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04/21/23 tumor and normal-tissue responses p. 1 of 102
Illinois Institute of Technology
Physics 561
Radiation Biophysics
Lecture 7:Radiation sensitivities of tumors and
normal tissues
Andrew Howard24 June 2014
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Plans For This Class In vivo assays of normal tissue Acute lethal response Teratogenesis
Nonstochastic effects (chapter 10) Late effects on normal tissue (chapter 11)
ch 10
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Skin
Exposure of epidermis can be modeled with MTSH kinetics. We find Do ~ 4.35 Gy, n = 12.Note Dq = D0ln n = 10.81 Gy.
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Acute Lethal Effects (< 1 Month)
Fig. 10.1 Populations of cells D & E categories are responsible for most acute effects:
– D: mitotic, translatable in & out– E: mitotic, translatable out (stem cells…)
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How do humans die of acute radiation exposure?
Acute lethal dose typically above 5 Gy Organ systems affected:
– Blood-forming organs:sensitivity mostly dependent on cycle time
Cell type cycle timesensitivity
Granulocyte (PMN) 4 days very high
Platelet 12 daysmoderate
Erythrocyte 40 days low– Gastrointestinal organs– Central Nervous System– Lymphocytes
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Acute Lethal Effects (< 1 Month) Blood-forming effects
– 3 weeks– Not very repairable
GI - If not fatal within 10 days, recovery likely Central Nervous Systems: resuts are dose-dependent
– 1 Gy Vomiting Results from direct stimulation of neurons?
– 100 Gy Massive disorientation Death
1 - 2 - 5 Gy
Lethality
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Lymphocytes: a special case
Mature lymphocytes are fairly radiosensitive
This is unusual for a nondividing, terminal cell type
Some kind of “interphase death”
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A possible explanation
There’s a lot of p53 gene product produced in lymphocytes: maybe they’re being stimulated into dying with moderate radiation exposure
http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb31_1.html
p53 core domainPDB structure 2BIN
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A detour into molecular biology The “central dogma” of molecular biology involves: Replication (cellular DNA can be duplicated before mitosis) Transcription (DNA codes for messenger RNA and other types of
RNA) Translation (mRNA codes for protein synthesis in the ribosomes) So in prokaryotic (anuclear) organisms, it’s simple:
DNAgene
transcriptionmRNA message
(To ribosome)
translation
protein Gene product
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How this works at the base-pair level DNA Duplex:
DNA strand: 5’-A-T-T-C-C-G-3’DNA strand: 3’T-A-A-G-G-C-5’
DNA-RNA hybrid, produced by transcription of DNA:DNA strand: 5’-A-T-T-C-C-G-3’RNA strand: 3’-U-A-A-G-G-C-5’
Resulting RNA strand that can code for protein:RNA strand: 3’-U-A-A-G-G-C-5’
Note that DNA nucleotides are dA, dC, dG, dT;RNA bases are A,C,G,U (thymine is 5-methyluracil)
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In eukaryotes, it’s more complex
DNA genetranscription
mRNA Full-length message
(To ribosome)translation
protein Gene product
mRNA processing (splicing)Processed messagenucleus
cytoplasm
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Numbers matter, too!
Suppose the original gene was 2000 base-pairs long.
The full-length message will therefore be 2000 bases.
The truncated (processed) mRNA might be 720 bases.
The resulting protein would be 720/3 = 240 amino acids long.
Note that only 36% of the DNA is actually coding for protein in this case.
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Can it get messier? Yes.
One full-length message can be spliceosomally processed in multiple ways to produce several viable protein products
DNA genetranscription
mRNA
mRNA splicevariant #2
mRNA splicevariant #1
mRNA splicevariant #3
Protein product #1
Protein product #2 Protein
product #3
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Bcl3: an example Bcl3 is an important gene in regulation of
apoptosis and therefore in carcinogenesis and other developmentally-related pathologies.
Exists in multiple splice variants, all derived from a single gene.
Some variants stimulate apoptosis, others inhibit it!
See Gil Ast, “The Alternative Genome,”Scientific American, April 2005, pp. 58-65.
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Why do we talk about this here? Alternative splicing may look like a topic that only a true
molecular biologist would care about, but it isn’t. If a certain gene is being stimulated into producing more of
its starting mRNA, the protein-dependent outcome of that stimulation may be complex or even counter-intuitive, depending on this phenomenon
If ionizing radiation were to turn on the BCL3 gene, would that increase or decrease apoptosis? Unclear
Even if we knew that, would we know whether that would be radioprotective? Not necessarily.
