Objectives: Know the characteristics of ionizing radiation that make it useful for RT

http://dmco.ucla.edu/McBride_Labhttp://dmco.ucla.edu/McBride_Lab

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Interaction of Radiation with Interaction of Radiation with Biological Matter: Biological Matter:

(what is biological dose?)(what is biological dose?)

Bill McBrideBill McBrideDivision of Cellular and Molecular OncologyDivision of Cellular and Molecular Oncology

Dept. Radiation OncologyDept. Radiation OncologyDavid Geffen School MedicineDavid Geffen School Medicine

UCLA, Los Angeles, Ca.UCLA, Los Angeles, [email protected]@mednet.ucla.edu

Room B3-109, x47051Room B3-109, x47051


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Objectives:Objectives:– Know the characteristics of ionizing radiation Know the characteristics of ionizing radiation

that make it useful for RTthat make it useful for RT– Define LET and RBE and what is meant by Define LET and RBE and what is meant by

quality of radiationquality of radiation– Know the difference between direct and indirect Know the difference between direct and indirect

action of radiation and the role of free radicalsaction of radiation and the role of free radicals– Recognize the impact of oxygen on initial Recognize the impact of oxygen on initial

radiation damage and of hypoxia in tumor RTradiation damage and of hypoxia in tumor RT– Understand how biological radiation dose and Understand how biological radiation dose and

physical radiation dose differphysical radiation dose differ


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Radiation TherapyRadiation Therapy

• Approximately 50% of cancer patients Approximately 50% of cancer patients receive RT with curative intentreceive RT with curative intent

• Approximately half of these are curedApproximately half of these are cured

Radiation Therapy has a long history!


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Roentgen with his wifeRoentgen with his wife’’s s hand, 1895hand, 1895

X-rays were rapidly adapted for use as a clinical treatment, initially for non-cancerous conditions, but soon for cancer, as well.


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FIRST CURE OF CANCER BY X-RAYSFIRST CURE OF CANCER BY X-RAYS 1899 - BASAL CELL CARCINOMA 1899 - BASAL CELL CARCINOMA

X-rays were used to cure cancer very soon after their discovery


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And rapidly became a standard And rapidly became a standard treatmenttreatment

Hammersmith Hospital, London, 1905


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Although side-effects were Although side-effects were encountered! encountered! This is a picture of a 70 year old This is a picture of a 70 year old person who was irradiated by person who was irradiated by Freund at the of age 5 in Austria Freund at the of age 5 in Austria 1896 for nevus pigmentosus 1896 for nevus pigmentosus piliferus. piliferus. L. Freund, Ein mit Rontgenstrahlen behandelter L. Freund, Ein mit Rontgenstrahlen behandelter fall von nevus pigmentosus piliferus. Wein. Med. fall von nevus pigmentosus piliferus. Wein. Med. Wochschr. 47, 428-434 (1987).Wochschr. 47, 428-434 (1987).


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EpilepsyEpilepsy

LupusLupus

Initially more non-cancerous diseases were treated that cancer (still popular in Europe)


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The Nobel Prize in Physiology or Medicine 1946The Nobel Prize in Physiology or Medicine 1946"for the discovery of the production of mutations by means of X-ray irradiation

Hermann J. Muller

However, its use for benign conditions has been limited in most countries for fear of radiation-induced cancer.

The carcinogenic effects of X-rays was discovered using fruit flies by Muller in 1946.


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Natural radioactivity was discovered by Becquerel, who was awarded Natural radioactivity was discovered by Becquerel, who was awarded the Nobel Prize in Physics in 1903 along with Marie and Pierre Curie the Nobel Prize in Physics in 1903 along with Marie and Pierre Curie "in recognition of the extraordinary services they have rendered by "in recognition of the extraordinary services they have rendered by

their joint researches on the their joint researches on the radiationradiation phenomena" phenomena"

“One wraps a Lumiere photographic plate with a bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. If one places between the phosphorescent substance and the paper a piece of money or a metal screen pierced with a cut-out design, one sees the image of these objects appear on the negative. One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduces silver salts.” Paris 1896

Maltese crossHenri BecquerelHenri Becquerel Marie CurieMarie Curie


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Natural RadioactivityNatural Radioactivity

particlesparticles– Positively charged, helium nucleusPositively charged, helium nucleus

particlesparticles– Negatively charged, electronsNegatively charged, electrons

-rays-rays– No charge, EMRNo charge, EMR


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Radioisotopes also were soon being used Radioisotopes also were soon being used to treat and cure cancer.to treat and cure cancer.

Radium applicators were used for many other conditions!

Radioactive plaques and implants are still in common use, for example in prostate implant seeds.

First cure of First cure of cancer by cancer by radium plaque - radium plaque - 19221922


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Therapeutic Benefit and R.T.Therapeutic Benefit and R.T.

There is always a need to derive a therapeutic There is always a need to derive a therapeutic benefit from RT. There are 2 main ways by benefit from RT. There are 2 main ways by which this is achieved: which this is achieved: 1. Physical means1. Physical means

– distributing dose by treatment planningdistributing dose by treatment planning

2. Biological means2. Biological means– dose fractionationdose fractionation


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Grenz Rays

Megavoltage

Orthovoltage

Superficial Therapy

Contact Therapy

20 KeV

50 KeV

150 KeV

500 KeV

1-25 MeV Major improvements in RT Major improvements in RT during the mid-1900s came during the mid-1900s came from improved penumbra from improved penumbra and decreased skin dose and decreased skin dose associated with higher associated with higher energy x-rays, cobalt, and energy x-rays, cobalt, and high energy photons. high energy photons.

