10/18/2015 Rad Bio: Radionuclides p. 1 of 70 Illinois Institute of Technology Physics 561 Radiation Biophysics, Lecture 10 Deposited Radionuclides; Exposures.

04/20/23 Rad Bio: Radionuclides p. 1 of 70

Illinois Institute of Technology

Physics 561 Radiation Biophysics, Lecture 10

Deposited Radionuclides;Exposures to Radiation

1 July 2014Andrew Howard


Class Overview Radionuclides, continued

– Routes of entry– Physics & chemistry of

nuclides– Dosimetry and activity– Tritium, noble gases– Alkali metals– Alkaline earths– Halogens– Uranium & plutonium

Exposure from natural and man-made sources

– Population dosimetry– Dose equivalent and

equivalent dose– Radiation weighting

factors– Natural sources– Man-made sources

04/20/23 Stochastic Damage; High-LET p. 3 of 70

Internally deposited radionuclides

Why radionuclides are studied in the context of internal deposition

• Exposure works differently from external exposure: acts over shorter length scales

• Often involves high-LET forms that would never have biological effects if they were external


How do they get in?

Ingestion - intake in food & water though GI tract & tracheal clearance

Inhalation - breathed-in radionuclides traveling through nasopharyringeal passages to the lung

Injection - only intentional(except in bad Hollywood movies)-only relevant in a few therapeutic contexts


Ingestion Intake through digestive system Various fates:

– Excretion Urine Feces

– Incorporated into blood,e.g. via glutathione conjugation

– Incorporation into lymph– Bile with radionuclides that have

collected into the liver out of the circulatory system can be secreted back into the digestive system


Inhalation

Respiratory system:3 compartments:– Nasopharyngeal (NP)– Tracheobronchial (TB)– Deep-lung parenchyma (P)

Deposition (graph sideways from book-fig. 15.2):)

% d

ep

osi

tion

Activity mean aerodynamic diameter, µm0.1 0.2 0.5 1.0 2.0 5.0 101

510

203050

7090

DT-B

DP DN-P


Inhalation: Fate of Radionuclides

Radionuclides enter respiratory system via nose & mouth

Travel through trachea Either travel farther down to bronchi & lungs or

are sent back up to be exhaled or swallowed Physical fate primarily function of size & shape Size Matters!


What happens to nuclides if they get into the deep lung?

Fate depends on chemistry If particles are moderately to very water-soluble,

they pass into the bloodstream readily– There’s a lot of surfactant (detergent) lining the lung

surface that helps to solubilize things– once in the blood, the compounds get metabolized

or cleared or both If the material is very insoluble it gets gobbled

up by macrophages– Particles go to lymph nodes inside macrophage– Ultimately the lymph empties into the blood


Physics and Chemistry of Inhaled Radionuclides

Shape matters, too! Biological response depends substantially

on shape because cells react very differently to needles as compared to cubes

– Asbestos: caused mostly by needle-shaped fibers, independent of their chemical nature

– Spheres of the same compounds would be harmless

– Macrophages respond peculiarly to needle-shaped particles

Surface area to volume ratios influence biological fate!


Chemistry of Radionuclides: General

Chemistry is neutron-independent, i.e. every isotope behaves identically (exception: 3H) . . . (until decay occurs)

Nuclides of elements without ordinary biological function are metabolized approximately like their nearest vertical neighbors in the periodic table

– Not entirely successful substitutions– Sometimes: Very small discrimination ratio

Alkali metals: Li, Na, K, Rb, Cs, Fr Elaborate mechanisms for handling K; none for Rb

so Rb tends to behave like K (but not like Na).