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Gestational Radiosensitivity
Fig. 10.10 provides a long list of radiosensitivity data for various organs and organ systems
In some cases the maximum sensitivities are early, in others much later
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Teratogenesis & Fetal Development Teratogenesis - Embryological abnormality
monster It’s traditional to argue that the fetus is highly
radiosensitive, but it actually isn’t, compared to other rapidly dividing cell systems.
We need to distinguish among:– Damage to gametes before fertilization– Somatic damage to growing organism– Damage to mother that influences development
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Measurement Problems associated with Teratogenesis
Confounding effects(how can one tell these phenomena apart?)
– Genetic damage before fertilization– Damage to embryo/fetus– Damage to placental system
High background– High incidence of birth defects in unexposed subjects– 5% of all births involve some significant abnormality
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Why does a high background matter? With low background, a small dose-response
effect can be discerned even in the presence of some experimental error
With high and nonuniform background, it’s hard to pick the signal out of the noise.
Response(fetal abnormalities)
DoseResults with low background Results with high background
Response
(Background)
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Stages of Development
Single cell rather sensitive Conceptus
– 2 cell: Rather resistant:both cells are totipotent, i.e. capable of differentiation into all necessary tissue types
– All cells remain totipotent in the first few cell divisions; differentiation begins just before implantation
– Severe damage to embryo can prevent implantation Early differentiation: Onset of abnormalities
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Types of fetal damage
Severe mental retardation– Occurs in 1-5 Gy range?– Is this really teratogenic or does it
involve damage to mother at higher doses?
Microcephaly– Might occur even in 1 Gy range– Data supporting that are disputable
Other types of damage described in mouse studies Microcephalic woman:
in utero in Hiroshima when the bomb fell
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Possible explanation for radiation-induced microcephaly
Fujimori et al. (2008) Biochem.Biophys.Res.Comm. 369:953
Authors argue that a gene product called ASPM is downregulated by in utero exposure to ionizing radiation
Related to problems withmitotic spindle regulation
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Developmental Toxicity
This is an attempt to get you to think about the collected information in fig. 10.10.
Fig. 10.10 provides a long list of radiosensitivity data for various organs and organ systems
In some cases the maximum sensitivities are early, in others much later
Most effects on the eye (other than cataracts) occur relatively late
Most functional disorders are very late Cephalic disorders occur all through gestation
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Stochastic Effects of Radiation One of two overarching categories of damage,
particularly as it produces long-term effects: Percent of population affected by the exposure
may be dose-dependent-BUT-
Severity of condition in an affected individual is independent of dose
Cancer is traditionally regarded as stochastic,but that may be an oversimplification
Nonstochastic damage is damage that does display dose-response relationships in an individual
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Nonstochastic Effects on Normal Tissue
Severity of condition does show dose dependence Possible threshold dose (no effect below DT) Most important mechanism: disruption of vascularization
(Casarett model, Fig. 11.1)
Dose
Re
spo
nse
DT
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Casarett model for microvasculature
Sequence of events beginning with irradiation and ending with loss of function, susceptibility to disease, and death
Shown in Fig. 11.1 Note cyclic effect of fibrosis and secondary
vascular regression
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Casarett model, graphicallyIrradiation of
SensitiveTissue Volume
Direct Cell Killing
“Indirect” effect
Endothelial changesin fine vasculature
Increased endothelialpermeability
Aging ChangesMononuclear infiltration
and active fibroblast proliferation(inflammation & fibrosis)
Progressive fibrosis;increased diffusional barriers
Reduced microcirculatorycapacity & inhibited diffusion
Reduced parenchymal functionsecondary to microvasculature
and diffusion changes
Replacement fibrosis &secondary vascular
regression
Loss of function,susceptibility to disease, and death
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Vascular endothelium as target
Endothelial cells lining capillaries are a cell-renewal system, so damage there will hurt the organ that those capillaries supply with blood.