More recently conformal More recently conformal RT, IMRT, IGRT, RT, IMRT, IGRT, Gammaknife, Cyberknife, Gammaknife, Cyberknife, tomotherapy, SRS, SRT, tomotherapy, SRS, SRT, protons, heavy ions, etc. protons, heavy ions, etc. have added considerable have added considerable variety to the choices for variety to the choices for physical radiation delivery physical radiation delivery and present radiobiological and present radiobiological challenges.challenges.


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18961896 Freund Freund - treated hairy nevus with fractionated doses- treated hairy nevus with fractionated doses19001900 Stenbeck - cured skin cancer with single dosesStenbeck - cured skin cancer with single doses19061906 Bergonie and Trubandeau introduced the Bergonie and Trubandeau introduced the ““LawLaw”” that radiosensitivity is related that radiosensitivity is related

to cell proliferation (NOT TRUE!) to explain why fractionated doses sterilized to cell proliferation (NOT TRUE!) to explain why fractionated doses sterilized rams without skin reactionsrams without skin reactionsRegaud - treated uterine cancer with fractionated dosesRegaud - treated uterine cancer with fractionated doses

19141914 SchwartzSchwartz- Fractionation is superior because of cell cycle redistribution- Fractionation is superior because of cell cycle redistribution

19191919 Coutard cures deep-seated H&N tumorsCoutard cures deep-seated H&N tumors19321932 Coutard shows fractionation superior to single doseCoutard shows fractionation superior to single dose19441944 Strandquist - empirical laws for changing dose per fractionStrandquist - empirical laws for changing dose per fraction19671967 Ellis - Nominal Standard Dose (NSD) formulaEllis - Nominal Standard Dose (NSD) formula1980s Linear Quadratic formula gains favor1980s Linear Quadratic formula gains favor

History of FractionationHistory of Fractionation


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The First Radiation Dosimeter!The First Radiation Dosimeter!

Early x-ray machines took a long time to deliver effective dose and gave skin reactions that could be circumvented by dose fractionation.


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From Amaldi and Kraft, “Radiotherapy with beams of carbon ions, Reports on Progress in Physics, 68, (2005)


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““In order to save machine time, a 3-day-a-week schedule was In order to save machine time, a 3-day-a-week schedule was initiated in 1962. This schedule was quickly abandoned in pre-initiated in 1962. This schedule was quickly abandoned in pre-operative irradiation because of increased wound healing problems. operative irradiation because of increased wound healing problems. Although acute reactions in the 3-day-a-week schedule for protracted Although acute reactions in the 3-day-a-week schedule for protracted radical irradiation were not excessive, radical irradiation were not excessive, late radiation sequelaelate radiation sequelae are are probably more pronounced as observed 2 or more years later.”probably more pronounced as observed 2 or more years later.”

Fletcher, 1966.Fletcher, 1966.

3 x 3.3 Gy3 x 3.3 Gy 5 x 2 Gy5 x 2 Gy

History has repeatedly shown that dose fractionation History has repeatedly shown that dose fractionation results in a therapeutic advantageresults in a therapeutic advantage


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Clinical RT is Changing, which PresentsClinical RT is Changing, which PresentsChallenges and Opportunities for RadiobiologyChallenges and Opportunities for Radiobiology

Conventional treatment:Conventional treatment: Tumors are irradiated to a specified dose with 2Gy fractions delivered, more or less Tumors are irradiated to a specified dose with 2Gy fractions delivered, more or less homogeneously, in a 6 week time periodhomogeneously, in a 6 week time period

• Varying this schedule impacts outcomeVarying this schedule impacts outcome• Radiobiological modeling attempts to provide guidelines for customization of RT usingRadiobiological modeling attempts to provide guidelines for customization of RT using

– Radiobiological principlesRadiobiological principles derived from preclinical data derived from preclinical data– Radiobiological parameters derived from clinical altered fractionation protocolsRadiobiological parameters derived from clinical altered fractionation protocols

Modern treatment:Modern treatment: IMRT etc allows optimized non-homogeneous dose distributions, concomitant boosts, IMRT etc allows optimized non-homogeneous dose distributions, concomitant boosts, dose painting - dose painting - dose heterogeneitydose heterogeneitySRS, SRT, HDR, Protons, Heavy Ions - SRS, SRT, HDR, Protons, Heavy Ions - high dose/fx issueshigh dose/fx issuesMolecular and chemical targeting - Molecular and chemical targeting - dose adjustmentdose adjustmentMolecular prognosis and diagnosis promise individualized treatment plans and Molecular prognosis and diagnosis promise individualized treatment plans and biological biological treatment planningtreatment planning


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• Radiobiology has derived means of Radiobiology has derived means of understanding why dose fractionation gives a understanding why dose fractionation gives a therapeutic benefit. therapeutic benefit.

• New physical delivery methods need to New physical delivery methods need to incorporate and/or modify these concepts.incorporate and/or modify these concepts.

• In order to understand either conventional or In order to understand either conventional or newer treatment effects, one needs to know newer treatment effects, one needs to know the differences between physical and the differences between physical and biological radiation dose biological radiation dose


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What is Radiation?What is Radiation?

