Radiobiology Flashcards
Roughly how many SSBs and DSBs occur per Gy of radiation?
SSB 1000
DSBs 40
What are hierarchical tissues?
Have populations of stem cells, transit (maturing partially differentiated) and functionally fully differentiated cells.
Acute toxicity after radiation
Michalowski classified tissues as following either a ‘heirarchical’ or ‘flexible’ model. Within tissues 3
types of cells exist – 1) Stem cells (have unlimited proliferation potential as ‘telomerase’), 2) Functional
(fully differentiated e.g. granulocytes) and 3) Maturing partially differentiating cells (E.g. granuloblasts).
Hierarchical tissues (H-type)
* Have all three populations with stem cells constantly giving rise to maturing cells which eventually
fully differentiate to become functional cells.
* Rapid turnover.
* E.g. Early responding tissue – bone marrow, epidermis and intestinal epithelium.
Cell death after irradiation occurs mostly as cells attempt to divide. Therefore: H-type (rapid turnover)
* Damage becomes evident quickly.
* Cells on the road to differentiation are more radioresistant, stem cells are more radiosensitive.
* Stem cell population is killed.
What are flexible tissues?
Cells rarely divide but may be induced to by damage. Cells are functional but retain ability to re-enter cell cycle.
Respond more slowly to radiation
Michalowski classified tissues as following either a ‘heirarchical’ or ‘flexible’ model. Within tissues 3
types of cells exist – 1) Stem cells (have unlimited proliferation potential as ‘telomerase’), 2) Functional
(fully differentiated e.g. granulocytes) and 3) Maturing partially differentiating cells (E.g. granuloblasts).
Flexible tissues (F-type)
* Cells rarely divide (normal conditions) but may be induced to by damage.
* Cells are functional but retain ability to re-enter the cell cycle if required. I.e. no strict hierarchy (as
above).
* E.g. Late responding tissues – liver, thyroid, dermis.
Cell death after irradiation occurs mostly as cells attempt to divide. F-type (rarely divide)
* Radiation damage remains latent for a long period and is expressed slowly.
* Particularly if dose is small, because cells do not enter cell cycle immediately.
* Acute damage is repaired rapidly because of rapid stem cell proliferation (maybe reversible).
* Late damage may repaired to some extent but is not fully reversible.
Name some F-type tissues (flexible)?
Liver
Thyroid
Dermis of skin
What is serial behaviour of an organ? Name some serial organs.
- FSUs are structured serially.
- Damage of 1 FSU can lead to loss of function of whole organ.
- Sensitive to a hotspot/high-point doses.
- E.g. spinal cord, oesophagus, rectum, brainstem, optic nerve, bowel
Small critical functioning volume
(Functional subunits: In some tissues FSUs are discrete I.e. Kidney – nephrons, Liver – lobule. In others there is no
clear anatomical demarcation I.e. skin, mucosa and spinal cord. The survival of structurally defined FSUs depends on the survival of one or more clonogenic
cell within them. Tissues maybe formed of large numbers of FSUs but each is a small self-contained entity
independent of its neighbours. Surviving clonogens cannot migrate from one to the other. E.g.
Kidney has low tolerance – high no. FSUs but each structurally independent.
In contrast, clonogenic cells can repopulate structurally undefined FSUs by migration and repopulation. E.g. Skin.
What is parallel behaviour of an organ? Name some parallel organs
- FSUs are structured in parallel.
- Each FSU is able to function independently of the others.
- Damage to one/several FSUs may not affect overall organ function.
- Can tolerate a higher dose in that area, more sensitive to overall volume affected e.g. lungs, liver, kidney.
Large critical functioning volume: I.e. only need 30% of the organ working to maintain function. Sensitive to TOTAL VOLUME irradiation but can tolerate a much higher dose in smaller
volumes.
What is the equivalent uniform dose?
EUD is the absorbed radiation dose which when given homogeneously to a tumour results in the same degree of clonogen cell kill as a non-homogenous dose.
Reflects the overall cell kill produced by a nonuniform dose distribution and so potentially a better indicator of biological outcome than simple average physical dose. It shows that even a small cold spot can play a disproportionate role in reducing overall treatment effectiveness
However, it does not inherently allow for the possibility that clonogenic cells themselves may be non-uniformly distributed e.g. if there was increased cells density in the cold spot the tumour control probability would be further reduced.