Types of damage:– Direct: interphase death of cells in wall
(DNA damage leading to apoptosis)– Indirect: interference with cell renewal
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Mechanism I
Yazlovitskaya et al. (2008) Cell Death & Differentiation 1-13
cytosolic PLA2 activated by radiation
increases LysoPC activation of Flk1 phosphorylation of AkT Activates ERK1 & 2 Modifies cell viability
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Mechanism II
Seeds of its own destruction:
Inhibition of the survival pathway leads to cell death and radiosensitization
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Gastrointestinal systems where this mechanism predominates
Esophagus Stomach Small & large intestine Rectum (not the only mechanism)
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Other systems where this mechanism predominates
Skin (dermal layer) &other epidermal mucosal organs
Liver (except for hepatitis) Kidneys (many other mechanisms) Lung (other mechanisms) Brain Spinal cord (low-dose effects are of this type)
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Functional Subunits
Concept: The fate of an organ depends on individual functional subunits (FSUs).
When all the stem cells that give rise to the functioning cells in a functional subunit die, then the functional subunit can’t continue to operate
Examples:– In the kidney: the nephron– In the lung: the alveolus– In the pancreas: a single islet of Langerhans– In the small intestine: a gastrointestinal crypt
Can we generalize this to all tissues? Maybe not.
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Other types of late damage
The assertion: If vascular damage were the whole story for the late effects of radiation, then the time of onset of late damage should be more or less the same for all organs. That’s false!
Stromal and parenchymal damage– parenchymal cells are those involved in the
actual function of an organ, e.g. the cells in the liver that actually filter out damaging chemicals
– Stromal cells are the support cells that undergird and provide morphological support for the parenchymal cells
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Non-endothelial late effects Rectum: thinning & perforation of rectum Epidermal layer of skin: desquamation Kidneys: complicated, multi-causal;
tubular disfunction in glomerulus unrelated to vascular disorders
Lung: killing of type 2 alveolar cells Spinal cord: fast paralysis involves damage
to myelin sheath around cord Eye: improper differentiation of lens fiber
cells leads to cataracts
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Summary of organ-specific effects
See table in the html documents on Blackboard for full summary on nonstochastic effects
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And now for something mostly different
We’ll move away from nonstochastic late effects to a discussion of treatment modalities for tumors.
Here we’re focusing on the fact that tumors are somewhat DNA repair-deficient and therefore respond differently to ionizing radiation than healthy tissues do
We’ll look, in particular, at dose fractionation as a component of treatment planning for cancer
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Fractionation Radiotherapy can’t wait for research:
people need answers now Even in the 1930’s and 40’s it was recognized
that there was an advantage in treating tumors to fractionate the dose, i.e. if the total dose you wanted to deliver was 5 Gy, you got a better therapeutic ratio if you delivered it in several small doses rather than all at once.
We’ll now explore some quantitative models of the relationship between damage and fractionation
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Power Law and Timing
Witte:measured dose D required to reach the threshold for skin erythema as a function of dose rate or number of fractions n:
Power law:lnD = a + blnn, i.e.D = ea+blnn = ea eblnn = ea elnnb
D = Qnb, where Q = ea.
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Power-law treatments, continued Strandqvist: total time
of treatment T:D = UT1-p; 1-p for skin was about 0.2.
Cohen: 1-p is tissue specific (0.30 normal, 0.22 for carcinomas); this enables radiotherapy!
Strandqvist modelU=2 Gy1-p = 0.2
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Normalized Standard Dose Ellis: tolerance dose D for
normal tissue is related to the number of fractions N and the overall treatment time in days, T:
D = T0.11N0.24
The value of is called the Normalized Standard Dose or NSD; it can be determined separately for each tissue and each treatment modality.
# of treatments
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What are we really doing here?
This is curve-fitting in its most unapologetic form.
As far as I know there is no attempt to attach physical meanings to the exponent (1-p) in the Strandqvist model.
Nor is there a reason to think there’s anything physically significant about the 0.11 and 0.24 exponents in the Ellis formulation
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Time of treatment and number of fractions
Clearly time and number of fractions are (anti-)correlated variables
BUT this approach can be helpful in treatment planning, at least within the range of conditions for which the formulas are valid.
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Can we do better than this?