• Radiation is classified into two main categories:Radiation is classified into two main categories:

- Non-ionizing radiationNon-ionizing radiation- Ionizing radiationIonizing radiation


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ELECTROMAGNETIC RADIATIONSELECTROMAGNETIC RADIATIONS

Photon E = h(energy = Planck’s const x frequency)

= hc/ (c = speed of light, = wave length)

1010-9-9 1010-8-8 1010-7-7 1010-6-6 1010-5-5 1010-4-4 1010-3-3 1010-2-2 1010-1-1 11 1010 101022 101033 101044

raysrays

X-raysX-rays U.V.U.V.

vviissiibbllee

Infra RedInfra Red Radio WavesRadio Waves

MicrowavesMicrowaves Short WavesShort Waves

T.V.T.V.RadioRadio

RadarRadar

IONIZINGIONIZINGRADIATIONRADIATION NON-IONIZING RADIATIONNON-IONIZING RADIATION

(cms)E (eV) 1.24x107 1.24x102 1.24x10-13


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• Non-ionizing radiationNon-ionizing radiation– Is a particle or wave that has enough kinetic energy to raise the Is a particle or wave that has enough kinetic energy to raise the thermal energy of an outer shell electron and cause thermal energy of an outer shell electron and cause excitation excitation with emission of with emission of low energy EMR (infrared)low energy EMR (infrared)

• Ionizing radiationIonizing radiation– Ionizing radiation has enough kinetic energy to detach at least one electron from an Ionizing radiation has enough kinetic energy to detach at least one electron from an

atom or molecule, creating ionsatom or molecule, creating ions– Charged particles such as electrons, protons, heavy ions, alpha and beta particles are Charged particles such as electrons, protons, heavy ions, alpha and beta particles are

directlydirectly ionizing because they can interact directly with atomic electrons through ionizing because they can interact directly with atomic electrons through coulombic forces and transfer a major part of their kinetic energy directlycoulombic forces and transfer a major part of their kinetic energy directly

– In contrast, photons (x rays, In contrast, photons (x rays, rays) and neutrons are chargeless and therefore more rays) and neutrons are chargeless and therefore more penetrating. They arepenetrating. They are indirectlyindirectly ionizing. They have sufficient kinetic energy to free an ionizing. They have sufficient kinetic energy to free an orbital electron producing a orbital electron producing a ‘‘fastfast’’ recoil or Compton electron that is, in turn, directly recoil or Compton electron that is, in turn, directly ionizingionizing

• Energy is deposited in Energy is deposited in ““packetspackets””, , which is why, when it is deposited in DNA, ionizing radiation is which is why, when it is deposited in DNA, ionizing radiation is an efficient cytotoxic agentan efficient cytotoxic agent

• Ionizing radiation has an energy in excess of 124 eV, which corresponds to a Ionizing radiation has an energy in excess of 124 eV, which corresponds to a < about 10 < about 10-6-6 cm. cm.

-ray’-ray

excitation

ionization

particle

excitation and ionization


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N hScattered Photon

e- recoil (Compton) electron

N

e- photoelectron

e- Auger electrons

Characteristic X-rays

K LM

N 1.02MeV

0.51MeV photon

0.51MeV photon

Electron-positron pairN

e-

e+

h

•InIn pair productionpair production -The threshold energy for The threshold energy for

pair production ispair production is 1.02 MeV 1.02 MeV -The photon disappearsThe photon disappears-An electron-positron pair is An electron-positron pair is

producedproduced

•In the Compton Effect In the Compton Effect -photon interacts with aphoton interacts with a loosely loosely

bound bound ““freefree”” orbital electron orbital electron which is emitted from the atom which is emitted from the atom as a Compton (recoil) electron as a Compton (recoil) electron and the photon is scatteredand the photon is scattered

•In the Photoelectric EffectIn the Photoelectric Effect-photon interacts with aphoton interacts with a

tightly bound electrontightly bound electron it is it is absorbed losing all its energy absorbed losing all its energy to the electronto the electron

Interaction of Photons with MatterInteraction of Photons with Matter• In general, the interaction can result inIn general, the interaction can result in

– Photoelectric effect, which predominates at low photon energies.Photoelectric effect, which predominates at low photon energies.– Compton (incoherent) scattering effect, which predominates at intermediate energies.Compton (incoherent) scattering effect, which predominates at intermediate energies.– Pair production, which predominates at high photon energies. Pair production, which predominates at high photon energies.

• The probability for a photon to undergo any one of the various interaction phenomena with an atom of The probability for a photon to undergo any one of the various interaction phenomena with an atom of the absorber depends onthe absorber depends on

– the atomic number Z of the absorber as well as the energy of the photonthe atomic number Z of the absorber as well as the energy of the photon

Arthur H. Compton

Nobel Prize in Physics 1927


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Ion formation – H2OIon formation – H2O++ and e- and e-

Excitation and H and OH radical formationExcitation and H and OH radical formation

ION RADICAL LIFETIMEION RADICAL LIFETIME

FREE RADICAL LIFETIMEFREE RADICAL LIFETIME

BREAKAGE OF BONDSBREAKAGE OF BONDSCHEMICAL REPAIR / MISREPAIRCHEMICAL REPAIR / MISREPAIRENZYMIC REPAIR / MISREPAIRENZYMIC REPAIR / MISREPAIR

EARLY BIOLOGICAL EFFECTSEARLY BIOLOGICAL EFFECTS

LATE BIOLOGICAL EFFECTSLATE BIOLOGICAL EFFECTS

1010-18-18

1010-12-12

1010-6-6

101000

101066

SECSSECSAbsorption of energyAbsorption of energy

Physical effectsPhysical effects

Chemical lesions Chemical lesions

Chemical repairChemical repairEnzyme repair/lesionEnzyme repair/lesion

Cellular effectsCellular effectsTissue effectsTissue effectsSystemic effectsSystemic effects

Days-YearsDays-Years

Hrs-DaysHrs-Days

Mins-HrsMins-Hrs

Ionization produces ions, ion radicals, and free radicals Ionization produces ions, ion radicals, and free radicals concentrated along tracks and especially at Bragg peak of concentrated along tracks and especially at Bragg peak of

primary and secondary electrons. They are highly reactive and primary and secondary electrons. They are highly reactive and cause damage to biological matter cause damage to biological matter

• IonIon - atom or molecule that has lost an electron and is charged. - atom or molecule that has lost an electron and is charged.• Free radicalFree radical - atom or group of atoms that contains an unpaired electron and is highly reactive - atom or group of atoms that contains an unpaired electron and is highly reactive• Aqueous electronAqueous electron - has lost kinetic energy and has been captured by water - a powerful reducing - has lost kinetic energy and has been captured by water - a powerful reducing

agent.agent.

