Tumor Responses to RT
Bill McBride
Dept. Radiation Oncology
David Geffen School Medicine
UCLA, Los Angeles, Ca.
wmcbride@mednet.ucla.edu
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Determinants of Tumor Cure
• Size of the clonogenic pool (stem cells)
• Intrinsic radiosensitivity
– S.F. 2Gy (pro-apoptotic tendency?)
• Repair
– T1/2 (HR, NHEJ, SLDR, PLDR, fast and slow repair?)
• Rate of repopulation/regeneration during therapy
– Tpot (L/I., Ki67?)
• Reoxygenation (extent of hypoxia)
– PO2 (dependence on tissue type, vascularity?)
• Redistribution
– Growth fraction (dependence on cell type, growth factors?)
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Determinants of Tumor Cure (continued)
Heterogeneity:
• Biological
– Number of clonogenic “stem cells”
• Intrinsic radiosensitivity
• Proliferative potential
– Tumor microenvironment
•
•
•
•
Hypoxia
Metabolism
Host cell infiltrates
Interstitial pressure
– Genetic
• Oncogenes
• Tumor suppressor genes
• Single Nucleotide Polymorphisms (SNPs)?
• Physical
– Dose heterogeneity
– Geographic miss
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TD50 Assay
1. Inject varying numbers of tumor cells into mice
2. Determine the number of cells that are needed to form tumors in 50% of mice.
To grow, tumors must have
arisen in that specific strain of
mice, or the mice must be
immune deficient. Even then,
not all tumors will grow, and
most need an inoculum size of
at least 104 cells
100
Percent of
mice with 50
tumors
0
Concept: Only cancer
“stem” cells will grow
10 102 103 104 105 106 107
Size of tumor inoculum
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Renewing stem cell
stem cell
Non-stem cell
Tumor cure
Tumor regeneration
from stem cell pool
The cancer stem cell hypothesis suggests that there are a small
number of clonogenic stem cells in a tumor and that, if they are
therapy-resistant, they are responsible for recurrences, and
accelerated tumor repopulation during therapy.
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MCF-7 Breast Cancer Stem Cells are Radioresistant
and are enriched Following Irradiation
“Stem” cells
At least some human tumors have a clonogenic subpopulation with
stem-like characteristics that can be grown in cytokines as spheres
and that are radioresistant and are selected for by fractionated
irradiation. Phillips et al J Natl Cancer Inst 98:1777, 2006
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TCD50 Assay
1. Inject mice with enough cells to form a tumor
2. Irradiate when 6mm diam
3. Determine the dose of radiation that is needed to cure 50% of mice.
100
Threshold-sigmoid curve
that goes from 10% to
90% cure over about
10Gy in a clinical
fractionation scheme
(which is hard to do in
mice).
Percent of
mice with
50
tumors
0
0 10 20 30 40 50 60 70 80
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Gy
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Tumor Control Probability
• In order to cure a tumor, the last surviving clonogen must be killed,
and even then it is a probability function of dose.
• TCP = e-x = e-(m. SF)
or e-m.e-(ad+bD2) or e -(m. e -(D/D0))
– Where x is the number of surviving clonogenic stem cells,
– m is the initial number of clonogens
• If there is an average of 1 cell surviving TCP=37%
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Heterogeneity in Radiosensitivity
100
80
TCP (%)
N=10
9
SF 2=0.7
60
SF 2=0.6
SF2=0.5
40
SF 2=0.4
SF 2=0.3
20
0
0
10
20
30
40
50
60
70
80
90 100 110 120 130
DOSE (Gy)
Rafi Suwinski
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Heterogeneity in Clonogen Number
100
SF2Gy = 0.5
80
N=10
TCP (%)
9
10
N=10
60
11
N=10
40
Average
20
0
0
10
20
30
40
50
DOSE (Gy)
60
70
80
90
Rafi Suwinski
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100
METASTASES RISK
PERCENT REDUCTION IN
Micrometastatic Disease
SF2Gy =0.5
80
N=10
N=103
5
N=10 8
60
40
20
0
0
10
20
30
40
50
60
70
DOSE (Gy)
Heterogeneity in tumor volume
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Tumor Growth and Regression
The kinetics of tumor growth and regression depend upon
•
•
Cell cycle
Growth fraction (G.F.)