The normal tissue complication probability is a function of…
1, FSU inactivation probability
- Number of FSUs
- Doses received by the FSUs
- Assumed hierarchial structure (serial, parallel)
- Minimum numbers of FSUs to remain intact to ensure functionality
- Consequential functional links between groups of FSUs
How is the generalised EUD calculated?
Equation which uses a biological parameter “a” which is derived from clinical observation and chosen to reflect the desired radiobiological property.
a <1 = lower doses are given higher weight so that the cold spots influence EUD to a larger extent. Useful if cold spots on target
a =1 : cold and hot spots given equal value - good for parallel organs.
a >1 : larger doses are given higher weight so that hot spots influence the EUD to a larger extent- good for serial organs.
What is a clonogenic tumour cell?
Has the capacity to generate a new tumour
Requires viability AND capacity for sustained cell division
Cells that form colonies exceeding 50 cells (7 generations) - this excludes cells with limited growth potential.
Radiobiologically speaking, a cell is “killed” if it is rendered unable to divide and cause further cell growth - so senescence counts as death.
When do cells lose their reproductive ability after radiation?
Cells do not die immediately after dose of radiation. Some undergo division before dying but the loss of reproductive integrity that is thought to be the critical response to radiotherapy occurs within a few hours
Some clonogenic normal tissue cells can regenerate.
What is a clonogenic assay?
Used to assess the response of cells to ionising radiation. They may be used to assess cell survival following radiation by measuring colony formation after a particular dose. Several types of assay exist. A control assay, where no radiation is given, must be used to determine the efficacy of the test and adjust the observed outcome. This is known (for in vitro colony assays) as plating effectiveness.
Name some types of clonogenic assay?
In vitro - plates. Take a precise number of tumour cells, plate and irradiate with set doses, measure survival
and colony formation based on radiation dose. Specimen is taken from a tumour or from normal tissue. Prepared into single-cell suspension. Seeded onto culture dish, covered in growth medium, kept at 37 degrees @ aseptic conditions. If able to divide each cell develops into a colony. 2 plates are produced, 1 is irradiated and one is control. They are compared. Under normal conditions, all cells should survive. They do not, plating efficiency describes the % of cells seeded that grow into colonies (PE = colonies observed/ number of cells plated). Surviving fraction of irradiated plate is calculated = colonies counted/cells seeded x plating efficiency
Spleen colony assays
- If bone marrow cells are injected into a synergistic mouse recipient, colonies in the spleen will form from the surviving stem cells. The number of colonies can be counted to assess cell survival of bone marrow cells following irradiation. This technique is also used for some types of mouse lymphoma cells.
Lung colony assays
This is similar to a spleen colony assay. Transplanted mouse tumours will often grow more readily in the lung and the number of colonies that form on the lung surface is related to the number of surviving tumour cells.
Limiting dilution assays
Different sizes of tumour colonies may be inoculated into the subcutaneous tissue of a creature (usually a mouse). The number and size of colonies which grow is then observed. By taking into account the size of the inoculated tumour colony, highly accurate readings of cell survival can be obtained (down to 1 x 10-6 surviving cells)
If you have a single suspension of tumour cells and split them into 2 parts, one was irradiated and one was not.
They are then plated out.
Plate 1: 100 control cells -> 20 control colonies
Plate 2: 400 irradiated cells -> 8 irradiated colonies
What is the plating efficiency and surviving fraction?
Plate 1: efficiency = 20/100 = 0.2
Plate 2 efficiency = 8/400 = 0.02
Surviving fraction = plating efficiency treated/plating efficiency of control = 0.02/0.2 = 0.1 = 10%
After treatment a linear survival curve would have what shape?
Sigmoid shape
A semi-logarithmic survival curve would have what kind of shape?
Has a shoulder at top then exponential decrease
X axis = linear scale of dose (note is actually exponential but is plotted on a logarithmic scale so looks linear)
Y axis = exponential scale
What is the LD50?
Lethal dose that kills 50%
What is linear energy transfer?
The density of ionisation in particle tracks.
LET is the average energy (in KeV) given up by a charged particle traversing a distance of 1 micrometre.