Chemistry and Metabolism

Periodic Table of the Elements

Actinide Series

Lanthanide Series


Dose to Specific Organs

Distribution over time can be complex It takes some time for each organ to receive

its dose, and as things clear (both physically and biologically) the concentrations will diminish

Note in fig. 15.3 that the 131I dose to the thyroid is not predominant over the others because it’s such a small organ


Fig.15.3, my version


Complex Case: 99mTc

Three emissions with different activities and energies

Effective value depends on tabulating individual contributions

Absorbed dose depends on applying this value in a Monte Carlo analysis of deposition in various organs: see tables 15.2 and 15.3 in the text

Further variability (beyond limitations of these models) come from the fact that real people aren’t identical to the “standard man”


Dosimetry of specific nuclides

We describe equivalent dose asHT,R = wRDT,R

where DT,R is the absorbed dose in tissue T from radiation type R, wR is the radiation weighting factor for this radiation type, and HT,R is the equivalent dose actually experienced by the tissue

This is similar to the concept of RBE except that it emphasizes that tissues respond differently to different types of radiation.


Tritium TP=12.3y Mostly in the form of water Turnover: TB = 10 days, like ordinary water Low-energy beta and it’s cleared quickly so

the hazard is pretty low. Some 3H can get incorporated into

macromolecules—that could be more severe. Hydrogens in different organic molecules

have different exchange rates; T~10-10s for hydrogens attached to N or O; T ~ minutes or forever for C-H hydrogens.


Example from Amino Acids

Serine has C-H, N-H, and O-H bonds in it.

Hydrogens attached to carbons are essentially non-exchanging

Hydrogens attached to N and O exchange in microseconds or faster

Exchange lives of some NH and OH are longer because they’re solvent-protected.


Exchangeable Hydrogens in DNA

The hydrogens on the ribose ring of DNA are not exchangeable: in fact, there are very few exchangeable H’s in DNA!(e.g. amine H’s)


Noble Gases: Krypton and Radon

85Kr common in nuclear power– Not incorporated in the body much

because it’s not very reactive– Therefore not a serious biological hazard

Radon is important:we’ll talk about it in chapter 16.


Alkali Metals Sodium isotopes are used in diagnostics 40K is an important background irradiator: see

chapter 16 Cesium (and rubidium) are produced in fission.

– 137Cs ended up in the atmosphere as a component of fallout

– Behave like potassium– TB ~ 50-150 days– TP ~ 30 y so biological clearance dominates– excretion has 2-component model


Two-Component Model

Model looks likeA(t) =A0[p1exp(-1t) +p2exp(-2t)]

Withp1 + p2 = 1


Alkaline Earths

These are typically divalent (2+) Be, Mg, Ca not very important except as research

subjects Sr has practical significance

– 90Sr is major component of fallout from nuclear weapons testing.

– Tp = 28 y (- to 90Y90Zr) so it’s dangerous

– 89Sr is important too (Tp = 50.5 d, - to 89Y)

– Sr is a Ca analog and tends to concentrate in tissues where Ca2+ is supposed to concentrate


Alkaline earths, concluded

140Ba (Tp =12.8d, - to 140La140Ce) common in fallout and reactor output; but it has a short half-life

226Ra (Tp=1600Y, to 222Rn218Po214Pb214Bi214Po210Pb210Bi206Tl206Pb) is important too


Sr and Ra retention and effects

Ra retained somewhat less than Sr,but a reasonable amount stays around for years

Did childhood leukemias increase in the US because of fallout in the 1950’s? Unclear; it’s hard to get unambiguous evidence of environmental effects on human health for anything except smoking.

Radium dial painters got sarcomas of bone and carcinomas of the sinus epithelium

224Ra was used in treatments after WWIIIt can be used to quantitate Pu carcinogenicity


Halogens: Iodine

Iodine is concentrated in the thyroid 131I is an important fission product

– Short physical half-life (8 days, - to 131Xe)

– Moves quickly through the food chain via milk

125I used in imaging & brachytherapy:Td=59.4 d, EC to 125Te


Iodine, concluded

Major releases of 131I:– Chernobyl (1986)– Windscale nuclear

plant in England (1957)– Flawed episode of West

Wing, season 7(“Duck & Cover”)

