Path 4400 Week 10: Ionizing Radiation

fold increase from sea level to low orbit, lunar, Van Allen Radiation belt and interplanetary space travel

low orbit lunar, interplanetary: ~ 1000x
Van Allen: 100 000 x

radiation symbols

e- = electron
e+ = positron
v = neutrino
ṽ = anti-neutrino ṽ
𝛼2+ = alpha particle (helium nucleus)

Particle Radiation - Electrons/positrons

- naturally in environment
- Beta-decay: C-14 (t1/2 = 5730 y) --> e- + ṽ + N-14
--> used for C dating, decays when organism dies
- positron-decay: C-11 (t1/2 = 20 min) --> e+ + v + B-11
--> uncommon in environment but used in med
- Beta-decay: K-40 (t1/2 = 10^9 yrs) --> e- + ṽ + Ca-40
--> in bones, radioactive

Particle Radiation - alpha particles (helium nucleus)

helium w/o 2 e- on outside
- U-238 (t1/2 = 4.5 x 10^9yrs) --> 𝛼2+ + Th-234 (t1/2 = 24 d)
- Rn-222 (t1/2 = 4d) --> 𝛼2+ + Po-218 (t1/2 = 3mths)
--> can breath it in
- Th-232 (t1/2= 14 x 10^9; entire life of earth) --> 𝛼2+ + Ra-228 (t1/2 = 6 y)

Particle Radiation - other

neutrons, protons

EM Spectrum from longest wavelenth to shortest

radio, microwave, infrared, visible light, ultraviolet, soft x-ray, hard x-ray, gamma ray

definitions - eV, c, E

eV: energy acquired by an electron accelerated across potential difference of 1 volt
- 1eV = 1.602 x 10^-19 joules (J)

c = lambda (wavelength) x v (frequency) = 3.0 x 10^8 m/sec
--> speed of light

E (energy) = v (frequency) x h (Planks constant = 6.62 x 10 ^-34)

EM radiation - ionizing radiation

Ionizing > 4eV
- less than 4eV and there is no change to the tissue

visible light: 1.77 to 3.1 eV
Infrared B: 0.6 eV
Microwave: 10^-4 eV

Linear Energy Transfer (LET)

amount of energy deposited per distance travelled
LET = dE/dI

dE = average energy locally imparted to the medium
dI = distance in medium over which dE is deposited

- units in thousands of electron Volts per micron (keV/u)

LET of 5.0 MeV 𝛼 particles

- from background radiation
- 100 keV/u
- can cause ds breaks

quality factor (Q) - X-rays, gamma rays, electrons/positrons, 𝛼 particles

X-rays, gamma rays, electrons/positrons: 1
𝛼 particles: 20
--> the damage in the same amount of energy deposited is 20 x higher than Q=1

Radiation Dose Units

Radiation Dose: D
--> 1 Gray = 1 Gy = 1 Joule/Kg of tissue (energy deposited in tissue

Equivalent Dose: H
--> 1 sievert = 1 Sv = DxQ (NOTE: gray is equivalent to Sv except wth alpha particles ie if Q does not equal 1)

Effective Dose Equivalent: He
He = Di (dose of individual tissue) x (Qi) x Wi (weighting factor)
--> to determine incidence of cancer/detrimental effects due to exposure

weighting factor used in He

- given in Q
- some tissues more sensitive than others

Mechanism of ionizing injury

- direct: breaks covalent bonds (ex. DNA strand breaks)
--> deposition of energy crosses DNA molecule
- indirect: ionization --> production of ions & free radicals which break covalent bonds (ex. reaction of hydroxyl radical w DNA)
--> diffuse away from site of creation

Ionizing Radiation - DNA as the target

- most important type of damage to DNA: breaks in backbone
--> ss break: easily and quickly repaired by enzymatic processes
--> ds Break: caused by single event or by two ss breaks less than 10 bp apart

Ionizing Radiation injury - micronuclei

- occur in dividing nucleated cells (ex lymphocytes)
--> spontaneous fragmentation & micronuclei occur naturally but occurs much more with exposure (increase in micronuclei induction as radiation dose increases)
- damage originates from lagging chromosomes or chromosome fragments at anaphase

Ionizing Radiation injury - comet assay

- micro-electrophoretic assay in which DNA damage can be assessed at the single cell level
--> DNA damage: ss and ds breaks
--> tail contains DNA fragments
- increase in mean tail length as the radiation dose increases (not v accurate tho, lots of noise)

Stochastic effect of ionizing radiation and model

- Hormesis: at low levels, body has mechanisms that makes radiation beneficial
- linear no-threshold hypothesis: any form of ionizing radiation causes damage
--> may be over-estimate --> some people may not get diagnostic procedure/treatment

source of annual effective dose of radiation - general

- most in Medical X-ray and nuclear
- some in Radon (basements, naturally occurring)
- some in Cosmic, especially if live high above sea level or fly a lot

deterministic effects of whole body irradiation

0-2 Gy: no effect, no treatment
2-7.5: bone marrow depression, treat w Abs, transfusions, nursing
--> 4: LD50 w no treatment
--> 7: LD50 w Abs and nursing
7.5-10: potential for bone marrow transplants as treatment
10-12: GI Death