1010-16-16

1010-14-14


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The Gray is the The Gray is the PhysicalPhysical Unit of RadiationUnit of Radiation

• 1 GRAY, the unit of absorbed dose (1 joule / Kg), 1 GRAY, the unit of absorbed dose (1 joule / Kg), – Causes 1-2 x 10Causes 1-2 x 1055 ionization events / cell ionization events / cell – 1% in DNA1% in DNA– A single cobalt 60 ray will deposit about 1mGy in a cellA single cobalt 60 ray will deposit about 1mGy in a cell

• Rad (Radiation Absorbed Dose) is the old unit = cGyRad (Radiation Absorbed Dose) is the old unit = cGy


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Direct and Indirect Direct and Indirect ActionAction of of RadiationRadiation

• Indirectly ionizing radiation can Indirectly ionizing radiation can actact directly or directly or indirectlyindirectly on biological targets on biological targets

• If the ion pairs and free radicalsIf the ion pairs and free radicals are produced in are produced in a a biologic targetbiologic target (DNA) then this is (DNA) then this is direct actiondirect action

• If water or other atoms or molecules are ionized, If water or other atoms or molecules are ionized, diffusible free radicals can act as intermediaries to diffusible free radicals can act as intermediaries to cause damage - this is cause damage - this is indirect actionindirect action


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p+

e-

photon

p+

photon

INDIRECT ACTION

DIRECT ACTION

Direct and Indirect Action of Ionizing Radiation on DNA

4 nm

2 nm

e-R.

H2O

OH.


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• Since HSince H22O is the major component in cells, the most common O is the major component in cells, the most common

ionization event is radiolysis of water, producing reactive oxygen ionization event is radiolysis of water, producing reactive oxygen species (ROS)species (ROS)• The most relevant water is within 2nm of the DNA and tightly The most relevant water is within 2nm of the DNA and tightly

boundbound

• ROS produced include: HROS produced include: H. . - reducing; OH- reducing; OH.. - oxidizing; HO - oxidizing; HO22.. - oxidizing - oxidizing

(O(O22 + H + H..); H); H22OO22 - oxidizing - oxidizing• The net effect is oxidation of cellular constituentsThe net effect is oxidation of cellular constituents

• About 60% of DNA damage caused by x-rays is due to ROSAbout 60% of DNA damage caused by x-rays is due to ROS

• About 75% of the indirect action of radiation is due to hydroxyl About 75% of the indirect action of radiation is due to hydroxyl radicals (OHradicals (OH..))

Reactive Oxygen Species (ROS)Reactive Oxygen Species (ROS)


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Free OHFree OH. . radicals generateradicals generate organic radicalsorganic radicals by: by:

– Addition Addition R + OHR + OH.. ..ROHROH

– Hydrogen abstractionHydrogen abstraction RH + OHRH + OH.. R R.. + H + H22OO

– Electron transferElectron transfer R + OHR + OH.. RR.. + OH + OH --

Where R is the organic moietyWhere R is the organic moiety


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Free Radicals and their Free Radicals and their Scavengers MatterScavengers Matter

• Biological effects of ionizing radiation are determined in large part by free Biological effects of ionizing radiation are determined in large part by free radicals radicals

• Free radicals are involved in many biological processes, including cellular Free radicals are involved in many biological processes, including cellular respirationrespiration

• We have defenses against free radicalsWe have defenses against free radicals– Endogenous free radical scavengers - most relevant within 2nm of the DNAEndogenous free radical scavengers - most relevant within 2nm of the DNA– Anti-oxidantsAnti-oxidants

• eg superoxide dismutase, especially in mitochondria, and catalaseeg superoxide dismutase, especially in mitochondria, and catalase

• Free radical scavengers can protect normal tissue from radiationFree radical scavengers can protect normal tissue from radiation– eg Amifostineeg Amifostine

• Depleting free radical scavengers will radiosensitizeDepleting free radical scavengers will radiosensitize• What interacts with free radicals, in particular radicals in biological What interacts with free radicals, in particular radicals in biological

materials will be important in determining outcome at this levelmaterials will be important in determining outcome at this level

• Oxygen interacts with free radicalsOxygen interacts with free radicals


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Oxygen MattersOxygen Matters

• Binds H radicals forming hydrogen peroxideBinds H radicals forming hydrogen peroxideHH.. + O + O22 HO HO22

.. (+HO (+HO22.. ) H ) H22OO2 2 (+O(+O22))

• Binds electrons to give superoxideBinds electrons to give superoxide

ee-- + O + O22 O O22-- + (H + (H22O) HOO) HO22

.. + OH + OH--

• Binds organic radicals to form peroxidesBinds organic radicals to form peroxidesRR. . + O + O22 RO RO22

.. (radical peroxide) (radical peroxide)RORO22

.. + R + R’’ H ROOH + R H ROOH + R’’ (hydroperoxide) (hydroperoxide)RORO22

.. + R + R’’.. ROORROOR’’ (peroxide) (peroxide)

Oxygen Oxygen ““fixesfixes”” the radical lesions in DNA in a form that can not the radical lesions in DNA in a form that can not be easily chemically repaired and therefore is a be easily chemically repaired and therefore is a very powerful very powerful radiosensitizer.radiosensitizer.