Explicit accounting for damage in terms of repairability:
– Sublethal– Potentially lethal– Nonrepairable
Model suggests that the limiting slope of lnS vs D as you fractionate a lot is determined by the single-hit (nonrepairable) mechanism
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Effect of FractionationFig. 11.3: Repair capability;
limiting slope determined by fraction sizes < W
Sur
vivi
ng fr
actio
n
W X Y Radiation Dose
Limiting slope forfraction sizes < W orlow dose rates
Effective slope,fraction size X
Effective slope,fraction size Y
Limiting slopefor largedose fraction
0.1
0.01
0.001
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Douglas & Fowler
Used mouse-foot skin reaction to fractionated doses: ≤ 64 fractions , constant overall time
For an isoeffect, the following equation held: n( + 2) = where n = # of fractions, = dose per fractionnote: I’m using where Alpen uses D, to reduce potential confusion with the overall dose.
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Douglas-Fowler Assumptions
Repair occurs after single doses Biological outcome depends on surviving
fraction of critical clonogenic cells Every equal fraction will have same
biological effect
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Survival fraction,Douglas&Fowler formulation
lnS = n(Fe/a) Note that a is not . For an appropriate choice of a, Fe = 1/(n)
Single-dose cell survival is S = exp[(Fe/a) So we do an isoeffect plot of Fe vs. :
Fe = b + c
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Douglas & Fowler Survival Fraction, Continued
Thus lnS = n(b/a + c2/a) cf. Standard LQ model, assuming constant
effect per fraction: lnS = -n() Defining E = -lnS, E/(n) = +
1/(n) = /E + /E plot vs Fe = 1/(n to get /E, /E. In practical situations we may not be able to
measure E directly
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Fig. 11.4: finding /E, /E
/E = intercept = 1.75 Gy-1
/E = slope = 27 Gy-2
= ratio = 0.0648 Gy
1/to
tal D
ose
, Gy-1
Dose per fraction, Gy0
2
4
6
0.05 0.10 0.15
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Applicability We don’t have to be using an LQ model to work
with the Douglas-Fowler formulation; we just need a nonzero slope of lnS vs. D at low dose.
Thus MTSH doesn’t work:With MTSH, S= 1 - (1 - exp(-D/D0))n
For n > 1,dS/dD = -n(1-exp(-D/D0))n-1 at D = 0, dS/dD = -n(1-e0)n-1= -n(0)n-1 = 0.
For n = 1, S = exp(-D/D0)dS/dD = (-1/D0)exp(-D/D0)at D = 0, dS/dD = (-1/D0)e-0 = -1/D0 ≠ 0.
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Withers extension of Fe model
Define flexure dose as the dose per fraction below which no further protection is provided by interfraction repair.
It turns out the flexure dose is a multiple of (units are correct: is in Gy)
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Withers extension: results
Let’s pick a reference total dose Dref and a reference dose per fraction ref.
Then-lnSref = Nref(ref + ref
2),where Nref is the reference number of doses(Dref= Nrefref)
Then for a different total dose D and different dose per fraction , D = N ,-lnS= N (+ 2)
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Withers result In order for the reference regimen to have the same
effect as the test regimen, S = Sref, or -lnS = -lnSref
ThereforeNref(ref + ref
2) = N( + 2), i.e. Nrefref + Nrefref
2 = N + 2
But Nrefref = Dref and N = D, so Nrefref
2 = Drefref and N2 = D Thus Dref(ref) = D()
D/Dref = (ref)/ () = ( + ref)/()
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Withers plot (fig. 11.5)Comparison of three different Isoeffect curves,
depending on (with ref = 2 Gy):
Yellow: α/β=10 GyRed:α/β=3.33 GyBlue: α/β=1.66Gy
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Homework for later in July
[This is a variation on problem 1 of chapter 11 in the book. I don't understand the wording of Alpen's problem, so I made up my own version]
Suppose that the Ellis power law equation (11.2) is valid in a particular tissue. A typical tumor dosing regimen consists of twenty treatments over four weeks using weekdays only, i.e. 26 days from the first Monday through the last Friday. Thus if the total dose delivered is 60 Gy, we deliver 3 Gy in each of the 20 treatments.
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Homework for later in July, continued
(a) Assuming NSD=17Gy, calculate the tolerance dose associated with this regimen. Will we be able to deliver this treatment regimen without damage to the normal tissue?
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Homework, concluded
(b) If we wish to shorten the treatment time to three weeks (19 days from the first Monday to the last Friday) we will have to deliver larger doses per day, e.g. 60/19 = 3.16 Gy/day if we include weekends. If we allow more than one dose delivery per day we can reduce the dose delivered in each treatment back to lower levels, though (1.052 Gy/treatment). Calculate the number of doses we will have to deliver over the 19-day period if we wish to ensure that the full 60 Gy will be tolerated. Determine the dose per treatment.