• G.F. is the proportion of proliferating cells
• G.F. = P / (P + Q) where P = proliferating cells and Q = nonproliferating cells (quiescent/senescent/differentiated cells)
•
Cell loss factor
• Cell Loss Factor  measures loss of cells from a tissue
• If = 0, Td = Tpot where Td is the actual volume doubling time
and Tpot is potential volume doubling time
• = 1 - Tpot / Td
• if G.F. = 1 then Tpot = Tc
• Under steady state conditions, constant cell number is maintained
by the balance between cell proliferation and cell loss i.e.  = 1.0.
In tumors (and embryos)  < 1.0
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Tumor Kinetics
Human SCC
Tc
Cell cycle time
36 hrs
G.F.
Growth fraction
0.25
Tpot
Pot. doubling time
6 days
Actual doubling time
60 days
Td
Cell loss factor
0.9
(36hr x 4)
(1-6/60)
Rate of tumor growth and rate of tumor regression after therapy are determined
largely by the cell loss factor, that varies greatly from tumor to tumor
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Tumor Growth and Regression
•
•
•
Slow growing tumors may regress rapidly
Slow regression is not an indication of treatment failure
Rapidly growing tumors would be expected to regress and regrow
rapidly
•
In general, the rate of tumor regression after Tx is not prognostic
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Tumor Regeneration
Relative tumor
volume
20Gy X-rays
Control
Irradiated
Growth delay
Surviving clonogens
measured in vitro
Time
Tumors can
regenerate at the
same time as
they regress!
Rat rhabdomyosarcoma
Hermans and Barendsen, 1969
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The regrowth rate of
surviving clonogens varies
with the surviving fraction
- Lewis Lung Carcinoma
(Stephens and Steel)
Control
15 Gy
25 Gy
35 Gy
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EVIDENCE FOR ACCELERATED
REPOPULATION IN TUMORS
• After RT, tumors recur faster than than would be
expected from the original growth rate
• Split-course RT often gives poor results
• Protraction of treatment time often gives poor
results
• Accelerated treatment is sometimes of benefit.
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Accelerated Tumor Repopulation
T2 T3
T2 T3
local control
no local control
70
Total Dose
(2 Gy equiv.)
local control
55
no local control
Withers et al, 1988
Maciejewski et al., 1989
40
Treatment Duration
• T2 and T3 SCC head and neck (excluding nasopharynx and vocal cord). TCD50 values
are consistent with onset of repopulation at 4 weeks followed by accelerated
repopulation with a 3-4 day doubling time, implying a loss in dose of about 0.6 Gy/dy
• If the red line is correct, onset may be about day 21 and repopulation may not be
constant. It may increase from 0.6 Gy/dy around week 3-4 to even 1.6 – 1.8 Gy/day
around week 6-7.
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Tpot in a Large Multicenter HNSC Trial
476 patients
(Begg et al 1999)
• It was thought that shortening treatment time by accelerated
hyperfractionation and that this might be predicted by Tpot , but a
large multicenter trial was unable to confirm this
• But note that Tpot in HNSCC was 3-5dys for most patients,
confirming the potential for very rapid growth
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Sources of Heterogeneity
• Biological Dose
– Number of clonogenic “stem cells”
• Intrinsic radiosensitivity
• Proliferative potential
– Tumor microenvironment
• Hypoxia
• Metabolism
• Physical Dose
– Need to know the importance of dose-volume
constraints
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•
History
– 1909
• Schwarz - radium dose on human skin
– 1930-1950
• Gray, Mottram, Flanders - oxygen effects in biology
– 1955
• Thomlinson & Gray - tumor cords
– 1960-1965
• Powers & Tolmach - survival curves in vivo
• Churchill Davidson - HBO in patients
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Hypoxia in Tumors
• Chronic hypoxia is a result largely of
– Limited O2 diffusion due to
• oxygen consumption (”diffusion limited hypoxia”)
• irregular vascular geometry
• Acute/transient/intermittent hypoxia is a result largely of
– Chaotic vasculature and interstitial pressure
• vascular stasis
• flow instabilities
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Chronic Hypoxia
• Within areas of need, oxygen is released from red blood cells and enters
tumor tissue by diffusion. It is metabolized by respiring cells. As a result,
at distances greater than about 100 µm from the nearest blood vessel
insufficient oxygen remains to maintain cell viability.