Radioactive iodine can be competed away with iodine in table salt


Uranium

Naturally occuring even though all its isotopes are radioactive

Precursor of other high-Z elements 238U is common— long half-life 235U is 0.7% of natural mixture

– percentage can be reduced (“depleted”)– Or enhanced (“enriched”)– Undergoes fission when bombarded with slow

(“thermal”) neutrons


Uranium, continued

Plentiful in reactors and weapons Toxic to kidney, independent of radioactivity Decay modes:

– 235U: emitter to 231Th … 207Pb, T1/2 = 7*108 y

– 238U : much more common (99.3%) emitter to 234Th … 206Pb, T1/2 = 4.5*109 y

In one year, 1g of 238UO2 produces 5.70*10-13 moles of 234ThO2 = 0.295 ng, generating 3.44*1011 depositions along the way = 1090 dps = 0.295 µCi.


Plutonium Two common isotopes:

238Pu: T1/2 = 86.4yand 239Pu : T1/2 = 24890 y

Inhaled Pu in lung:cancer, some lymphatic-system damage


Is Pu the most toxic substance? Is it the most toxic substance in the world?

– Available exposure routes limited– I wouldn’t want to eat it, but there are worse

toxicants even among metals; and biohazardous substances (e.g. aflatoxin) are much nastier on a per-g or per-mole basis

Exposure through fallout: 400 megacuries worldwide.But it still has to get inside us to do damage.


How nasty is Pu, really?

It’s probably about as carcinogenic as radium if it’s deposited in the skeleton

Lung tumors are likely with inhalation of large quantities;possibility of lavage exists as a mitigation

Depositions of Pu elsewhere might be cancerous if they stay around long enough

Kidney toxicant, like Uranium? Probably:the actinides have chemistries that are similar one to the other


Important nuclides, Z ≤ 80Nuclide mode product final T1/2 comment

3H - 3He 3He 12.33y reactor byproduct40K EC 40Ar 40Ar 1.28*109y naturally occurring85Kr - 85Rb 85Rb 10.8 y reactor byproduct87Rb - 87Sr 87Sr 4.75*1010y naturally occurring89Sr - 89Y 89Y 50.5d fallout90Sr - 90Y 90Zr 28.8y significant in fallout99m Tc 99Tc 99Ru 6h medical imaging125I EC 125Te 125Te 59.4d imaging reagent131I - 131Xe 131Xe 8.0d fallout: found in milk137Cs - 137Ba 137Ba 30.1y weapon byproduct140Ba - 140La 140Ce 12.8d* weapons, reactors

* Misstated in Alpen as 128d


Important radionuclides, Z > 80

Nuclide mode product final T1/2 comment219Rn 215Po 207Pb 4.0s gas; see next chapter222Rn 218Po 206Pb 3.8d gas; see next chapter223Ra 219Rn 207Pb 11.4d on 235U chain224Ra 220Rn 208Pb 3.66d on 232Th chain226Ra 222Rn 206Pb 1600y on 238U chain232Th 228Ra 208Pb 1.4*1010y pseudo-stable!233U 229Th 209Bi 1.59*105y detectable235U ,SF 231Th 207Pb 7.04*108y detectable; fissionable238U 234Th 206Pb 4.47*109y predominant U isotope238Pu 234U 206Pb 87.7y fallout; space vehicles239Pu , SF 235U 207Pb 24110y fallout, reactorsSF= spontaneous fission


Human exposures to ionizing radiation This is the last chapter in Alpen, but clearly it’s not the

final part of what we will discuss in this course We want to offer you a fuller understanding of some

special topics, including hormesis, as well as a brief introduction to general biochemistry.

Those additional lectures are intended to provide you with a better context for the material that Alpen offers.

Anyway …the point of the remainder of this lecture is to consider human exposures to ionizing radiation, whether they’re natural or anthropogenic.


What can and can’t we control?