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Stochastic Effects
These are defined as effects for which the percentage of the population affected by the exposure is dependent on dose
BUT the severity of the [medical] condition in an individual is independent of dose.
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Does cancer really work that way? Not entirely Fry (1976):
– Harderian gland tumors seldom invasive after low doses of low LET radiation
– More invasivity and metastasis after higher doses of low LET radiation
Ullrich & Storer (1979):maybe there’s a threshold dose
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Traditional View of Population Dose-Response Relationships
Notion is that there’s a nonzero slope at D=0, rather than a threshold:
Pro
babi
lity
of C
ance
r In
cide
nce
Dose
Background IncidenceNonzero slope at D=0
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Radiation Carcinogenesis in Animals Earliest tool in understanding radiation-induced
cancer Consider mice with leukemia brought on by ionizing
radiation (fig. 12.1):
Inci
den
ce(%
of p
op
ula
tion)
Dose, Gray
1 2 3 4
Raw incidence
Corrected for mortality50
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The Background Problem
(made-up data): Error bars make it impossible to
figure out which line is correct
Inci
denc
e, %
Dose, Gray
1
2
3
0.2 0.4 0.6
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In fact, it’s worse!
Substantial error in the dose values too in many cases!
Inci
denc
e, %
Dose, Gray
1
2
3
0.2 0.4 0.6
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Extrapolation to low dose The only reliable experimental
measurements are made at doses much higher than the levels for which we want to set regulatory limits. Therefore we extrapolate, somehow:
Exc
ess
Inci
denc
e,%
of
popu
latio
n
Dose, Gray
2 4 6
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Differential Sensitivity
Some individuals within a population are more susceptible than others
– To tumors– To other conditions
Why?– Defective DNA repair mechanisms– Problems in cell signaling– Lifestyle agents
(smoking, drinking, lack of exercise)– Genetic differences among individuals
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How does differential sensitivity affect dose-response relationships?
Dose
Tu
mo
r in
cid
ence
Response in normal population
Responsesin sensitive populations
Additive
Supra-additive
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Differential Exposure
Mean dose = 1 Gy Maximum dose = 10 Gy Minimum dose = 0 Gy Mode of dose distribution =
1.2 Gy
Fra
ctio
n r
ece
ivin
g g
iven
dos
e
Dose, Gy0 1
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Upton’s Summary of the Animal Data
Neoplasms of almost any type can be induced by irradiation of a suitable animal in a suitable way.
Not every type of neoplasm is increased in frequency by irradiation of animals of one strain.
Carcinogenic effects are interconnected through a variety of mechanisms.
Some mechanisms involve direct effects on the tumor-forming cells; others don’t.
High-LET radiation produces dose-dependent rather than dose-rate-dependent effects
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Upton, continued Development of tumors is multicausal and multistage;
effects of radiation may be modified by other agents. Low to intermediate doses produce no tumors unless
promoted by other agents. At high doses the effect is suppressed by sterilization
of potentially transformed cells; this causes saturation. Time distribution of appearance of tumors varies with
type of tumor, genetics and age, conditions of irradiation.
Dose-response curves vary significantly.
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Events from transformationto mutated cells (fig. 12.2)
Many factors influence events up through malignancy
Radiation event:dose, dose rate, quality
Mutagenic eventsin cell
Killing orsterilizingof the cell
Nonproliferating
Oncogenes &Tumor Suppressor
Genes
Cells with oncogenic mutations RepairViral
Activation
Repair
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Mutations through Malignancy Additional influences seen
Cells with oncogenic mutations
HormonesCell Cycle State
Proliferative stimuliOther mutations,
radiation,and/or chemicals
Malignancy withfull autonomy of growth
Neoplasia
Clonal selectionAltered immune state
Mitosis
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Tumors: Definitions
Tumor: abnormal, de-differentiated cellular proliferation
– Benign: small mass reaches a certain size and then stops growing
– Malignant: those capable of uncontrolled growth & metastasis
Cancer: a malignant tumor Carcinogen: a chemical or physical agent
that increases the likelihood of cancer
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Cancer: Prevalence and Significance
550,000 cancer deaths per year in the US 20-40% caused by environmental and workplace
agents Others caused by smoking, diet, and natural causes Teasing apart these statistics is tricky:
– Probability of any individual getting cancer under a particular set of circumstances is small
– Multistage model makes multiple causes likely
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Inci
den
ce,
% o
f po
pul
atio
n
1.5 3.5 4.7
20
40
60
Observed
Adjusted for
Intercurrent mortality
Fig.12.1: myeloid leukemia in mice
Tumors and Radiation Stochastic late effects (cf. earlier in this lecture)
– Are these effects truly stochastic?– Even with cancer, there exists some dose-response effects in
the individual
Dose, Gy
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Tumors and Radiation (Cont’d) Is there a threshold?