• Adjacent to areas of necrosis, one may
find a region 1-2 cell layers thick where
oxygen tensions are hypoxic. Within a
solid tumor mass, mitotic index and
viability decrease with distance from the
nearest blood vessel (Tomlinson and
Gray; Tannock, Cancer Res 30: 2470,
1970)
• Hypoxia does NOT correlate with tumor
volume
BLOOD
VESSEL
Proliferation, O2,
pH, cell viability
100 m
HIGH.................LOW
V
Proliferation
Hypoxia
VV
V
V
V
V
V
V V VVV
VV V
V VV
Necrosis
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• The vascular network
that develops in tumors
is structurally abnormal
• Vessels are dilated,
tortuous, elongated,
with A-V shunts and
blind ends
• Pericytes are frequently
absent
• The basement
membrane is thin
• Vessels are more
permeable giving
increased interstitial
pressure
• The abnormal
vasculature results in
spatial and temporal
heterogeneity in blood
flow that in turn produce
regions of temporary or
acute hypoxia, acidity
and nutrient depletion
Acute Hypoxia
Brown & Giaccia, 1994
Normal
Tissue
Konerding et al.,
1998
Neoplastic
tissue
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THE OXYGEN EFFECT
•
•
•
Oxygen is a powerful oxidizing agent and therefore acts as a radiosensitizer
if it is present at the time of irradiation (within msecs)
The magnitude of the OER is critically dependent upon oxygen tension. The
greatest increase occurs between 0-20 mm Hg with further modest
increases to air (155 mm Hg) and above (760 mm Hg=100% oxygen).
Its effects are measured as the oxygen enhancement ratio (O.E.R.)
–
O.E.R. = the ratio of doses needed to obtain a given level of biological effect under anoxic
and oxic conditions = D(anox)/D(ox)
For low LET radiation the O.E.R. is 2.5-3.0 and in the higher range at higher doses
For neutrons, O.E.R is about 1.6
–
–
1.0
3.0
S.F.
hypoxic
2.5
O.E.R.
oxic
0.1
2.0
O.E.R.= 2.67
1.5
air
1.0
0
100% oxygen
10 20 30 40 50 200
760
Partial Pressure of Oxygen (mm Hg) at 37o C
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0.01
0
2
4
6
8
10
Dose (Gy)
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RBE and OER as a function of LET
4
8
RBE
(for cell kill)
Fast
Neutrons
6
Alpha
3
Particles
2
4
2
0
OER
RBE
OER
Co-60 Diagnostic
gamma rays X-rays
1
0.1
10
100
1
0
1000
Linear Energy Transfer (LET in keV/mm)
OER is the inverse of RBE because it depends on the indirect action of
ionizing radiation
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Demonstrating hypoxic regions/cells
within tumors
• Differential radiation sensitivity
• Eppendorf polarographic electrode
• Immunohistochemistry
– Misonidazole
– Hypoxyprobe™ immunohistochemistry with pimonidazole
– HIF-1 and products
• PET imaging
– 18F-fluoromisonidazole (FMISO-PET)
– EF5 - etanidazole
– Cu(II)-diacetyl-bis(N4-methylthiosemicarbazone (Cu-ATSM)
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Tumor Cell Survival : In vivo-in vitro assay
•
IRRADIATE
tumor
If solid tumors in mice are irradiated with single doses of radiation under
hypoxic conditions or in air and an in vitro clonogenic assay performed,
normally a dog-leg curve is obtained in air indicating a radioresistant
population whose magnitude can be estimated by extrapolation onto the Y
axis. After Rockwell and Kalman, 1973
After 24hrs make cell suspension
Plate cells
DOSE (Gy)
0 2 4 6 8 10 12 14 16 18 20 22
1
Hypoxic
-1
Fraction 10
-2
10
S.F.
