Radiobiology Flashcards
1
Q
Which of the DNA bases are single-ring (pyrimidine) and which are double-ring (purine)?
A
Pyrimidine = thymine & cytosine Purine = adenine & guanine
2
Q
What is the D0 dose?
A
The dose of radiation that induces an average of one lethal event per cell, leaving 37% still viable
3
Q
Why is DNA in cells more resistant to radiation damage than free DNA?
A
- presence of molecular scavengers
- physical protection via protein (histone) packaging
4
Q
What do nuclear foci of phosphorylated 53BP1 indicate?
A
DNA DSB
5
Q
What is the extrapolation number, n? What type of curve results if n is large?
A
n is a measure of shoulder width
if n is large, the survival curve has a broad shoulder
6
Q
Which genes appear to be responsible for induction of senescence?
A
Rb, p53
7
Q
Following radiation, one cell line shows clear DNA laddering on electrophoresis, while another shows little evidence of DNA laddering. Which of these is more likely radiosensitive?
A
cell line with laddering –> indicates apoptosis
this is likely the more radiosensitive cell line
8
Q
What is the OER for protons?
A
2.5-3.0
equivalent to x-rays
9
Q
What is the average LET during the entrance plateau for protons? during the Bragg peak?
A
entrance: 0.5keV/um
Bragg peak: 100keV/um
10
Q
The probability of which type of effect increases with radiation dose: deterministic or stochastic?
A
Both
11
Q
What type of effect has a practical threshold dose and increases in severity with dose: deterministic or stochastic?
A
Deterministic
12
Q
Which type of effect is best described as “random”, with no threshold observed and no change in severity with dose: What type of effect has a practical threshold dose and increases in severity with dose: deterministic or stochastic?
A
Stochastic
13
Q
Which radiation-induced malignancy has the shortest latent period?
A
Leukemia
Peak incidence at 5-7 years
14
Q
T/F: Radiation-induced solid tumors occur in younger patients than spontaneous tumors.
A
FALSE
Radiation-induced solid tumors tend to be expressed later in life, at the same time as spontaneous tumors of the same type
15
Q
Which of the following risk models for malignancy following radiation exposure is favored by the BEIR committee:
absolute risk model
relative risk model
time-dependent risk model
A
Time-dependent risk model
16
Q
Name the two international committees on radiation protection.
A
UNSCEAR - United Nations Scientific Committee on the Effects of Atomic Radiation (scholarly)
ICRP - Int’l Council on Radiological Protection
17
Q
Name the two US committees on radiation protection.
A
BEIR - Biological Effects of Ionizing Radiation (US Nat’l Academy of Sciences)
NCRP - Nat’l Council on Radiological Protection and measurement
`
18
Q
T/F: The incidence of CLL increases following radiation.
A
False - AML/CML account for most of the increase
19
Q
The incidence of which types of leukemia increases most after radiation of a human population?
A
AML and CML
20
Q
Which populations are most susceptible to increased incidence of ALL and stem cell leukemia following radiation?
A
Children
21
Q
Which type of cancer has the highest relative risk of any malignancy following radiation of a population?
A
Leukemia (specifically AML, CML)
22
Q
The risk of leukemia rises in a dose-dependent manner up to what dose?
A
3 Sv
Risk per unit dose at 1 Sv is about 3X greater than 0.1 Sv
23
Q
Which population is most sensitive to induction of thyroid carcinoma following radiation?
A
Children
24
Q
What is the threshold dose to the developing fetus for radiation-induced mental retardation?
A
0.3Gy
25
What is the unit for absorbed dose (J/kg)?
Gy
1 Gy = 100 rad
26
What dose rate defines high dose rate brachytherapy?
HDR is >12Gy/h
27
What dose rate defines low dose rate brachytherapy?
LDR brachytherapy is ~2Gy/hr
28
At what timeframe following RT to the globe is retinal hemorrhage and degeneration expected?
3-6 months
These lesions progress over time
29
How much energy is dissipated per ionization event in ionizing radiation?
33eV
30
What is the velocity (c) of electromagnetic radiation?
3x10^10 cm/s (3x10^8 m/s)
31
All forms of electromagnetic have the same velocity (c = 3x10^8 m/s) but differ in frequency (v) and wavelength. How can frequency be used to determine photon energy?