What type of radiation has a high LET?
Alpha particles
What types of radiation has a low LET?
XR, gamma rays, protons (most of the time)
How does low LET Radiation cause damage?
1/3 direct DNA damage
2/3 indirect damage - ROS
Is low or high LET radiation more likely to cause cancer?
Low LET
Causes more mutations in surviving cells due to indirect damage and these can go on to form cancer.
High LET radiation more likely to kill cell then cause mutation in a surviving cell
As LET increases how does the survival curve change
Steeper without a shoulder
As LET increases what happens to the RBE?
RBE rises with increasing LET up to a certain point.
At very high values of LET you get overkill.
Very high LET radiation is inefficient as it deposits more energy per cell and so produces more DSBs per cell than is needed to kill it, so has less energy to kill other cells.
What is the optimum LET for cell kill?
Around 100kev/micrometre
As LET increases what happens to the OER?
Decreases as less depedend on oxygen.
What is the RBE?
Ratio of dose needed of one type of ionising radiation relative to another to yield the same biological effect. Usually reference radiation is on top: usually 250 kVp x-rays or 60Co γ-rays.
What is RBE measured in?
Sieverts (I think this is incorrect it should have no units as it is a ratio)
How do you calculate relative biological effectiveness?
Dx/Dr
Dx = reference absorbed dose of radiation of standard type X Dr = absorbed dose of radiation of type R that causes the same biological effect
What does alpha tell us on linear quadratic model?
Indicates sensitivity to low doses of radiation
What does beta on LQ model tell us?
Indicates sensitivity to high doses of radiation
What does the alpha/beta ratio tell us? Is it a marker of radiosensitivity?
does NOT relate to radiosensitivity
Indicates the sensitivity to changes in dose per fraction
Determines the bendiness of the survival cruve
Would a radiosensitiser affect the alpha/beta ratio?
No not the ratio but it would affect the alpha and beta seperately
If a cell has a low alpha/beta ratio, is it sensitve to dose per fraction?
Very sensitive to dose per fraction
Some tumours, notably prostate cancer and melanoma, have low α/β values. This makes them more sensitive to large fraction sizes and resistant to small fraction sizes
If a cell has a high alpha/beta ratio is it sensitive to dose per fraction?
No
What is the alpha/beta ratio of H+N, breast and prostate cancer?
H+N = 10
Breast = 4.5
Prostate =1.5
What is the alpha/beta ratio of spinal cord/brain/lens, skin, and other late organs?
Spinal cord/brain and lens = 2
Skin = 10 acute, 3 late
Other late organs = 3
According to the LQ model what is the effect of radiation proportional to ?
αD + βD^2
What is the mean inactivation dose?
An estimate of the average radiation dose to inactive a cell. Calculated as the area under the survival curve (survival probability).
If you reduce the overall treatment time then what happens to acute tox, late tox and tumour control?
Acute tox: worse
Late tox: same
Tumour control: better
If you give more fractions (hyper fractionate) with same dose and treatment time then what happens to acute tox, late tox and tumour control??
Acute tox: same
Late tox: better
Tumour control: same
If you increase the total dose then what happens to acute tox, late tox and tumour control?
Acute tox: worse
Late tox: worse
Tumour control: better
If you treat 2 x a day then what happens to acute tox, late tox and tumour control?
Acute tox: same/worse (needs 6hrs between)
Late tox: worse - due to repair
Tumour control: same /better
What happens to cells during acute toxicity after radiotherapy?
Differentiated cells - cell depletion, hypoplasia, desquamation
Vascular endothelium- increase permeability, oedema, erythema
Mediated by cytokines - paracrine signalling.
- IL1, IL6,
- TNF-alpha- promotes apoptosis
Healing of depleted mature cells via migration of stem cells from outside the area and division of surviving stem cells.
How long does repair take in late responding tissues?
Longer than acute responding
Does total dose have more effect on early or late responding tissues?
Early but effects both
The latent time for chronic radiation effects is proportional to what…
Inversely proportional to dose
Higher dose the quicker the onset and progression
Describe the early and late radiation effects on skin?