Calculation of stochastic risk of fatal cancer

Fatal cancers for 1 Sv whole body (from table) x annual dose x population = fatal cancers from 1 yr of exposure in popn over their life time

Calculation of stochastic risk of fatal cancer for individual over years

Fatal cancers for 1 Sv whole body (from table) x annual dose x # of years = chance that person will die of cancer

where does most electricity in Ontario come from

nuclear energy

evidence against linear no-threshold hypothesis

- hyperthyroidism treatment w radioactive iodine does not increase cancer risk (actually lower than treatment w drugs and surgery)
- no bone sarcomas in radium dial painters for doses below 10 Gy or 10 Sv (we get 5mSv)
- risk of lung cancer is greater in those exposed to lower doses of residential radon

medical imaging applications of Radionuclides

- tagged onto biologically relevant molecules, then injected --> distribution measured
--> ex sugar b/c many metastatic tumours upregulate glucose usage
- want to decrease radioactive dose so use stuff w a shorter half life
--> may need to make on site. Ex N-13 has half life of 10 min (must make, test, inject, image in 10 min)

Short-lived PET tracers:

C-11 (20 min), N-13 (10 min), O-15 (2 min)
- v v sensitive (detect 1 in trillion)

longer-lived PET tracers

Ga-68 (68 min), F-18 (109 min), Zr-89 (3.3 days)

Prostate PET/MRI with radiotracers

- 18F-PSMA allows visualization of cells displaying antigen found on prostate cancer cells
- add heavy metal nucleus that emits 𝛼 particles after 10 day half life to an antigen specific for cancerous tissue

fission power reactor - uranium-235

1 molecule of U-235 (0.7% abundance) + thermal neutron (0.03eV) --> fission fragments + 2 plus neutrons + energy (200MeV)

* note, U-233 and Pu-239 will also fission on thermal neutron absorption

origin of fission and fusion energy - concept, deuterium, uranium

- binding energy of constituents of nucleus (to take out and put one constituent)
--> If you need to put energy in to take something out, energy will be released when putting it back in
- deuterium (H): releases tons of energy in fusion (but not fission b/c not strongly binding)

fission power reactor - energy from ethanol combustion and ratio of Uranium fission to ethanol combustion

- ethanol combustion: 1.5 x 10^-2 eV
- ratio: (200 x 10^6)/(1.5x 10^-2) = 1.33 x 10^10
--> ten billion times more energy in fission of uranium

fission power reactor - problem

- long lived fission fragment waste
--> 6% is Cs-137 (30 yr half life)
--> 3% is I-131
--> both are cancerous

fission power reactor - reactor waste

- fission fragments
- Plutonium-239 (t1/2 = 24,000 yrs)

fission power reactor - reactor waste (plutonium synthesis)

U-238 (99.3% abundance) + thermal neutron --> U-239 (unstable)
U-239 --> Np-239 + e- + ṽ (t1/2 = 23 min)
Np-239 --> Pu-239 + e- + ṽ (t1/2 = 2.3 days)

Pu-239 fissions by fast neutrons, can be used to produce nuclear weapons

fission power reactor - Thorium (general)

- more abundant than uranium
- Th-232: 100% isotopic abundance, t1/2 = 14 billion years
- does not fission when absorbing a thermal neutron

fission power reactor - Thorium (energy production

Th-232 + thermal neutro --> Th-233
Th-233 --> Pa-233 + e- + ṽ(t1/2 = 23 min)
Pa-233 --> U-233 (what we want) + e- + ṽ (t1/2 = 27 days)
U-233 + thermal neutron --> fission fragment + 2 plus neutrons + energy (200 MeV)

U-233 does not produce Pu-239 (half life of 160,000 yrs) --> reactor waste only problem for 100s of years, not 10s of thousands

Fission Production of Cs-137

U-235 + thermal neutron --> Te-137 + other fragment + neutrons + energy
Te-137 --> I-137 + e + ṽ (t1/2 = 3.5 s)
I-137 --> Xe-137 + e + ṽ (t1/2 = 24 s)
Xe-137 --> Cs-137 + e + ṽ (t1/2 = 4 min)

Cs-137: t1/2 = 30 yrs, 6% yield (decays to Ba-137)

Fission production of I-131

U-235 + thermal neutron --> Sn-131 + other fragments + neutrons _ energy

Sn-131 --> Sb-131 + e + ṽ (t1/2 = 63 s)
Sb-131 --> Te-131 + e + ṽ (t1/2 = 23 min)
Te-131 --> I-131 + e + ṽ (t1/2 = 25 min depending on how made)