Photon energy = hv
...where h= Planck's constant, 6.626E-34 J-sec
c= frequency (v) x wavelength (lambda)
32
What happens to the energy of electromagnetic radiation as the wavelength increases
Energy DECREASES as wavelength increases
c = frequency x wavelength
frequency @ energy (E = hv, where h = Planck's constant)
33
What is the minimum energy of ionizing radiation
124 eV
| wavelength <10E-6 cm
34
Which forms of particulate radiation have a net + charge?
protons, a-particles, heavy charged particles
35
Is electromagnetic radiation directly or indirectly ionizing?
indirectly --> gives up energy in the absorbing material to result in fast-moving charged particles
36
What is the kinetic energy of an electron liberated by an incident photon in the photoelectric process?
in photoelectric process, all energy from incident photon is given up to electron
kinetic E = hv - (electron binding E)
37
What is the primary free radical formed by indirect action of radiation?
H2O+ --> OH* + H3O
38
Which DNA molecules are single-ringed?
Pyrimidines (thymine, cytosine)
39
Which DNA molecules are double-ringed?
Purines (adenine, guanine)
40
What is D0? What proportion of cells are left viable after irradiation with this dose?
Dose that results in average 1 lethal event/cell
Leaves 37% viability
~2-3Gy for mammalian cells
41
About 95% of energy deposition events for x-rays are spurs. What is the energy & diameter of this event size, and how many ion pairs does it produce?
Spurs <100eV
4nm diameter
~3 ion pairs
42
Blobs are energy deposition events in the 100-500eV range. Which type of radiation produces blobs most commonly?
densely ionizing radiation (ie neutrons, a-particles)
7nm diameter
avg 12 ion pairs
43
What is gH2AX?
gH2AX is the phosphorylated form of histone protein H2AX
H2AX is rapidly phosphorylated in response to damage and accumulates at sites of DNA damage
44
What is 53BP1?
53BP1 is a nuclear protein that becomes phosphorylated in response to stress, forming nuclear foci at the site of DNA DSBs
45
How many bases are removed by base excision repair?
single base
46
T/F: defects in base excision repair (BER) lead to enhanced radiosensitivity.
FALSE
Defects in BER lead to increased mutation rate but NOT increased radiosensitivity
47
Which enzymes are involved in base excision repair?
DNA ligase
APE1
DNA-polB
48
What is xeroderma pigmentosum? Does this disorder result in increased radiosensitivity?
XP is a defect in nucleotide excision repair
Does NOT result in increased radiosensitivity but does enhance sensitivity to UV-induced damage
49
At what phase of the cell cycle does homologous recombination repair occur?
Late S/G2 (requires sister chromatid template)
50
ATM, Rad51, and BRCA1/2 are involved in what type of DNA repair?
Homologous recombination
Recruited to sites of DSB
ATM also involved in NHEJ
51
T/F: Defects in HRR and NHEJ results in increased radiosensitivity
TRUE
52
Defects in which DNA repair process result in microsatellite instability?
Mismatch repair
Ex: hereditary nonpolyposis colon cancer
53
Radiation exposure during which cell cycle phase results in chromosome aberrations?
Interphase --> prior to DNA duplication
Ex: dicentric, ring, symmetric translocation, small deletions
54
Radiation exposure after late interphase results in what type of aberration?
Chromatic aberration (break in single chromatid arm)
55
Which chromosomal aberrations are lethal?
Dicentric
Ring
ChromaTID lethal aberration = anaphase bridge
56
Which chromatid aberrations are lethal?
Anaphase bridge
57
What is the difference in dose required to produce reproductive death vs functional cell death?
loss of function ~100Gy
| loss of replicative capacity ~2Gy
58
T/F: Cell survival curves assess cell death following radiation.
FALSE: Survival curves assess loss of replicative capacity, clonogenic cells = "survivors"
59
What is the plating efficiency?
colonies/# seeded x100
60
How is the surviving fraction calculated for a cell survival curve?
```
# colonies/
#seeded * plating efficiency/100
```
61
What differentiates the slope of survival curves for high LET vs low LET radiation?
low LET - survival curve is log-linear at low dose (a component), exponential at mid-range doses (B component), and log-linear again at higher doses (a=B)
high LET - survival curve is log-linear at all doses; survival fraction is an exponential function of dose
62
In the linear quadratic model, what equation describes survival? What about the curve predicted by this equation does not correspond with clinical observation?