Early:
Dry skin -> erythema -> dry desquamation -> hyperpigmentation -> wet desquamation -> re- epitheliasation -> hair growth (over 1-10 weeks)
Late: subcutaneous fibrosis, telangiectasia, atrophy, dyskeratosis.
What are the risks of irradiating the teeth?
Radiation caries common - due to direct radiation effects at the dentine-enamel border zone and also effects on salivary glands
Risk of osteoradionecrosis of jaw
How does radiotherapy affect the salivary glands?
Dose of 40Gy saliva production stops
Dose of > 60Gy no recovery
Describe the acute and chronic effects of radiation on the intestine?
Acute
- increase in motility followed by atonic phase
- loss of epithelium and villi owing to proliferative impairment in the crypts causing water, electrolyte and protein loss -> diarrhoea
- Risk of sepsis
Late
- chronic ulcers + fibrosis -> stenosis, ileus
- telangiectasia - bleeding
How does radiation affect the liver?
Liver is radiosensitive but only dose limiting when the whole liver is irradiated.
Acute radiation hepatitis- 2-6 weeks after, usually presents as veno-occlusive disease
Chronic hepatopathy- 6m -years, progressive fibrosis
How are the kidneys affected by radiation?
Radiation nephropathy- takes years
- manifests as proteinuria, hypertension, impairment in urine concentration. Usually with anaemia
Glomerular endothelial injury starts a cascade leading to glomerular sclerosis and later tubo-insterstitial fibrosis- upregulation of plasminogen activator inhibitor 1 and enhanced fibrin deposition.
LOSS OF TOLERANCE OVER TIME- dose tolerated by kidney reduces with increasing time from radiotherapy due to continuous progression of damage.
What happens to bladder when irradiated?
Early: hyperaemia and mucosal oedema
Chronic- progressive mucosal breakdown from superficial ulceration to fistulae. Fibrosis and telangiectasia
How is the brain affected by radiation?
Transient demyelination - somnolence syndrome or leukoencephalopathy- occurs within 6 months
Radiation necrosis - 6m -2yrs
Changes that occur in the first 6-12 months are in white matter only, after that grey matter also shows changes with pronounced vascular lesions (telangiectasia and focal haemorrhages)
How is spinal cord affected by radiotherapy?
Lhermittes sign: usually due to reversible demyelination- severeal months after treatment and lasts months- years. Does not predict later development of myelopathy
Later myelopathy:
- 6-18 months- demyelination and necrosis of white matter
- 1yr-4yrs- vasculopathy
Does the spinal cord recover its tolerance?
Yes - around 50% after 1 year. As long as it received sub tolerance dose first time.
How are peripheral nerves effected by radiation?
Dose of 60Gy in 2Gy # assoc with < 5% risk but increases steeply after this.
Plexopathy characterised by mixed sensory and motor deficits
Develops from 6m - years
PathogenesisL vascular degeneration, fibrosis and demyelination
How is the heart affected by radiation?
Low doses: reversible functional ECG changes
High doses: pericarditis + effusion, 50% occur in 1st 6 months and rest within 2 years. Usually asymp and clears within a year.
Cardiomyopathy -10-20yrs later, conduction blocks or reduced EF
Cardiovascular disease
Pathogenesis: diffuse interstitial and perivascular fibrosis, loss of cardiomyocytes
How is the eye effected by radiotherapy?
Keratoconjuncitvitis: resolves after radiotherapy
Lens: epithelial degeneration in the equator zone where proliferation occurs, damaged fibres develop vacuolation and partly retain nuclei -> posterior subcapsular radiation cataract. 6m -> years.
What is lethal damage?
Hits to critical targets are sufficient to cause death at next mitosis.
Irreversible, irreparable damage- direct effect of radiation
What is sublethal damage?
Hits to critical targets not sufficient to cause death.
Repairable if given time, nutrients etc, unless additional radiation dose given
Corresponds to initial slow of survival curve
Effect of indirect damage - low LET radiation. Corresponds to beta on alpha beta curve so low alpha beta has more sublethal damage.
Found when single dose of radiation split into two doses. Found less cell death in split dose due to repair inbetween.
What is potentially lethal damage?
Hits are sufficient to cause cell death if enters cell cycle but may be repairable with period of cell cycle arrest.
May become lethal if misrepair occurs.