Significant sources of risk from exposure to ionizing radiation to the population as a whole

– Natural background– Diagnostic applications of ionizing radiation

Anything else?– Therapeutic X-rays and isotopes:

few people so population dose is tiny– Consumer applications (e.g. Cathode Ray

Tube TV receivers): tiny per-person dose


The Meta-question:Are low doses worse than zero doses?

Clearly most people don’t get exposed to distinctly high doses of ionizing radiation

We’re going to spend some time thinking about how to quantitate and how to assess risk from various sources, particularly as part of natural and man-made background

But:Do we really know that these background levels pose a net risk; or is the hormesis concept operating here?


Population Dosimetry Biologically effective dose from n different sources:

We can describe the dose equivalent as H = i=1n DiQiNi

– Di is dose from source or radiation type i– Qi corrects for LET-dependent biological effectiveness– Ni corrects for nonuniformities in distribution and anything else

This is an old way of doing things... Reference source of exposure:

Whole-body exposure to rays Adjustments to dose depend on

– Type of radiation– Portion of person irradiated


Why Consider Equivalent Dose?

It enables us to compare radiation types & exposure modalities on a quasi-equal footing

Unit of equivalent dose: Sievert corresponds to the Gray for dose. Rem corresponds to rad.

1 Sv = 1 Gy if using reference exposure

Equivalent dose HT = RwRDT,R

(Might underestimate health risks of the reference source!)

High LET sources have high radiation weight factors, up to a point (see last week’s lectures)!


Radiation Weight Factors, wR

Radiation type wR

Xrays, rays 1e-, e+, µ 1Neutrons

– <10keV 5– 10-100keV 10– 100-2000 keV 20– 2-20MeV 10– > 20 MeV 5

Protons (non-recoil), > 2MeV 2 particles, fission fragments 20Relativistic heavy ions 20


Effective Dose

Effective dose E is defined so that the probability of cancer and genetic effects is the same no matter where and how uniformly the deposition occurs:E = TwTHT = TRwTwRDT,R

Implicit here is the concept that wR is independent of wT, which isn’t completely true; but it’s close enough given how vague the values are!


Tissue Weight Factors, wT

wT=0.01 wT=0.05 wT=0.12 wT=0.20

Bone surface Bladder Bone Marrow Gonads

Skin Breast Colon

Liver Lung

Esophagus Stomach

Thyroid

Remainder


What are we trying to do here?

These tissue weighting factors are taking into account the portion of the radiation that ends up in the relevant tissue

That, in turn, depends on the tendency to concentrate certain atoms in particular organs

But it also depends on the actual size (fraction of total body weight) associated with that organ

Consider iodine in the thyroid and calcium in bone; but most other nuclides are more promiscuous in their distribution.


Committed Equivalent Dose

ICRP idea looking at time-integral over time of equivalent dose rate in a specific tissue T following intake

Thus for a single nuclide absorbed at time t0,

HT() = ∫t0t0+ (dHT(t)/dt)dt

Usually is taken as 50 years for occupational exposures and 70 years for general public

Smaller values apply to already-aged populations


Committed Effective Dose

Add up the committed equivalent dosesover all the tissues irradiated

E() = T wTHT() =

T wT∫t0t0+ (dH(t)/dt) dt

Units for all of these are Sieverts wR is like Q in the dose equivalent definition

wT is like the N value in the dose equivalent definition


Collective Dose Collective Dose S: the aggregate dose associated with

an exposure received by a population of N individuals Thus if we know <E> and N then

(collective dose) S = <E>Nis proportional to expected number of cases of disease (assuming linear response and 0 threshold).