S = e^(-aD-Bd^2)
resulting survival curve is constantly bending; does not account for final log-linear portion
63
What is the cost common form of cell death following radiation?
mitotic cell death
64
What are the morphologic hallmarks of apoptotic cell death?
condensation of nuclear chromatin into crescents around the periphery, eventual cell separation into membrane-bound apoptotic bodies
65
T/F: Bcl-2 suppresses apoptotic cell death.
TRUE
Apoptotic cell death following RT is p53-dependent and suppressed by Bcl-2
66
Cellular senescence is an irreversible cell cycle arrest characterized by induction of which proteins?
p53
| Rb
67
T/F - D0 for cells derived from patients with ataxia telangiectasia is less than for normal mammalian cells.
TRUE
D0 for normal cells ~1-2Gy, for AT cells ~0.5Gy
68
Which phase of the cell cycle varies most in the amount of time required?
G1
69
Which component of the a/B model is directly proportional to dose? What does this represent?
a = component proportional to dose
represents single-event killing
70
What component of the a/B model results from multiple-event cell kill? How does this component relate to dose?
B - exponential function of dose (D^2)
71
What is D10 in cell survival curves
Dose required to kill 90% of cells
| D10=2.3XD0
72
What is the x-ray transition point?
midpoint of G2 where radiosensitivity shifts from markedly radioresistant (early G2) to radiosensitive (late G2)
73
During which phase of the cell cycle does radiosensitivity shift from highly radioresistant to highly sensitive?
G2
early G2 --> ver resistant
late G2 --> very sensitive
74
OER varies between cell cycle phases. What is the OER in early G2 vs S?
G2 - 2.3-2.4
| S - 2.8-2.9
75
In which cell cycle phase do irradiated cells tend to arrest?
G2
allows for chromosomal damage repair prior to mitosis
76
Under which conditions is potentially lethal damage (PLD) most likely to be repaired?
post-radiation conditions suboptimal for growth
77
What is the difference between sublethal damage and potentially lethal damage? Which is more likely to be repaired?
SLD - can be repaired under normal circumstances (more likely to be repaired)
PLD - can be modified by environmental conditions post-radiation (ie suboptimal growth conditions --> PLD repair)
78
What effect does sublethal damage (SLD) repair have on the survival curve?
correlates with width of the shoulder region
better SLD repair --> wider shoulder
79
Sublethal damage is primarily what DNA lesion? What impact does dose fractionation have on repair?
SLD ~~ DSB
SLD repair = repair of DSB before they can interact to form lethal aberrations
Fractionation --> DSBs produced by the first dose may be repaired by the time the second dose is delivered
80
Is SLD repair more likely to occur with low-LET or high-LET radiation?
low-LET
81
What is the dose-rate effect?
As dose rate is lowered and exposure time is extended, biologic effect of a given dose is reduced
82
What impact does reduction of dose rate have on the survival curve?
Curve becomes shallower, shoulder disappears
83
What is the average photon energy and half life of iridium-192?
130 keV
| 74.2 d
84
What is the HVL (mmPb) for Ir-192?
2.5
85
What is the HVL (mmPb) for cesium-137?
5.5
86
What is the OER (range) for high-dose X-rays?
2.5-3.5
87
What is the OER for densely ionizing high LET radiation such as a-particles?
There is no OER, there is no shoulder!
88
What is the OER for neutrons?
~1.6 - falls between x-rays (2.5-3.5) and high LET (no OER)
89
What is the mechanism of the OER?
Under hypoxia, DNA is chemically restored by reaction with sulfhydryl (SH) molecules
When O2 is present, formation of peroxide groups results in non restorable damage
90
What concentration of O2 is required to result in radiosensitivity ~half-way between 0-100%
0.5% O2 (3mmHg)
91
What are the units for LET?
keV/um
92
The classification of radiation as densely or sparsely ionizing refers to the spatial separation of ionization events. What happens to the LET of a given type of radiation as energy increases?
as energy increases, LET
DECREASES!!!
93
How does fractionation affect RBE?
RBE is greater for a fractionated regimen than a single dose (ie more separation of curves) because RBE is larger with smaller doses
94
How does SLD repair impact RBE?