– Probably not (but is this a red herring?)– Not at the population level
Serious Inquiry: the ED01 experiment Brown & Hoel, Fundamental & Applied Toxicology 3: 458 (1983)
Po
pula
ti on
re
spo
nse
Dose
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Upton’s rules (remember?) Irradiation can produce almost any kind of neoplasm if we do it
right Not every type of neoplasm has its incidence increased by
irradiation of animals of any one species or strain Carcinogenic effects depend on a variety of mechanisms Some effects are direct, some are indirect Incidence rises more steeply with dose for high-LET radiation
than for low-LET radiation Irradiation interacts with other causative agents Promotion may depend on other agents
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How do Cancers Begin?: The Clonal Theory
In general, mutational events in a single cell are sufficient to begin the process of tumorigenesis
Often several mutations must arise in order for cancer to be a likely outcome
Generally the mutation must be in one of the 50 or so genes that control cell replication and differentiation
The mutagenic events are never enough to guarantee development of cancer(but that still leaves open the possibility that radiation could cause cancer all by itself, if it can act as a promoter too …)
Mutations must be followed by promotional events, which stimulate uncontrolled cell division
Events from transformationto mutated cells (fig. 12.2)
Many factors influence events up through malignancy
Radiation event:dose, dose rate, quality
Mutagenic eventsin cell
Killing orsterilizingof the cell
Nonproliferating
Oncogenes &Tumor Suppressor
Genes
Cells with oncogenic mutations RepairViral
Activation
Repair
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Mutations through Malignancy Additional influences seen
Cells with oncogenic mutations
HormonesCell Cycle State
Proliferative stimuliOther mutations,
radiation,and/or chemicals
Malignancy withfull autonomy of growth
Neoplasia
Clonal selectionAltered immune state
Mitosis
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Modifying Factors
Immune system
Hormonal effects
Oncogenes
Oncogenic viruses
Environmental factors
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How Cancers Develop: The Multistage Theory
Initiation– DNA damage– e.g. Intercalators
Promotion– Generally not mutational– Involves changes in control systems, e.g. arachidonic acid
cascade– Tumors are present and capable of metastasis but
haven’t necessarily metastisized Progression
– Development of metastatic tumors
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Potentiation of Effect of Radiation by Smoking
Inquiry into lung-cancer incidence among uranium miners and nearby office workers. Smokers and nonsmokers were surveyed.
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How do we study radiation-induced carcinogenesis?
Induction and progress of cancer in experimental animals
Transformation of cells grown in tissue culture Human epidemiological studies
– Accidental exposures:Radium-dial workers, Chernobyl victims,foot fluoroscopes
– Medicinal exposures– Atomic bomb victims
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What Constitutes a Cancer?
Morphological change Cell immortality (escape from apoptosis) Tumorigenicity, i.e. spread of
undifferentiated cells
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Oncogenes Genes that are activated or show enhanced
expression in tumors Limited data showing connection between
human radiation-induced tumors and activation of oncogenes
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ED01 study
We mentioned this a bit earlier Study run by scientists at the National
Institute of Environmental Health Sciences in Research Triangle Park, North Carolina
BALB-C mice analyzed for liver tumors Test compound was 2-acetylaminofluorene,
a known carcinogen in rodents:
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ED01 study, continued
24000 mice in various exposure groups Endpoints and elements of study:
– Time to tumor incidence– Dose “fractionation” (but this is a chemical)
Sophisticated statistical analyses:– Initial analyses around 1981– Re-analysis a few years later
Compared various dose-response models
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ED01 quantitation
Analyze tumor incidence according toP(t,d) = 1 - exp(-F(t,d)), wheret = time and d = dose.