Multiple populations (subgroups i = 1, . . . N):S = iHiPi

Thus if at collective dose level E, the probabilityP(cancer) = 10-5, then in a population N = 106 we expect ~10 cancer cases

This notion works well with stochastic endpoints


Risk Factors

Weighting factor for any organ is the ratio of the risk for that organ to the total risk

These estimates involve– Fatal cancers– Genetic risk– Life shortening

Purpose:risk-weighted dose estimate for a mixture of types of radiation or for radiation of parts of the body


Total Stochastic Detriment, part I

Lifetime risk coefficients, 10-2 Sv-1

Organ Popu- wT Rad wT

lation WorkersBladder 0.29 0.040 0.23 0.042Bone Marrow 1.04 0.143 0.83 0.150Bone Surface 0.07 0.010 0.06 0.011Breast 0.36 0.050 0.29 0.052Colon 1.03 0.142 0.82 0.148Esophagus 0.24 0.033 0.19 0.034Liver 0.80 0.110 0.64 0.116


Total Stochastic Detriment, Concluded

Organ Popu- wT Rad wT

lation WorkersOvary 0.15 0.021 0.12 0.022Skin 0.04 0.006 0.03 0.005Stomach 1.00 0.138 0.80 0.145Thyroid 0.15 0.021 0.12 0.022Remainder 0.59 0.081 0.47 0.085Gonads (genetic) 1.33 0.183 0.80 0.145Grand Total 7.25 1 5.53 1


So let’s look at specifics Up to now we’ve been arming you with a

few analytical tools. Now we’ll look at actual sources of

background– Natural– Anthropogenic

Medicinal Occupational


A few natural sources matter: Soil-borne radionuclides Airborne radionuclides Cosmic rays from deep space

Total Exposure from these sources:

around 0.7 - 3 millisieverts per year Dominated by U-Th series (mostly Rn),

40K, 87Rb, Cosmic rays Varies significantly with altitude

External Natural Sources


Why 40K and 87Rb? Very long half-lives 40K:

– T1/2 is 1.3*109y, i.e. about 0.25* age of the earth– K common in earth’s crust so there’s a lot of 40K– 40K is only 0.01% of total K, though– 2 decay modes: EC or + + to 40Ar or - to 40Ca

87Rb (- to 87Sr)– even longer T1/2: 5.0*1010y (3* age of universe)– 27.2% of the natural abundance– But there isn’t much Rb in the soil


Radon as a natural source

Substantial concern in recent years about radon such that wR = 20 for Rn alphas and

decay products. n.b.: should we actually consider indoor

radon a natural source? Pay attention to fig. 16.1!

– Nuclear industry, consumer products, air travel are

trivial for the general population– Most of the natural background is Rn daughters


Distribution of Doses Natural background and

medical dose dominate This is a redrawing of Alpen’s

fig. 16.1, corrected.

An

nua

l dos

e, m

Sv

2.4

NaturalBack-ground

Medical

Fallout

NuclearIndustry

ConsumerProducts Air Travel

1


What are the natural sources?

Series Primordial radionuclides starting mostly from 238U, 232Th, and 235U;especially 222Rn from 226Ra

Nonseries primordial radionuclides: 40K, 87Rb Cosmogenic radionuclides:

Elements in earth’s crust or in atmosphere interact with cosmic ray: mostly 14C, 3H, 22Na (and 7Be)


Outdoor, Extracorporeal Sources

Mostly uranium-thorium seriesbut also some 40K

Varies widely from place to place:150-1400 µGy per year,depending on where you are

What matters is gamma emitters here: wR=1 because nothing else gets into the skin

Typical 222Rn exposure is 232µGy per year; with wR=20, that’s almost 5 millisieverts—high compared to Alpen’s published totals


How variable is the background? An extreme case:

Ramsar, Iran: 260 mGy (not mSv!) natural

background Mostly 226Ra in hot springs Some U, Th in minerals No evidence that the local

population is suffering from this exposure


Indoor, Extracorporeal Sources

Depend slightly on the method of construction– What is the source of the building material?– How leaky is the building (especially for 222Rn);

Well-insulated buildings deliver a high body burden because we’re trying not to have to heat them so much--so the gas stays in the house

– Therefore, as building practices improve, the Rn exposure increases

Almost all uranium-thorium series stuff


Inhaled Radionuclides

222Rn is the inhalation culprit Study in New York gave some big numbers

for total body burdens received:1.9-3 mGy/year, I.e. around 38-60 millisieverts!