Tissues with high capacity for SLD repair have a higher RBE because shoulder is larger
95
What happens to RBE as LET increases 0-10keV/um?
RBE increases slowly
96
What happens to RBE as LET increases 10-100keV/um?
increases rapidly
97
What happens to RBE as LET increases >100keV/um?
RBE decreases >100keV/um
98
Why is 100keV/um considered the optimal LET in terms of RBE?
average separation btw ionization events at 100keV/um is 2nm (20A), which corresponds to the diameter of the DNA double-helix
99
Two types of radiation are compared, one with LET = 95keV/um and the other with LET = 150keV/um. Which has higher RBE?
95keV/um
RBE decreases >100keV/um
100
At what LET does OER reach 0?
200keV/um
101
What is the approximate OER for a radiation with LET = 200keV/um?
0
102
What happens to OER as LET increases from 0-60keV/um?
OER slowly decreases
103
1 Sv = ___rem
1 Sv = 100rem
104
1Gy = ___rads
1Gy = 100 rads
105
What are the stages of acute radiation syndrome?
Prodromal, latent, manifest illness
106
At what dose does cerebrovascular syndrome occur? When does death occur?
~40-100Gy
Results in death in 24-48hrs
107
Whole body exposures >___Gy are considered absolutely lethal.
>10Gy - death is certain
108
At what acute radiation dose does gastrointestinal syndrome occur? When does death occur?
>10Gy
Death in 3-10d
109
At what acute radiation dose does hematopoietic syndrome occur? When does death occur?
2.5-5Gy
Peak incidence of death @30d
110
At what does range should bone marrow transplantation be considered following acute whole body radiation exposure?
8-10Gy
>10Gy, death is inevitable because of GI death, below 8Gy the patient is likely to survive with antibiotics alone
111
What is the excess risk of childhood malignancy following radiation to the fetus?
6% increase in risk per Gy
112
Sulfhydryl compounds can act as radio protectors for what type of radiation?
Sparsely ionizing
113
What feature of sulfhydryl compounds allows for facilitation of DNA repair?
Donation of H atom
114
What limits the use of cysteine as a radioprotector?
causes nausea and vomiting at doses required for radioprotection
115
What is the only FDA-aproved radioprotective drug?
Amifostine - may also reduce mutagenesis
116
How is amifostine converted to its active metabolite?
dephosphorylation by alkphos (ALP)
then can readily enter cells, prior to depths does not penetrate cells well
117
What dose is required to induce permanent sterility in males vs females?
Males --> 6Gy single dose; fractionated doses cause more gonadal damage than single dose (2.5-3Gy)
Females --> 12Gy prepuberty, 2Gy premenopausal
118
What dose is required to induce oligospermia in males? Azoospermia and temporary sterility?
0. 15Gy --> oligospermia
| 0. 5Gy --> azoospermia, temporary sterility after 6wk latent period
119
What is the doubling dose (ie dose of radiation that doubles the baseline spontaneous mutation rate) to induce heritable effects in humans?
Doubling dose = 1Gy
120
What is the rate of risk of a hereditary mutation at a population level?
0.2%/Sv
121
What is the embryonic stage most sensitive to radiation? What is the threshold dose for death at this stage?
Preimplantation - most sensitive
Threshold dose = 100mGy (10cGy)
122
What effect does radiation have on the fetus at 8-15 weeks gestation?
Mental retardation and microcephaly
123
What is the does limit to the embryo following declaration of pregnancy?
0.5mSv/month
124
What are the characteristics of a deterministic effect?
Threshold dose
Dose-related severity
Probability increases with dose
125
Is tissue fibrosis deterministic or stochastic?
Deterministic
126
What type of radiation induced malignancy has the shortest latent period?
Leukemia - 5-7yrs
127
What types of leukemia are most commonly induced by radiation?
AML, CML
CKK does not appear to be affected by radiation
128
At what age are humans most susceptible to thyroid cancer induction by RT?
childhood
129
At what age are humans most susceptible to lung cancer induction by RT?
middle age
130
Where are the dividing cells in the ocular lens?
Limited to pre-equatorial region of epithelium; progeny differentiate and migrate towards posterior pole
131
What impact does dose have on the latent period to cataract formation?
Higher dose --> shorter latent period
132
T/F: High LET radiation is more effective at cataract induction
True
133
Can a cataract be definitively classified as radiation-induced?