-3
10
HYPOXIC
14 Days
-4
10
-5
10
-6
10
AIR
OXIC
Colony assay
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Tumor Hypoxia
• If murine tumors are
irradiated with varying sized
single doses of radiation
under clamped (hypoxic)
and normal conditions and
the % of tumors controlled
plotted, the TCP curve is
shifted to the right by
hypoxia and the O.E.R. can
be calculated.
Moulder and Rockwell, 1984
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Eppendorf Polarographic Fine Needle
pO2 Probe
Probe
Casing
300 mm
Insulating
glass
Gold Wire
12 mm
Membrane
• A 700 mV polarizing voltage is applied against the Ag/AgCl anode. The
measured current is proportional to the local oxygen tension
• No longer sold, but other versions are possible
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Eppendorf Polarographic Probe
Proportion of measures
50%
40%
40%
30%
NFSA
NFSA TNF
30%
20%
NFSA
NFSA IL7
20%
10%
10%
0%
0%
<6 <12 <18 <24 <30 <36 <42 <48
mmHg
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<6 <12 <18 <24 <30 <36 <42 <48
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Bioreductive Drugs
• Misonidazole forms adducts in hypoxic cells
in vitro and in vivo with thiol groups in
proteins, peptides and amino acids. Hypoxia
(pO2 < 10 mmHg) is required for binding.
• FMISO-PET is one of 2 commonly used
PET tracers (the other being Cu-ATSM),
but it accumulates slowly. Other
imidazoles are under study.
• EF5 is a fluorinated derivative of etanidazole
Pimonidazole staining of human CRC tumor
• Pimonidazole is generally injected in vivo and
the adducts stained using antibodies.
• Intracellular Cu-ATSM is a non-nitroimidazole
that has been shown to be bioreduced and
trapped in hypoxic cells and is used for PET.
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Hypoxia and proliferation in a solid tumor
Biopsy of head/neck squamous cell carcinoma
blood vessels
proliferating cells
(IdUrd +)
Hypoxia
(pimonidazole +)
necrosis
From: Albert Van
der Kogel
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Pimonidazole (green) and vascular staining (red)
in human head and neck tumor
Chronic
Acute
From Bussink et al., 2001
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Hypoxia-induced gene expression
• Transcription factors
– AP-1, NF-kB, SP-1 activation
• which can mediate radioresistancy
– p53 induction
• which can cause apoptosis with hypoxia-driven p53 mutant
selection and increasing genetic instability
– HIF-1a and products eg VEGF, CA IX, OPN etc
• HIF-1alpha is a target for prolyl hydroxylation by HIF prolylhydroxylase, targeting it for rapid degradation in normoxic
conditions. Under hypoxia, HIF prolyl-hydroxylase is inhibited,
since it utilizes oxygen as a cosubstrate, stabilizing HIF-1α.
This upregulates several genes to promote survival in lowoxygen conditions, including glycolytic enzymes and VEGF,
which promotes angiogenesis.