This is actually equal to the occupational limit, so something is wrong: either we need to get radon out of our houses or we need to revise the occupational limits upward.


Nonseries Radionuclides

40K and 87Rb get into the body through ordinary metabolism. Beta emitters, wR=1.

Typical doses for 40K:– 180 µSieverts/year to gonads– 60 µSieverts/year to bone– 270 µSv per year to bone marrow

Typical doses for 87Rb:– 10 µSieverts/year to gonads– <10 µSieverts/year to bone– 10 µSv per year to bone marrow


Cosmic Rays and Cosmogenic Radionuclides

Varies a lot by location– The higher in altitude you are,

the more you get– Also, the portion contributed by neutrons

goes up as you go higher in altitude– Some variation by latitude and longitude


Cosmic Radiation and AltitudeDose and equivalent dose rate

go up with altitude

Tis

s ue

Do s

e E

q ui v

a le n

t R

ate

mS

v/y

1

2

0.5

Altitude, Kilometers0

0.25 0.25

0.5

1.0

2

Abs

orbe

d do

se r

a te

in a

ir, m

Gy/

y

4 km

SeaLevel

Chicago

Albu-querque

Mt. Whitney


World Summary (cf. fig. 16.4)

All but 5% of U-Th series is 222Rn daughters

An

nua

l equ

iva

len

t

dos

e, m

Sv

Cosmogenic nuclides

Cosmic rays 40K + 87Rb U-Th internal

U-Th external

1

Human exposures, worldwide


Artificial sources



Another perspective

Courtesy Emory University Radiation Protection program


Effective Dose, mSv Yr-1, US+Canadafrom natural sources

Source Lung Gonads Bone Marrow OtherTotal

WT value 0.12 0.25 0.03 0.03 0.03 1.0

Cosmic rad 0.03 0.07 0.008 0.03 0.13 0.27

Cosmogenic 0.001 0.002 — 0.004 0.003 0.01radionuclides

Terrestrial:

External 0.03 0.07 0.008 0.03 0.14 0.28

Inhaled 2.00 — — — — 2.00

Nuclides 0.04 0.09 0.03 0.06 0.17 0.40In the body

Totals 2.1 0.23 0.05 0.12 0.44 2.96


Man-Made Sources Major contributors to population dose are:

– Medical diagnostic procedures– Smoking (!) 210Pb, 210Po

Individual burdens:– Diagnostic– Smoking– Therapeutic use of x-rays,

Recall discussion ofadditive vs. multiplicative risk

I encourage you to read the details about anthropogenic sources, but I won’t test you in detail


Diagnostic X-rays

US bone marrow dose ~ 1mGy = 1 mSv US genetically significant dose ~ 0.3 mGy Methods getting better but more procedures are done Weighted annual effective dose ~ 0.36 mSv Age and gender-specific adjustments bring that down

to about 0.23 mSv. Values in 3rd-world countries are lower in spite of the

patients’ experience of higher doses per treatment


Nuclear medicine

US value ~ 140 µSv; lower elsewhere Includes nuclides for cardiac function tests, PET Effective dose is about 0.6 * diagnostic X-ray value

Dose,mGy or mSv

0 0.2 0.4 0.6 0.8

Genetically significant dose, mGy

Effective Dose Equivalent, HE, mSv

Bone marrow dose, mGy

Nuclearmedicine

DiagnosticX-Ray


Nuclear Power

Fuel cycle: mining, milling, refining, UF6,enriched UF6 (sublimes at 56.5ºC)

Fuel fabrication, power generation, reprocessing,waste disposal, fuel storage, transportation

These are different from medical background in that they’re regional rather than global

A sub-population is particularly at risk: namely, miners and neighbors of the facilities that handle these compounds

10/18/2015 Rad Bio: Radionuclides p. 1 of 70 Illinois Institute of Technology Physics 561 Radiation Biophysics, Lecture 10 Deposited Radionuclides; Exposures.

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