No - but certain cataracts (ie nuclear cataracts) are NOT radiation induced
134
What is the threshold dose for cataract formation?
2Gy
135
What is the half-life of 64-Cu (used in Cu-ATSM imaging)?
12.7 hours
136
Which radio tracers can be used to image for angiogenesis?
Galacto-RGD and Dimeric-RGD
137
Which radio tracer can be used to assay vascular perfusion?
Cu-PTSM
138
What is 18F-FLT used to image for?
cellular proliferation
139
What is the half-life for 18F (ie 18F-FDG, 18F-FLT)?
109 min (1.83hrs)
140
What is the formula for HU?
HU = 1000 x (u-u,water)/u,water
where u = x-ray absorption
141
What is the HU of air?
-1000
142
What is the HU of soft tissue?
0-100
143
What is the HU of fat?
-100-0
144
What is the HU of bone?
>500
145
How much iodine is required to produce a change of 100 HU (ie iodinated contrast agents)?
10mM
146
What is the spatial resolution of PET?
4-6mm
147
What is the amount of cosmic radiation experienced at sea level? How does this change as altitude increases?
0.26mSv at sea level
doubles for each 2000m increase in altitude
148
What is the highest source of internal radiation?
Potassium-40 - ~0.2mSV/year
149
Which national agencies have the ability to enforce laws regarding radiation protection?
Nuclear Regulatory Commission
| EPA
150
What is used to calculate the equivalent dose (Sv)?
absorbed dose (gy) x Wr
Wr = radiation weighting factor, takes into account dose rate and RBE
151
What is the Wr of electrons?
1 (same as photons)
152
What is the Wr of protons?
2
153
What is the Wr of neutrons
Continuous curve depending on neutron energy
154
What is the Wr of a-particles?
20
155
What is the difference between equivalent dose and effective dose? What are the units?
Equivalent dose takes into account Wr based on type of radiation
Effective dose also takes into account the relative contribution of each tissue/organ to two total detriment
Units - Sv
156
What is the committed equivalent dose?
equivalent dose integrated over 50yrs
157
What is the cumulative lifetime occupational exposure limit recommended by the NCRP?
10mSv x age
158
What is the annual occupational exposure limit recommended by the NCRP?
50mSv/yr
159
What is the public effective dose limit recommended by the NCRP for continuous exposure? Infrequent exposure?
1mSv/yr frequent, 5mSv/yr infrequent
160
What air concentration of radon results in a dose of 1mSv/yr to bronchial epithelium?
20Bq/m3
161
What is the action level of radon?
148Bq/m3
162
What is the formula for surviving fraction?
```
# colonies/
#seeded * plating efficiency/100
```
163
What is the tissue rescue unit?
The minimal # of functional subunits required to maintain tissue function
164
What are Michalowski's H- and F-type populations?
Heirarchical - ie consists of stem cells, maturing, and functional cells
Flexible - can be triggered to divide by damage
165
Which interleukins are induced by radiation?
IL-1, IL-6
IL-1 is radioprotective in hematopoietic cells`
166
Which cytokine is implicated in late complications of RT?
TNFa
167
What is the transit time for an epidermal cell from basal layer to desquamation from surface?
14d
168
Which type of lymphocyte is most radiosensitive?
B cells > T cells
169
When is oral cavity desquamation expected? How does this compare to skin?
12d
Skin slightly longer - 14d
170
At what timepoint following RT is villous atrophy observed in the bowel?
2-4d
171
Which segment of bowel is most radiosensitive?
small bowel
rectal tolerance dose ~70Gy!
172
During what time period does acute pneumonitis occur?
2-6mo
173
What are the target cells for RT damage in lung?
endothelial cells
| type II pneumocytes
174
In the brain, which type of brain matter will show histopathologic change in the first year? After 12 mo?
First year --> white matter
| >12mo --> gray
175
There are two syndromes of late damage to the spinal cord following RT. What occurs between 6-18mo, and at 1-4 years?
6-18mo --> demyelination and white matter necrosis
| 1-4 years --> vasculopathy
176
What is the target tissue underlying the 6hr separation of fractions?
CNS - sublethal damage repair is slow in CNS
177
Which vascular structures are most radiosensitive? Least radiosensitive?