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Oxygen Enhancement RatioOxygen Enhancement Ratio (OER)(OER)

Dose required to produce a specific biological effect in the absence of oxygenDose required to produce a specific biological effect in the absence of oxygen

Dose required for the same effect in its presenceDose required for the same effect in its presence==OER varies with level of effect but can be 2.5 - 3 foldOER varies with level of effect but can be 2.5 - 3 fold

1) Culture Cells1) Culture Cells

((

3) Count cells in hemocytometer 3) Count cells in hemocytometer

4) irradiate under oxic or hypoxic conditions 4) irradiate under oxic or hypoxic conditions

0 Gy 2Gy 4Gy 6Gy0 Gy 2Gy 4Gy 6Gy

5) Plate cells and 5) Plate cells and grow for about 12 daysgrow for about 12 days

.. .. ....

...... ..

6) Count colonies6) Count colonies

Dose (Gy)Dose (Gy)

S.F.S.F.

0 2 4 6 8 100 2 4 6 8 10

1.01.0

0.10.1

0.010.01

oxicoxichypoxichypoxic

Physical Dose = Biological DosePhysical Dose = Biological Dose

2) Suspend Cells2) Suspend Cellstrysinization)trysinization)


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• Hypoxic areas Hypoxic areas occur almost solely in tumorsoccur almost solely in tumors and are more radioresistant than oxic and are more radioresistant than oxic areas. areas.

• Hypoxia contributes to treatment failureHypoxia contributes to treatment failure• Reoxygenation occurs between radiation dose fractions giving a rationale for dose Reoxygenation occurs between radiation dose fractions giving a rationale for dose

fractionationfractionation• The oxygen effect is greater for low LET than high LET radiationThe oxygen effect is greater for low LET than high LET radiation

Giacca and Brown

Pimonizadole (oxygen mimetic) staining colorectal carcinoma

The effects of hypoxia were first discovered in 1909 by Schwarz who showed that strapping a radium source on the arm gave less of a skin reaction than just placing it there. This was used to give higher doses to deep seated tumors.

Clinical Relevance of HypoxiaClinical Relevance of Hypoxia


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RADIATION QUALITY ANDRADIATION QUALITY AND BIOLOGICAL BIOLOGICAL

EFFECTIVENESSEFFECTIVENESS


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gamma raysgamma rays

deep therapydeep therapyX-raysX-rays

soft X-rayssoft X-rays

alpha-alpha-particleparticle

HIGH LETHIGH LETRadiationRadiation

LOW LETLOW LETRadiationRadiation

Separation of ion clusters in relation toSeparation of ion clusters in relation tosize of biological targetsize of biological target

LINEAR ENERGY TRANSFERLINEAR ENERGY TRANSFER

LET is average energy (dE) imparted by excitation LET is average energy (dE) imparted by excitation and Ionization events caused by a charged particle and Ionization events caused by a charged particle traveling a set distance (dl) - traveling a set distance (dl) - LET = dE/dl (keV/ LET = dE/dl (keV/ m)m)


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• A dose of 1 Gy will give 2x10A dose of 1 Gy will give 2x1033 ionization events in 10ionization events in 10-10 -10 g (the size g (the size of a cell nucleus). This can be of a cell nucleus). This can be achieved by:achieved by:

– 1MeV electrons 1MeV electrons • 700 electrons which give 6 700 electrons which give 6 ionization events per ionization events per m.m.

– 30 keV electrons 30 keV electrons • 140 electrons which give 30 140 electrons which give 30 ionization events per ionization events per m.m.

– 4 MeV protons 4 MeV protons • 14 protons which give 300 14 protons which give 300 ionization events per ionization events per m.m.

• The biological effectiveness of The biological effectiveness of these different radiations vary!these different radiations vary!

-ray

’-ray

excitation

ionization

particle

excitation and ionization


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Relative Biological Relative Biological EffectivenessEffectiveness (RBE) of the (RBE) of the

Radiation MattersRadiation Matters

Dose of 250 kVp x-rays required to produce an effectDose of 250 kVp x-rays required to produce an effectDose of test radiation required for the same effectDose of test radiation required for the same effect

==

S.F.S.F.

1.01.0

0.10.1

0.010.01

0.0010.001

DOSE GyDOSE Gy

High LETHigh LETLow LET, HDRLow LET, HDR



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Linear Energy Transfer (LET keV/Linear Energy Transfer (LET keV/m)m)

RBERBE(for cell kill)(for cell kill)

1000100010010010101100

22

44

66

88

RBERBE

DiagnosticDiagnosticX-raysX-rays

Fast Fast NeutronsNeutrons Alpha Alpha

ParticlesParticles

overkilloverkill

0.10.1

Co-60Co-60gamma raysgamma rays

00

11

22

33

44

OEROER

OEROER

OER is the inverse of RBE because OER depends considerably on the OER is the inverse of RBE because OER depends considerably on the indirect action of ionizing radiationindirect action of ionizing radiation

RBE is maximal when the average distance between ionization events = RBE is maximal when the average distance between ionization events = distance between DNA strands = 2nmdistance between DNA strands = 2nm

RBE and OER as a function of LETRBE and OER as a function of LET


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DNA is the Primary, but not the only, Cellular DNA is the Primary, but not the only, Cellular Target for RadiationTarget for Radiation

• Microbeam irradiation of cell cytoplasm Microbeam irradiation of cell cytoplasm does not generally cause cell death, but does not generally cause cell death, but irradiation of the nucleus doesirradiation of the nucleus does

• Tritiated thymidine incorporated into cells Tritiated thymidine incorporated into cells can kill themcan kill them

• Radiation-induced chromosomal Radiation-induced chromosomal abnormalities correlate with cell death abnormalities correlate with cell death and carcinogenesisand carcinogenesis

• However, irradiation of the cytoplasm is However, irradiation of the cytoplasm is not without biological consequencesnot without biological consequences


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Lesion size Lesion size about 15-20 about 15-20 nucleotidesnucleotides BlobBlob