What is the law of bergonie and tribondeau?
From 1906
States rapidly diving cells are more sensitive to radiation
What is hyperfractionation?
Reducing dose per fraction <1.8Gy
What is hypofractionation?
Increases dose per fraction >2Gy
What is accelerated fractionation?
Rate of dose accumulation exceeds 10Gy/week
Why is the time between fractions important for late toxicities?
Take longer to repair
Need sufficient time to repair or worse late toxicity
What is SF2?
Clinically useful assessment of radiosensitvity
Surviving fraction after 2 Gy
Does NOT predict SF at other doses
Is SF2 related to alpha beta ratio?
No
Same ratio can be assoc with a high or low SF2
What is D0?
Another measure of radiosensitivity. D0 is the dose that produces an average of one lethal lesion per cell in a population of irradiated cells; Because the radiation-induced ionizations are random, discrete events , the probability to be killed follows a Poisson distribution with the assumption that one hit is enough to kill. In this instance (at D0), 37% of the targets will not receive a lethal hit and will survive.
the radiation dose that reduces survival to e^-1 of its previous value on the exponential portion of the survival curve.
What is radiation tolerance of normal tissues determined by?
Number of tissue stem cells
Their intrinsic radiosensitivity
What mechanisms allow repopulation?
Asymmetry loss
- seen during lag period
- once repopulation is started this effect is counteracted.
- usually a stem cell divides into one stem cell and one differentiated cell -> called asymmetrical
- for repopulation- stem cell divides into two stem cells- symmetrical division
Acceleration of stem cell proliferation - shorten stem cell cycle time
Abortive divisions- cells that have had high doses of radiation undergo a few more cell cycles before dying.
What is the most common form of cell death after radiotherapy?
Mitotic castastrophe
When does apoptosis occur after cell death and what happens?
Occurs in very radiosensitive tissues eg lymphoma/testicular.
Mainly intrinsic pathway -> activation caspase 9
- p53 activation and induction of pro-apoptotic proteins BAX and PUMA
- some cells almost never undergo apoptosis as despite activation it is not enough to activate cytochrome c
- other cells readily undergo apoptosis
Apoptotic sensitvity may be altered by mutation in p53
What is autophagy?
Can protect cells and can lead to senescence or death (through PTEN, mTOR)
Formation of double membrane bound structure that grows and engulfs cytoplasmic components forming a cytoplasmic filled vacuoles - autophagosomes.
What is necrosis?
Uncontrollable, irreversible chaotic form of cell death
Can be induced by radiation
Activated by extreme change in conditions eg pH, ion imbalance, energy loss
What is senescence and how is it controlled?
Usually p53 dependent
- via p21- temporary
- via p16 permanent
Cells usually experience initial period of exponential growth followed by permanent arrest termed replicative senescence - Hayflick limit.
Premature senescence can happen as response to radiation - characterised by flattened cytoplasm and increased granularity.
What is mitotic castastrophe?
Most cells try to undergo mitosis with DNA damage
Chromosomes can’t properly separate
Eventually after at least one mitosis the damage will b e so severe it will cause cell death
Formation of micro-nuclei
Multi-nucleated giant cells containing uncondensed chromosomes with aberrations and micronuclei.
What is a chromosome aberration?
Single chromosome is broken before duplication in S phase, may cause:
- broken ends may rejoin in original position
- fragment of chromosome may be lost
- broken ends may attache to incorrect broken ends -> lethal/stable
Reciprocal translocations
What is a dicentric chromosome?
Lethal chromosome damage
- short arm of two separate chromosomes are deleted
- 2 chromosomes (long arm with centromere each) attach to each other rather than their separated short arms
- new chromosomes have two centromeres then get duplicated.
May manage to complete mitosis however, loss of genetic material in the acentric fragment (micronuclei) in subsequent mitosis might lead to cell death.
What is a ring chromosome?
Lethal chromosome damage
Both ends are lost from same chromosome
- chromosome attaches its two ends together and forms a ring
What is an anaphase bridge?
Happens after chromosome is duplicated
Both chromatids suffer DSBs and sticky ends join together
At anaphase unable to separate the fused arms after centromere breaks.
What is an acentric fragment?
A segment of chromosome that lacks a centromere
What is bystander effect?