• In general these surrogate markers do not correlate well with
hypoxia, probably because more than hypoxia stabilizes HIF-1
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Contribution of hypoxia to tumor progression
• Enhances resistance to radiation and chemotherapy because of
classic oxygen effect
• Induces expression of genes that
– confer resistance to radiation and other pro-apoptotic insults
– triggers genetic instability
– cause angiogenesis and potentiate metastasis
From Giaccia, 1999
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Regulation of hypoxia-induced gene expression
HER2 IGFR EGFR
Src
LY294002
PI3K
PTEN
AKT
Rapamycin
FRAP
HIF-1a synthesis
FIH-1
HIF-1b
HIF-1a
protein
HIF-1a mRNA
Target gene
expression
HYPOXIA
Prolylhydroxylation
VHL
Ubiquitination
VEGF
Angiogenesis
IGF-2
Proliferation
Glucose
Metabolism
transporters
p53
HIF-1a degradation
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Inflammatory
Cytokines
apoptosis/
necrosis
IL-1a, IL-8
BNip3
(BCl2 family)
HIF-1
VEGF
VEGFR
EPO
angiogenesis
EGF
EGFR
PDGF-B
IGF-1
IGF-2
proliferation
Redox
regulation
Heme oxygenase 1,
metallothionein,
diaphorase,
GSH,
carbonic
Anhydrases
CA9
pH
regulation
Glucose transporters Glut1,3
Glycolytic enzymes
ALDA, PGK1, PKM, PFKL, LDHA
energy
metabolism
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Clinical Relevance of Tumor Hypoxia
• Evidence for hypoxia in human tumors
– Hyperbaric chambers anecdotally show benefit
• normobaric oxygen/carbogen has also been applied and, at times, combined
with nicotinamide, a B6 vitamin analog thought to counteract the acute hypoxia
(ARCON)
– Anemia correction has benefit especially in cervix ca
• Note that erythropoietin has a deleterious effect in HNSCC due to stimulating
tumor growth
– Nitroimidazoles - immunohistochemistry and PET
– Microelectrode measurements - several studies have correlated hypoxia with poor
local response and survival
• Nordsmark et al. Radiother Oncol 41, 31, 1996 showed local tumor control correlates with
pre-treatment oxygen levels in head and neck ca.
• Brizel et al IJROBP 38:285, 1997 showed DFS correlates with hypoxia in T3 and T4 and
large node mets from head and neck
• Hockel et al Cancer Res 56:4509, 1996 showed hypoxia correlated with local invasion
and survival in cases treated with RT or only with surgery
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Hypoxia and Local Tumor Control
Small Hypoxic Fraction
Large Hypoxic Fraction
• Local tumor control correlates
with pre-treatment oxygen levels
in head and neck ca., as
measured with an Eppendorf
electrode. Tumors were
stratified by whether the fraction
of pO2 values less than 2.5 mm
Hg was above or below the
median (15%).
66-68 Gy was given in 33-34 Fx.
• Nordsmark et al Radiother
Oncol 41, 31, 1996
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Tumor Hypoxia and DFS
• DFS in cervix ca depends on
pO2, irrespective of type of
treatment, surgery/RT.
Hockel et al, Sem. Radiat.
Oncol. 6:30, 1996.
• This suggests that hypoxia is
linked to tumor aggression
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Radiosensitizers
From Zeman, 2000
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Radiosensitizers
• Radiosensitizers such as nitroimidazoles can
“mimic” oxygen and fix damage
– Associated with some toxicity and there were only rarely efforts
to determine if the tumors were hypoxic in advance of treatment
– However there have been positive trials
• DAHANCA 5 trial using
nimorazole in treatment of
advanced squamous cell
carcinoma of the head and
neck
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And a meta-analysis by Jens Overgaard has shown
significantly improved survival and loco-regional control
Journal of Clinical Oncology, 25: pp. 4066-4074, 2007
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Hypoxic Cytotoxins
• Quinones
– Mitomycin C
• Nitroaromatics
• Benzotriazine di-N-oxides
– Tirapazamine
• Phase III clinical trials with
cisplatin
• Phase II with RT
• Currently off the market!
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Tumor Reoxygenation
• Since well oxygenated cells are more sensitive than hypoxic
cells to ionizing radiation, one might reasonably expect that the
hypoxic fraction (i.e. the proportion of hypoxic cells) to increase
during the course of radiation therapy
• In fact, Putten & Kallman and others demonstrated that the
proportion of hypoxic cells present within a tumor varies a lot,
but does not increase during a course of fractionated radiation
therapy showing REOXYGENATION exists. Multiple
mechanisms exist and the variation seems considerable from
tumor to tumor.
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Hypoxic
Fraction
Days post RT
N.B. All single dose!
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Angiogenesis
•
•
•
Development of new blood vessels from pre-existing capillaries.