7 nm diam.7 nm diam.12 ion pairs12 ion pairs

OH OH .. e eaquaqu

OH OH .. e eaquaqu

OH OH .. e eaquaquOH OH .. e eaquaquOH OH .. e eaquaquOH OH .. e eaquaqu

OH OH .. e eaquaqu

OH OH .. e eaquaqu

OH OH .. e eaquaqu

OH OH .. e eaquvaquv

OH OH .. e eaquaqu

OH OH .. e eaquaqu

OH OH .. e eaquaqu

SpurSpur4 nm diam4 nm diam3 ion pairs3 ion pairs100 eV energy100 eV energy95% of energy deposition events95% of energy deposition events

The lesions in DNA that are associated with cell The lesions in DNA that are associated with cell death and carcinogenesis after radiation exposure death and carcinogenesis after radiation exposure

are are largelarge

The high cytotoxic efficiency of ionizing radiation can be ascribed The high cytotoxic efficiency of ionizing radiation can be ascribed to the deposition of low levels of energy in small packets within to the deposition of low levels of energy in small packets within the DNA that cause lesions large enough to be fatalthe DNA that cause lesions large enough to be fatal


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SINGLE STRAND BREAK

1000 / CELL / GRAY

BASE CHANGE (eg C - U)BASE LOSS

1000 / CELL / GRAYBASE MODIFICATION(eg thymine/cytosine glycol)

SUGAR DAMAGE(abstraction of hydrogen atom)

INTRASTRANDCROSSLINK

0.5 / CELL / GRAYINTERSTRANDCROSSLINK

DNA-PROTEINCROSSLINK

1 / CELL / GRAY

*

DOUBLE STRAND BREAK

30/ CELL / GRAY


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• Not all ionization events are lethal!!Not all ionization events are lethal!!• As a rough guide the fraction of cells surviving 2Gy As a rough guide the fraction of cells surviving 2Gy

(SF(SF2Gy2Gy) is about 0.5) is about 0.5

• If the S.F. 2Gy is 0.5, what is the S.F. after 60Gy? If the S.F. 2Gy is 0.5, what is the S.F. after 60Gy?

= 0.5= 0.53030 = 0.9x10 = 0.9x10-9-9

• If the S.F. 2Gy is 0.7, what is the S.F. after 60Gy?If the S.F. 2Gy is 0.7, what is the S.F. after 60Gy?

= 0.7= 0.73030 = 2.2x10 = 2.2x10-5-5


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What is the Lethal Lesion?What is the Lethal Lesion?


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Repairable Sublethal DamageRepairable Sublethal Damage

X- or X- or -radiation is sparsely ionizing; most -radiation is sparsely ionizing; most damage can be repaireddamage can be repaired

4 nm4 nm

2 nm2 nm


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SingleSingle lethal hitAlso known as - type killing

4 nm4 nm

2 nm2 nm

Unrepairable Multiply Damaged Site

It is hypothesized that the lethal lesions are large double strand breaks with Multiply Damaged Sites (MDS) that can not be repaired. They are more likely to occur at the end of a track


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At high dose, intertrack repairable Sublethal Damage may Accumulate forming unrepairable, lethal MDS

Also known asAlso known as - type killing - type killing


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S.F.S.F.

1.01.0

0.10.1

0.010.01

0.0010.001

DOSE GyDOSE Gy

Low LET, HDRLow LET, HDR

Low Dose RateLow Dose Rateallows continuous SLDRallows continuous SLDR

Dose Rate MattersDose Rate Matters



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Chromatin Structure MattersChromatin Structure Matters

• Each cell contains about 2m of DNAEach cell contains about 2m of DNA

• The basic structure is the The basic structure is the nucleosome, nucleosome, which is 146 base pairs of DNA wrapped around 2 copies of histones H2A, H2A, H2B, H3, and H4H2B, H3, and H4

• Nucleosomes are in turn wrapped around other proteins to Nucleosomes are in turn wrapped around other proteins to form form compacted chromatincompacted chromatin

• Chromatin is maximally compacted during mitosis Chromatin is maximally compacted during mitosis

• Transcription requires decompaction to facilitate initiation Transcription requires decompaction to facilitate initiation (binding of transcription factors and RNAP II) and (binding of transcription factors and RNAP II) and elongationelongation

840nm840nm

minibanminibandd

- 30nm- 30nm


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Chromatin Structure and Chromatin Structure and Radiation ResponsesRadiation Responses

• Compact chromatin is more radiosensitive than non-compactedCompact chromatin is more radiosensitive than non-compacted–Mitotic cellsMitotic cells

• are 2.8 times more sensitive to DNA breaks than interphase cells are 2.8 times more sensitive to DNA breaks than interphase cells • have a lower OER (eg 2.0 compared with 2.8) have a lower OER (eg 2.0 compared with 2.8) • do not have much of a do not have much of a ““shouldershoulder”” on their survival curve on their survival curve

–Actively transcribing genes are less sensitive to damageActively transcribing genes are less sensitive to damage• Decompaction and compaction require acetylation and deacetylation of Decompaction and compaction require acetylation and deacetylation of

histones by acetyltransferases (HAT) and deacetylases (HDAC) histones by acetyltransferases (HAT) and deacetylases (HDAC) • HDAC inhibitors are entering the clinic as anti-cancer agents and can radiosensitizeHDAC inhibitors are entering the clinic as anti-cancer agents and can radiosensitize

• Radiation Damage to DNA is not randomly distributed. Radiation Damage to DNA is not randomly distributed. • It varies with cell cycle phase and level of gene expressionIt varies with cell cycle phase and level of gene expression


S.F.S.F.