Capillaries - most radiosensitive
arteries - middle
veins - least radiosensitive
178
At what point in the cell cycle is the cell committed to S phase, and no longer responding to growth signals?
G1 restriction point
| regulated by Rb
179
What is the most important cell cycle checkpoint following radiation damage?
G2/M
180
What do Chk1 and Chk2 inhibit?
Chk1 --> inhibits Cdk1
| Chk2 --> inhibits Cdk2
181
Which cyclin do p53 and p21 inhibit?
Cyclin D/E
182
What is the equation for mitotic index?
MI = 0.693 x Tm/Tc
Where Tm is length of mitosis and Tc is total length of cell cycle
183
What is the equation for cell loss within a tumor population?
1 - Tpot/Td
where Tpot is potential tumor doubling time and Td is actual tumor doubling time
184
In which type of solid tumor is apoptosis common?
Carcinoma
Apoptosis is rare in sarcomas
185
What is the nominal standard dose?
total dose for tolerance of connective tissue as related to number of fractions (N) and overall time (T)
Total dose = (NSD)*T^0.11*N^0.24
186
How do the dose-response curves for early and late responding tissues differ?
Curve is steeper/more curved for late responding tissues
187
Can the bladder and kidneys be retreated following a first course of RT?
NO - bladder and kidney are not capable of recovery from late functional damage and do not tolerate retreatment
188
BED calculations assume a 2Gy/fx schedule. What is the assumed a/B for early responding tissues? Late responding tissues?
Early responding tissues - a/B = 10
| Late responding tissues - a/B = 3
189
What is HIF1 & where is it expressed? Where is HIF2a expressed?
HIF1 = global regulator of hypoxia-inducible gene expression; expressed in most cell types
HIF2a = expressed primarily in endothelial cells, glial cells, type II pneumocytes, heart, kidney, pancreas, and liver
190
How is HIF expression regulated?
In the presence of O2, HIF is hydroxylated by prolyl hydroxylases and ubiquitinated by VHL. Under hypoxia, PHD cannot hydroxylate HIF.
191
How does HIF1a regulate gene expression?
Binds to HIF1B in the nucleus and promotes transcription of genes involved in angiogenesis (ie VEGF), erythropoiesis, and glycolysis
192
What effect does mutation of VHL have on HIF?
HIF is stabilized in VHL-mutated individuals, as VHL usually targets HIF for destruction. VHL-mutated individuals are prone to development of highly vascularized tumors
193
In addition to HIF, what cellular response can be induced in response to prolonged hypoxia?
Unfolded protein response (UPR)
194
What is PERK?
PERK is a stress sensor in the ER involved in the Unfolded Protein Response pathway induced by hypoxia. PERK is important for surviving long term hypoxia.
195
What is the dose limiting toxicity of misonidazole?
Neurotoxicity - peripheral and CNS
196
Order the following drugs in terms of neurotoxicity:
Nimorazole
Etanidazole
Misonidazole
Misonidazole - most toxic
Etanidazole
Nimorazole - least toxic
197
Which of the quinone antibiotics is recognized as a hypoxic cytotoxin?
Mitomycin C
198
The efficacy of chemotherapy drugs is limited by the growth fraction of the tumor. What happens to the growth fraction as tumor size increases?
Growth fraction decreases as tumor size increases
199
3 of the following drugs are preferentially toxic to aerobic cells. Which is preferentially toxic to hypoxic cells?
- Bleomycin
- Procarbazine
- DOX
- Actinomycin-D
Doxorubicin
Bleo, proverb, and dactinomycin are all preferentially toxic to oxygenated cells
200
What impact does oxygenation have on cellular toxicity of 5-FU, methotrexate, cisplatin, and CCNU?
NONE
201
What is the mechanism of cellular damage due to hyperthermia?
Protein denaturation
202
Does hypoxia affect sensitivity to hyperthermia? What other factors impact sensitivity to hyperthermia?
Hypoxia - no
Factors that impact sensitivity:
- changes in lipid component of cell membrane
- acidic pH
- nutritional deprivation
203
How is hyperthermia measured?
Cumulative equivalent minutes (CEM)
ie measure of thermal dose CEM 43C T90 refers to the # of minutes 90% of the tumor was at 43C
204
Following irradiation, cells tend to stall in the ___ phase of the cell cycle.
G2