Although tumors smaller than approximately 1 mm can receive
sufficient oxygen and nutrients by diffusion, continued growth
depends upon the development of an adequate blood supply. In
the absence of angiogenesis, tumors do not increase in size
and remain localized
Angiogenesis also occurs during
– wound repair
– pregnancy
– certain times in the menstrual cycle
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Major Steps in the Angiogenic Process
• In response to various tissue-derived pro-angiogenic signals,
endothelial cells in nearby blood vessels
– Degrade their basement
membrane and invade the
adjacent extravascular space
– Endothelial cells behind the
leading edge proliferate to replace
the migrating cells
– Newly generated endothelial cells
migrate through connective tissue
toward the source of proangiogenic signals
– Endothelial cells assemble into a
new vessel, form a lumen, lay
down a basement membrane and
join other vessels to allow flow.
Hypoxia
CO
2
COX-2
NO
Tumor suppressor genes
Oncogenes
Proliferation
Lumen formation
Angiogenic factors:
VEGF, FGF, PDGF,
EGF, HGF, TGF
Differentiation
Migration
Degradation of basement membrane
Ischemia and reperfusion
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Tumor Vasculature and Metastasis
• Weidner et al (N Engl J Med 324: 1, 1991)
demonstrated that the likelihood of
developing metastasis increased directly as
the density of tumor associated blood vessels
increased
15/15
% with metastasis
• There is a relationship between
microvascular density and the probability of
metastasis, relapse free survival and/or
prognosis.
5/7
9/20
1/7
# microvessels/unit area
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Anti-Angiogenesis
Vascular Targeting
Preventing the growth of
new tumor-associated
blood vessels
Induction of selective and
irreversible damage to
established tumor-associated
blood vessels
Blood vessel
Inhibition of tumor growth
• chronic exposure
Blood vessel
Induction of tumor necrosis
• acute exposure
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Advantages of vascular targeting
• Since many thousands of tumor cells depend upon each blood
vessel for the delivery of oxygen and nutrients, theoretically even
limited damage to tumor vasculature may occlude a vessel and
cause “an avalanche of tumour cell death”.
• Since cells being targeted are in contact with the blood stream,
delivery problems that limit the efficacy of therapies directed
toward tumor cells are not an issue
• Since endothelial cells are genetically stable and nontransformed, treatment-related resistance is less likely to emerge
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Questions:
Tumor Responses to Radiotherapy
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78. The probability of tumor cure (TCP) in a
series of tumors that have on average 1 cell
surviving is
–0
– 0.37
– 0.5
– 1.0
#2 – it follows a Poisson distribution – events in space
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79. If a tumor contains 109 clonogenic cells and
RT reduces survival by 10-9, what is the
probability of tumor cure
– minimal
– 37%
– 50%
– 90%
#2 – P of cure = e-x , where x is the average number of
surviving clonogens, with 1 cell on average surviving the
TCP will be 37% - e-1
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80. If a tumor contains 109 clonogenic cells and
RT reduces survival by 10-10, what is the
probability of tumor cure
– 10%
– 37%
– 50%
– 90%
#4 – P of cure = e-x , where x is the average number of
surviving clonogens, in this case 0.1 - or 90.5%
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81. Tumorgenicity is a stem cell property of
tumors that is best assessed by
– TD50 assay
– TCD50 assay
– In vivo - in vitro assay
– Tumor regrowth assay
#1 – varying numbers of cells are injected and the
number required to cause 50% to grow is the tumor
dose 50.
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82. Which of the following is not a property of
stem cells in tumors
– Tumorgenicity
– Pluripotentiality
– Expression of developmental markers
– Radiation resistance
– Cause of tumor regression
#5 –cancer stem cells are a subpopulation of cancer
cells that are responsible for tumor recurrence.
Regression is probably more a property of the non-stem
cell population.
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83. The rate of tumor regression after RT is
determined primarily by
– Tumor cell cycle time
– Tumor growth fraction
– Tpot
– Labeling index
– Cell loss factor
#5 – the cell loss factor is the major influence on the rate
of tumor growth and regression
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84. If the cell cycle time of a tumor is 48hrs and
the growth fraction 10%, what is the potential
volume doubling time?