P, the tumor incidence fraction, behaves like1-S in our survival curve studies
Some analyses suggest that 2-AAF is primarily a promotor, not an initiator
– So it isn’t a great model for what radiation does…– But it still illustrates the importance of careful statistics!
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Experimental Systems for Studying Rad-induced Tumors
We need these because we can’t deliberately do high-dose experiments on humans!
CHO cells– Chinese Hamster Ovary– Good for looking at early effects--Initiation– Difficult to model the promotional events.– Transformation results in loss of contact inhibition
Mouse embryo fibroblasts– Immortalized– Still display contact inhibition
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CHO Cells (Continued) Key assay: resistance to contact inhibition
radiationor chemicals
No radiationor chemicals
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Mouse Embryo Cells:
Experiment: growing total confluence Lose contact inhibition? Can induce tumors in syngeneic animals
Limitation in both systems:– Fibroblasts (mesenchymals)– Most human tumors are epithelial
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Mutagenesis
Many chemicals, as well as radiation, can be shown to cause mutations.
It is therefore logical to test for mutagenicity as a first-stage inquiry into the likelihood that a compound or a radiation treatment might be carcinogenic
Standard mutagenic test:The Ames test (developed by Bruce Ames), in which Salmonella cells are exposed to a chemical and mutation rates in the cells are measured.
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Is an Ames Test a Good Substitute for These Complex Systems?
No! 1,8-dinitropyrene is the
most mutagenic substance known in the Ames test; yet it is only weakly tumorigenic in rats.
NO2
NO2
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Why might we care about dinitropyrene?
Most mutagenic substance known in Salmonella strain TA98: 72900 revertants/nanomole
Nitroarenes like this one were found to be present in used toner, i.e., combustion waste from Xerox toner
When this appeared, Xerox chemists reformulated their toner to drastically reduce the nitroarene content in the used toner.
Mermelstein (1981) Mutation Research 89:187-196. Löfroth et al(1980) Science 209:1037-1039 and
Mermelstein et al (1980) Science 209:1039-1043. So: all’s well that ends well!
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This is also a story about enzyme induction
Nitroarenes like dinitropyrene and other polynuclear aromatic hydrocarbons, (e.g. benzo(a)pyrene) are known to be inducers of enzyme activities
Some of these enzyme activities actually activate toxicants rather than detoxifying them
Most of the activity of these enzymes will detoxify; But if 1% makes things worse, we want to understand that
1% activation So we found that pretreatment with these compounds
could induce subsequent binding of other compounds to mouse DNA:Howard et al (1986), Biochem. Pharm. 35: 2129-2134.
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Animal Cell-Line Cancer Studies How similar are these rodent cell systems
(CHO, mouse) to human cells? Answer: Human cells:
– Are more resistant to spontaneous immortalization
– Tend to give more nearly linear responses to dose
– Radical scavengers and cold don’t protect as much:That suggests that direct mechanisms prevail in humans and indirect mechanisms are more important in rodents
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More on humans vs. rodents
High-LET studies indicate that repair is less effective in humans than in humans
Is the timescale a factor in that? Humans live a lot longer than rodents.
Promotion can be studied in animal cells, along with initiation
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Radiation Carcinogenesisin Human Populations
Occupational: radiologists, miners, dial painters Medical exposures:
– Ankylosing spondylitis– Nonmalignant disease in pelvis and breast– Multiple fluoroscopies in to chest (e.g. in TB patients)– Infants & children with enlarged thymus and ringworm– Children exposed in utero to diagnostic X-rays
Nuclear accidents and weapon detonations Environmental background (see last chapter)
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Dose-Incidence in Cancer Studies We seek a relationship relating post-exposure
incidence ID to dose D and normal incidence In Model might be: Linear: ID = In + 1D
Quadratic: ID = In + 2D2
LQ: ID = In + 1D + 2D2
Corrected for loss of clonogenic potential: ID = (In + 1D + 2D2)exp(-1D+2D2)
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Linear, Quadratic, LQ Models
We try to devise low-dose models based on high-dose data, where the three models are close together. It’s often difficult:
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Latency Definition (in the cancer context):
Time between the mutational events that began cellular transformation and the appearance of a medically observable malignancy
How long in humans?– A few years (blood or lymphatic cancers)– 15-30 years for solid tumors– Animals: scale these numbers to animal’s lifespan– These numbers are minima:
leukemia can take > 15 yrs
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