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Dose (Gy)Dose (Gy)

LATE SLATE S

EARLY SEARLY S

G1 PHASEG1 PHASEG2/M PHASEG2/M PHASE


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12.5Gy 14.0Gy

15.5Gy 17.0Gy

Withers, H. R. and Elkind, M. M. Radiology 91:998, 1968Withers, H. R. and Elkind, M. M. Radiology 91:998, 1968

Used the macrocolony assay in mouse Used the macrocolony assay in mouse jejunum to assessed the effects of 2 jejunum to assessed the effects of 2 radiation doses given varying times apart radiation doses given varying times apart to measure the time to and extent of repair, to measure the time to and extent of repair, redistribution, and repopulation redistribution, and repopulation (regeneration) between dose fractions.(regeneration) between dose fractions.

RedistributionRedistribution

RepairRepair

RepopulationRepopulation

700R 1500R

Colony derived from a single surviving clonogen


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ACUTE RESPONDING TISSUESACUTE RESPONDING TISSUES(responses seen during standard therapy)(responses seen during standard therapy)

GutGutSkinSkinBone MarrowBone MarrowMucosaMucosaLATE RESPONDING TISSUESLATE RESPONDING TISSUES(responses seen after end of therapy)(responses seen after end of therapy) BrainBrainSpinal CordSpinal CordKidneyKidneyLungLungBladderBladder

Tissue Type MattersTissue Type Matters

Dose (Gy)Dose (Gy)

SurvivingSurvivingFractionFraction

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Late RespondingLate RespondingTissuesTissues

Acute RespondingAcute RespondingTissues and Tissues and Many TumorsMany Tumors



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Dose (Gy)Dose (Gy)

Dose FractionationDose Fractionation

242420201616121288440000

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SurvivingSurvivingFractionFraction

Single doseSingle doseLate responding tissuesLate responding tissues Single doseSingle dose

Acute responding tissuesAcute responding tissues

Fractionated doseFractionated doseAcute responding tissuesAcute responding tissues

Fractionated doseFractionated doseLate responding tissuesLate responding tissues

Dose fractionation spares late responding tissues more than acute Dose fractionation spares late responding tissues more than acute responding tissues and many tumorsresponding tissues and many tumors


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The Aim is to Increase Therapeutic The Aim is to Increase Therapeutic Benefit!Benefit!

ProbabilityProbabilityof tumorof tumorcontrol/control/

of normalof normaltissue tissue

damagedamage

Dose (Gy)Dose (Gy)A B CA B C

1.01.0

00

therapeutic benefittherapeutic benefit

Normal tissue complication dose-response curves are steep!Normal tissue complication dose-response curves are steep!


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Biological effectiveness of RT varies withBiological effectiveness of RT varies with

• Size of Dose (D) - (alpha and beta)Size of Dose (D) - (alpha and beta)• Size of Dose Per Fraction (d) - (alpha and beta)Size of Dose Per Fraction (d) - (alpha and beta)• Time over which it is delivered (T)- (alpha and beta)Time over which it is delivered (T)- (alpha and beta)• Time between fractions (t)Time between fractions (t)• Volume irradiated (V)Volume irradiated (V)• Quality of Radiation (Q) - RBEQuality of Radiation (Q) - RBE• Presence/Absence of Oxygen - OERPresence/Absence of Oxygen - OER• DNA Repair efficiency and completenessDNA Repair efficiency and completeness• Cell cycle phase and level of gene activationCell cycle phase and level of gene activation• Tissue/Tumor TypeTissue/Tumor Type



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4Rs OF RADIOBIOLOGY RELEVANT TO 4Rs OF RADIOBIOLOGY RELEVANT TO CLINICAL DOSE FRACTIONATIONCLINICAL DOSE FRACTIONATION

• Repair of sublethal damageRepair of sublethal damage- spares late responding normal tissue preferentiallyspares late responding normal tissue preferentially

• Reassortment/Redistribution of cells in the cell cycleReassortment/Redistribution of cells in the cell cycle– increases acute effectsincreases acute effects– no influence on late effectsno influence on late effects– increases damage to tumorincreases damage to tumor

• Repopulation/RegenerationRepopulation/Regeneration– spares acute responding normal tissue preferentiallyspares acute responding normal tissue preferentially– no influence on late effects,no influence on late effects,– danger of tumor repopulationdanger of tumor repopulation

• ReoxygenationReoxygenation– no influence on normal tissue responsesno influence on normal tissue responses– increases tumor damageincreases tumor damage


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Questions onQuestions onInteraction of Radiation with Biological Matter: Interaction of Radiation with Biological Matter:

what is biological dose?what is biological dose?


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The lifetime of radicals in target molecules is The lifetime of radicals in target molecules is aboutabout

1.1. 1010-3-3 secs secs

2.2. 1010-6-6 secs secs

3.3. 1010-9-9 secs secs

4.4. 1010-12-12 secs secs


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Electromagnetic radiation is considered Electromagnetic radiation is considered ionizing if it has a photon energy greater thanionizing if it has a photon energy greater than

1.1. 1.24 eV1.24 eV

2.2. 12.4 eV12.4 eV

3.3. 124 eV124 eV

4.4. 1.24 keV1.24 keV


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The S.I. unit of absorbed dose isThe S.I. unit of absorbed dose is

1.1. BecquerelBecquerel

2.2. SievertSievert

3.3. GrayGray

4.4. RoentgenRoentgen


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Which of the following are not Which of the following are not charged particles?charged particles?