– 2 days
– 10 days
– 20 days
– 3 months
#3 – the the GF was 100% the Tpot would be 2 days. If
10% it will be 20 days.
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85. If the Tpot for a tumor is 3 days and the
actual volume doubling time is estimated as
30 days, what is the cell loss factor
– 0.1
– 0.3
– 0.5
– 0.9
– 1.0
#4 – 1- 3/30 = 0.9
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86. The increase in tumor control probability
with dose for clinically detectable tumors is
theoretically best described by which of the
following
– A log-linear curve
– A sigmoid curve with a dose threshold
– A sigmoid curve with no threshold
– A curve that is close to linear with no
threshold
#2 – a certain dose will be needed until you begin to see
cures. After that it will be determined by the killing of the
last surviving clonogens, which will be a sigmoid curve.
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87. The probability of eliminating metastatic
disease theoretically increases with dose and
is best described by which of the following
– A log-linear curve
– A sigmoid curve with a dose threshold
– A sigmoid curve with no threshold
– A flat curve that is close to linear with no
threshold
#4 – palpable tumors have a restricted range of clonogens
around 109-1010. Micrometastatic disease can be anywhere
between 1 cell and palpable tumor – a wide range. Any dose
could be effective up to what would be needed for palpable
tumor i.e. the curve will be flat.
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88. Which of the following is NOT correct about
accelerated repopulation
– It involves a decrease in the cell loss factor
– It can be promoted by treatment breaks
– It explains why tumor recur faster than
expected after RT
– It occurs only in tumors
#4 – accelerated repopulation/regeneration spares
normal tissues from the effects of radiation eg mucositis
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89. In T2-T3 HNSCC, what percent of a 2Gy
dose is estimated may be lost to accelerated
repopulation later in the course
– 5%
– 20%
– 33%
– 70%
#3 – This has been estimated by Maciejewski et al.,
1989 from the relationship between treatment time and
total dose for control to be 0.6 Gy/day, but may be
higher
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90. In T2-T3 HNSCC, when is accelerated
repopulation thought to be initiated
– 1 week
– 4 weeks
– 8 weeks
– 12 weeks
#2 – there is a lag period before dose is ‘lost’ to
accelerated repopulation that in HNSCC is about 4
weeks.
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91. In T2-T3 HNSCC, tumor doubling time may
become close to Tpot, which on average is
– 2-7 days
– 1-4 weeks
– 1-2 months
– 2-6 months
#1 – Tpot has been measured in HNSCC patients and is
in the order of 2-7days
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92. What occurs in tumors with distance from a
blood vessel
– Increased cell proliferation
– Poor oxygen diffusion
– Decreased oxygen levels due to high
consumption near the vessel
– Increased pH
#3 – the reason for areas of chronic hypoxia is that
oxygen is consumed by cells nearer the vessels.
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93. At what distance from a blood vessel does
radiobiologically relevant hypoxia occur
– 50 micrometers
– 100 micrometers
– 200 micrometers
– 500 micrometers
#2 – As shown by Tomlinson and Gray
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94. Hypoxic areas in tumors are best assessed
by
– Pimonidazole uptake
– Carbonic anhydrases, like CAIX
– Expression of hypoxia inducible factor
(HIF-1)
– VEGF expression
#1 Pimonidazole is an oxygen mimetic that binds in
hypoxic areas. The others are downstream events that
are less reliable measures of hypoxia as they are
induced by other stimuli also.
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95. Which of the following is true of tumor vasculature
– Its abnormal vasculature results in acute transient
areas of hypoxia
– It has a thin basement membrane but no increase
in permeability
– It is responsible for low interstitial tumor pressure
– Poor angiogenesis limits it, causing hypoxia
#1 – it is unable to sustain an open configuration for long
and constantly collapses causing transient hypoxia.