I-131: t1/2 = 8 days, 3% yield (decays to Xe-131)

I-131 contributes to thyroid cancer

Chernobyl - deposition of radionuclide

- surface ground deposition of Caesium-137 on territories of Belarus, Russia, Ukraine and to a lesser degree, Europe

Chernobyl - confinement

- confinement sarcophagus being built and will be slid over reactor
- will fully encapsulate Unit 4 reactor, destroyed in 1986

Ukraine/Russia Was and bombing of Chernobyl Reactors

- could be simular to radioactive contamination produced by dirty bomb (combines radioactive material w conventional explosives
- distribution of contaminants could be similar to distribution of Cs-137 after reactor explosion
- factors affecting radioactive contamination
--> initial distribution of each contaminant on ground and air
--> wind speed & direction at different altitudes
--> precipitation
--> could contaminate Belarus and Russia more than Ukraine

- pathways of exposure from radioactive material release

- external dose direct from radioactive materials deposited in the ground
- external radiation direct from clouds
- internal dose from breathing in sea spray and sand
- internal dose from inhalation of radioactive materials in the air
- internal dose from eating and drinking radioactive materials in food (Primary source)

detection of body contamination from radiation

- use a Whole Body Shadow Shield Detector
--> made of pre-WWII Iron and Lead (prior to increased use of nuclear material so low background radioactivity)
--> moving bed under Large High-Purity Germanium Detector and two Sodium Iodine Detectors
-Detects Gamma-Rays; calibrated once/year by Health Canada

avg accumulated doses to affected popn from Chernobyl fallout

liquidators: ~100 mSV
Evacuees from highly-contaminated zones: 33 mSv
Residents of "strict-control" zones: >50 mSv
Residents of other "contaminated" area: 10-20 mSV

normal person gets 5 mSV

Whole Body Irradiation causing stochastic effects - table with the dose used in equations

Values in detriment x 10^-2/Sv

Adult workers
- Fatal cancer: 4.0
- non-fatal Cancer: 0.8
- severe hereditary effects: 0.8
- total: 5.6

Whole popn
- Fatal cancer: 5.0
- non-fatal Cancer: 1.0
- severe hereditary effects: 1.3
- total: 7.3

Chernobyl - thyroid cancer

- greater incidence in Belarus in children and adolescents exposed to I-131
- still an increase in Ukraine as well but not as much

Estimate He for thyroid cancer (not death)
incidence: 5.5 per 100,000
Risk: 6x10^-2/Sievert
Let y represent dose in He

6x10^-2 x 10^5 x y = 5.5
x = 0.92 mSv (whole body equivalent dose)

sensitivity of the thyroid is 0.05, hence thyroid dose is 0.92/0.05 or 18 mGy

Chernobyl - congenital malformations and teratogen effects

- no evidence of congenital abnormalities
--> compared 4 places n Belarus w high and low levels of contamination
- no consistent proof that teratogenic levels of radiation exposure (0.1 Sv) were attained
--> BUT: there was an increase in induced legal abortions

biological effects of ionizing radiation - stochastic vs deterministic

Stochastic
- probability of occuring, rather than severity
- function of radiation dose w/o threshold
Deterministic
- severity varies w dose
- usually, there is a threshold

Chernobyl - Hereditary Effects

- lack of trans-generational effects
--> no elevation in total de novo mutations in children born of parents exposed to parental dose

Chernobyl - contamination of predatory and prey fish

- increase in both but much more in predatory fish
--> possibly because of bioconcentration/magnification
- non-predators have pretty much returned to normal levels but predators still high

Chernobyl - large mammals

- elk, deer, boar popn similar to that on nature reserves
wolf popn 7 x higher

Fukushima - Caesium

- Caesium levels in bottom dwelling and freshwater fish are much higher than other fish types
- Still radioactive with highest concentration in and near Fukushima (Ibaraki, Miyagi)

Fukushima - radiation near powerplant

- one mile away: 330 uSv/hr
--> 3000 x above background, annual dose in 10 hrs
- koi fish: 240,000 Bq/kg (consumption limit is 100)
- genetic abnormalities in butterfly offspring

atomic bomb testing and regeneration of human heart tissue

- integration of C14 from nuclear bomb tests into cardiomyocytes of fetuses (and adults)
--> measured cardiomyocytes [C14], lower than expected based on C14 levels at birth --> new cardiomyocytes incorporated less C14 because less in environment. If they didn't renew, wouldn't have changed
- 1%/yr at age 25, down to 0.45%/yr at 75
--> fewer than 50% exchanged in normal life span but pvsly thought they did not renew

atomic bomb testing and regeneration of brain cells and animal age

- same process as cardiomyocyte renewal
- proved adult hippocampal neurogenesis i humans --> structural and functional brain plasticity in higher cognitive function

- used radiocarbon in eye lens nuclei of sharks to determine age