1.1. ElectronsElectrons

2.2. NeutronsNeutrons

3.3. ProtonsProtons

4.4. Heavy ionsHeavy ions

5.5. Alpha particlesAlpha particles


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Which of the following is NOT a characteristic of the indirect Which of the following is NOT a characteristic of the indirect action of ionizing radiationaction of ionizing radiation

1.1. Production of diffusible free radicalsProduction of diffusible free radicals

2.2. Production of reactive oxygen speciesProduction of reactive oxygen species

3.3. Involvement of anti-oxidant defensesInvolvement of anti-oxidant defenses

4.4. A change in redox within a cell favoring reduction of A change in redox within a cell favoring reduction of constituentsconstituents


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Which of the following is true about the oxygen Which of the following is true about the oxygen enhancement ratio enhancement ratio 1.1. Is the same at all levels of cell survivalIs the same at all levels of cell survival2.2. Can be measured by the dog-leg in a cell Can be measured by the dog-leg in a cell

survival curve after single high dose irradiation survival curve after single high dose irradiation of tumorsof tumors

3.3. Is the ratio of doses needed for an isoeffect in Is the ratio of doses needed for an isoeffect in the absence to the presence of oxygenthe absence to the presence of oxygen

4.4. Is low for cells in S cell cycle phase compared to Is low for cells in S cell cycle phase compared to cells in G2/M phasecells in G2/M phase


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Which of the following is true about Linear Energy TransferWhich of the following is true about Linear Energy Transfer

1.1. It is a measure of the biological effectiveness of ionizing It is a measure of the biological effectiveness of ionizing radiationradiation

2.2. Correlates directly with the oxygen enhancement ratioCorrelates directly with the oxygen enhancement ratio

3.3. Is maximal at a relative biological effectiveness of 150 Is maximal at a relative biological effectiveness of 150 keV/micrometerkeV/micrometer

4.4. Is measured in keV/micrometerIs measured in keV/micrometer


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The Relative Biological Effectiveness of a The Relative Biological Effectiveness of a radiation isradiation is– Assessed by the dose required for to Assessed by the dose required for to

produce the same effect as 250kVp X-raysproduce the same effect as 250kVp X-rays– The ratio of the dose required of 250 kVp X-The ratio of the dose required of 250 kVp X-

rays to that of the test radiation for a given rays to that of the test radiation for a given isoeffectisoeffect

– Directly related to Linear Energy TransferDirectly related to Linear Energy Transfer– About 1 for alpha particle radiation About 1 for alpha particle radiation


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The lethal lesion caused in DNA by low LET The lethal lesion caused in DNA by low LET ionizing radiation isionizing radiation is

1.1. 15-20 nucleotides in size15-20 nucleotides in size

2.2. Caused by alpha-type eventsCaused by alpha-type events

3.3. Does not correlate with chromosomal Does not correlate with chromosomal aberrationsaberrations

4.4. Due to oxygen fixation Due to oxygen fixation


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Approximately how many DNA double Approximately how many DNA double strand breaks are caused per cell per Gray?strand breaks are caused per cell per Gray?

1.1. 1-101-10

2.2. 15-2515-25

3.3. 30-4030-40

4.4. 45-6045-60

5.5. 60-75 60-75


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If the fraction of cells surviving 2Gy irradiation is 0.5, If the fraction of cells surviving 2Gy irradiation is 0.5, what is a reasonable estimate of the percent of DNA what is a reasonable estimate of the percent of DNA double strand breaks that are effectively repaired?double strand breaks that are effectively repaired?1.1. 99%99%2.2. 95%95%3.3. 75%75%4.4. 50%50%


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If the fraction of cells surviving 2Gy is 0.4, If the fraction of cells surviving 2Gy is 0.4, what is the surviving fraction after 50 Gy what is the surviving fraction after 50 Gy given in 2Gy fractions?given in 2Gy fractions?

1.1. 1010-8-8

2.2. 1010-9-9

3.3. 1010-10-10

4.4. 1010-11-11


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Sublethal DNA damage is most likely to Sublethal DNA damage is most likely to accumulateaccumulate1.1. At high total doses given at high dose rateAt high total doses given at high dose rate2.2. At high total doses under hypoxiaAt high total doses under hypoxia3.3. After high LET radiationAfter high LET radiation4.4. After low fractionated doses of radiation After low fractionated doses of radiation


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Sublethal DNA damage is most likely to be repairedSublethal DNA damage is most likely to be repaired1.1. After high total doses given at high dose rateAfter high total doses given at high dose rate2.2. If cells are held in a non-proliferative stateIf cells are held in a non-proliferative state3.3. After high LET radiationAfter high LET radiation4.4. Between low fractionated doses of radiationBetween low fractionated doses of radiation


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Which of the following is true about chromatin structure in cellsWhich of the following is true about chromatin structure in cells

1.1. Compacted chromatin is more radiosensitive than non-Compacted chromatin is more radiosensitive than non-compacted chromationcompacted chromation

2.2. During mitosis cells decompact their chromatin and During mitosis cells decompact their chromatin and become radiosensitivebecome radiosensitive

3.3. Compact chromatin in S phase mediates radioresistancyCompact chromatin in S phase mediates radioresistancy

4.4. Compaction facilitates gene transcriptionCompaction facilitates gene transcription


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Which of the following is correct about Which of the following is correct about alpha-type cell killing following radiation alpha-type cell killing following radiation exposureexposure

1.1. It represents single lethal hitsIt represents single lethal hits

2.2. It is due to accumulated damageIt is due to accumulated damage

3.3. It requires intertrack interactionsIt requires intertrack interactions

4.4. It is not oxygen dependentIt is not oxygen dependent


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Which of the following radiobiological Which of the following radiobiological phenomena occurring between dose phenomena occurring between dose fractions has little or no effect on normal fractions has little or no effect on normal tissue radiation responses?tissue radiation responses?1.1. RepairRepair2.2. Redistribution of cells in the cell cycle Redistribution of cells in the cell cycle 3.3. RepopulationRepopulation4.4. ReoxygenationReoxygenation


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AnswersAnswers

1. NA2. 23. 34. 35. 26. 47. 38. 49. 210. 111. 312. 113. 314. 415. 416. 117. 118. 4

Documents

Objectives: Know the characteristics of ionizing radiation that make it useful for RT