
repopulation and blunt repairrepopulation and blunt repair Large non-cycling populations blunt Large non-cycling populations blunt
radiation responseradiation response Dose required to inhibit function is Dose required to inhibit function is
much higher than that for much higher than that for reproductive inhibition or failure.reproductive inhibition or failure.
Tissue EffectsTissue Effects At the tissue level the ultimate At the tissue level the ultimate
survival of the tissue depends on:survival of the tissue depends on:• The number of cycling cellsThe number of cycling cells• The ability of the tissue to repair the The ability of the tissue to repair the
injury.injury.• The ability of the tissue to repopulate The ability of the tissue to repopulate
the tissue with the original cell type. the tissue with the original cell type.
Tissue effectsTissue effects Repopulation is most important at Repopulation is most important at
low doses; low doses; Early responding tissues tend to have Early responding tissues tend to have
more repopulationmore repopulation Late responding tissues tend to have Late responding tissues tend to have
limited repopulation capabilitylimited repopulation capability• Therefore sensitive to larger doses of Therefore sensitive to larger doses of
radiation.radiation.
Tissue EffectsTissue Effects
Radiation DeliveryRadiation Delivery Treatment with a number smaller Treatment with a number smaller
doses improves normal tissue doses improves normal tissue response and increases total dose response and increases total dose that can be given to a tumorthat can be given to a tumor• Reduces hypoxiaReduces hypoxia• Promotes repopulation in late responding Promotes repopulation in late responding
tisuestisues• Promote reassortmentPromote reassortment• Promotes repair of DNA injuryPromotes repair of DNA injury
FractionationFractionation
FractionationFractionation Optimal dose is that which is just Optimal dose is that which is just
about midway through the repair about midway through the repair shoulder. shoulder.
Usually approximately equal to the Usually approximately equal to the Do doseDo dose
Must wait at least 6 hours for repair Must wait at least 6 hours for repair to be complete.to be complete.