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96. The concentration of oxygen that gives halfmaximal radiosensitization is
– 1mm Hg
– 3mm Hg
– 10mm Hg
– 100 mm Hg
#3 – this is half-maximal value. Relevant hypoxia is often
taken as <5mm Hg, but there is no standard.
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97. For low LET radiation, the OER for an
isoeffect is
– 1.0-1.6
– 1.7-2.3
– 2.3-3.0
– 3.0-3.7
#3 – a huge effect in keeping with oxygen being a great
radiosensitizer
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98. For 15MeV fast neutrons, the OER is
approximately
– 1.6
– 2.3
– 3.0
– 3.7
#1 – this may be attributed to a low LET component
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99. For alpha particle radiation, the OER is
closest to
– 1.0
– 1.7
– 2.3
– 3.0
#1 – OER falls inversely with RBE as LET increases
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100. The OER is lowest at an LET closest to
– 1 keV/mm
– 10 keV/mm
– 50 keV/mm
– 100 keV/mm
#4 – Due to direct action of ionizing radiation predominating
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101. Which tracer is NOT used to detect
hypoxic areas in tumors by PET
– FMISO
– Hypoxyprobe
– EF5 - etanidazole
– Cu-ATSM
#2 – hydroxyprobe is a kit that include a nitroimidazole
and an antibody for staining fixed tissues.
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102. Which of the following is NOT induced by
hypoxia
– Carbonic anhydrases
– VEGF
– Erythropoietin
– NF-B
– EGFR downregulation
#5 – in fact hypoxia tends to up-regulate EGFR
expression. The first 3 are downstream of HIF-1
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103. The generally accepted gold standard in
measuring hypoxia in a human tumor is
– Polarographic needle probes
– Hydroxyprobe
– Carbonic anhydrase levels
– Osteopontin levels
– HIF-1 expression
#1 – this may be replaced by 18F-miso as the needles
are invasive and difficult to use reliable.
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104. How does HIF-1 act?
– It is a transcription factor that is activated
by phosphorylation
– It is a transcription factor that is activated
by VHL (Von-Hippel Lindau) protein
– Oxygen inhibits prolyl hydroxylase that
targets it for VHL (Von-Hippel Lindau)
protein-mediated degradation
– Hypoxia inhibits prolyl hydroxylase leading
to HIF-1a stabilization
#4 – hypoxia stabilizes HIF-1a expression through
inhibiting prolyl hydrolases and hence its degradation
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105. Clinically, pretreatment hypoxia has NOT
been correlated with
– Distant failure following surgery
– Loco-regional failure in HNSCC following
RT
– Decreased tumor recurrence in clinical
trials of erythropoietin with RT in HNSCC
– Microvessel density
#3 – hypoxia drives tumor aggression and metastasis.
Attempts to use EPO to improve RT gave a 10%
decrease in local control because it stimulates tumor
growth
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106. Which of the following is true for nitroimidazoles
– They mimic oxygen and radiosensitize tumors
– They radioprotect normal tissue by scavenging
reactive oxygen species
– They are unable to sensitize acute hypoxic areas
– A metanalysis by Overgaard has shown that they
improve locoregional control to RT, but not
survival
– They can not kill hypoxic cells
#1 – they are oxygen mimetic radiosensitizers that can
be hypoxic cell cytotoxins. In the metanalysis survival
was improved.
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107. Which of the following is correct for
Tirapazamine
– It is a radioprotector
– It has been shown to effectively
radiosensitize tumors in phase III clinical
trails in HNSCC
– it is a hypoxic cell cytotoxin
– It radioprotects normal tissues
#3 – It is driven to be toxic by low oxygen concentrations
but has yet to be put into a Phase III trial with RT.
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108. Vascular targeting refers to
– The effects of agents like Avastin on tumor
angiogenesis
– The interaction of anti-angiogenesis factors
with RT
– The selective effects of agents on
established vasculature
– The effects of agents like erythropoietin on
oxygen delivery into tumors
#3 Vascular targeting sometimes is used loosely to refer to any
anti-vascular effects but really should be used for established
vasculature where the aim is to be cytotoxic, as opposed to
anti-angiogenesis factors which are more cytostatic
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