Cells respond to their neighbours being irradiated. Most relevant at low doses which cause damage to only a small number of cells
- via gap junctions
- via soluble factors, cytokines, ROS, NO
Responses can include DNA damage, mutations and cell death
List the radiosenstivity of each stage of cell cycle from most sensitive to least sensitive.
G2/M - most radiosensitive
- G1
- Early S
- Late S
What is the major transducer of DSBs? How does it do this?
ATM
Acts as inactive homodimers
DNA damage causes autophosphorylation which promotes dissociation of 2 components of dimer
Monomers then go on the phosphorylate proteins targets eg p53, Chk2, Chk1, BRCA1, Nbs1
What is the difference between ATM and ATR?
ATM activated by DSBs
ATR activated by single stranded DNA breaks at collapsed replication forms or distorted DNA.
ATR is slower onset and more sustained than ATM
What is p53s role in repair after radiotherapy?
Continiously synthesised and degraded by MDM2
Detects high levels of damage and activates apoptotic mechanisms
If moderate damage -> activated G1/S checkpoint for repair.
Commonly mutated in cancer
p53 mutant cells lack, G1-S checkpoint, also impaired apoptosis
What is Chk2?
Key target of ATM
Amplifies G1-S and G2-M checkpoint signals
Mutation in Chk assoc with familial breast cancer
What is Chk1?
Key target of ATR and ATM
Amplifies S phase and G2-M checkpoint signals.
What is the oxygen enhancement ratio?
Radiation dose in hypoxia / radiation dose in air
How does oxygen increase radiosensitvity?
Radiation induced free radicals are fixed (damage made permanent) by oxygen
Most indirect damage caused by water
Molecule splits to OH• and causes damage
DNA produces R• which is unstable and reacts with oxygen to produce RO2• then ROOH - damage fixed
In hypoxia:
R• reacts with H+ chemically restoring original form
What does a tumour make in response to hypoxia?
HIF- 1a - a transcription factor
Promotes angiogenesis, invasion and motility, less apoptosis and increased genomic instability
How can you make a tumour less hypoxic?
Blood transfusion - cervical >110
Carbogen gas (oxygen - chronic hypoxia) + nicotinomide (vit B3 - acute hypoxia, vasodilator) - in laryngeal + bladder
Nimorazole - mimics free radicals, specifically taken up by hypoxic cells, H+N
What causes cells to be chronically or acutely hypoxic?
Chronic hypoxic cells have a short half life and are continually replaced as displaced away from blood vessel
Acutely hypoxic cells occur when blood vessels transiently occluded
What is an abscopal effect in radiotherapy
Regression of distant mets after localised radiation enhanced by immune checkpoint blockade
What are the immune stimulating and suppressing factors with radiotherapy?
Lymphocyte depletion, increased Treg - at standard fractionation
Pro inflammatory factor release: DAMP
Tumour antigen presentation
Does depend on dose/fractionation
Why do gaps in radiotherapy matter?
Repopulation
Which cancers are priority 1?
Squamous cell tumours Adeno oesophagus SCLC NSCLC Medulloblastoma Treatment duration must not be prolonged by >2 days
What are priority 2 cancers?
Breast cancer
TCC bladder
Prostate cancer
Advised no prolonged interruption >2 days
What are category 3 patients
Palliative patients
What is accelerated repopulation?
Dog leg curve
Phenomonent that net clonogen doubling time during or shortly after irradiation exceeds clonogen doubling time in untreated tumour
? due to reoxygenation or due to nutrient supply improving
EGFR signalling has also been implicated
What are the options for making up for gaps in radiotherapy and which are used?
Give 1 extra fraction - maintain tumour BED
- worse so not done
Give 2 fractions per day
- same tumour control
- worse for late responding tissue due to repair time
- 2nd best way to compensate
Treat at weekends
- same tumour control
- same early and late responding tissues
- best way
What is EQD2 equation?
Equivalent dose in 2Gy fractions
EGD2 = D (d + α/β / 2 + α/β)
What equation can you use to compare different fraction sizes and calculate total dose required?
D1 = D2 x (α/β + d2 / α/β +d1)
Define biologically effective dose (equation)
BED = nd ( 1 + d/α/β)
Can you add BEDs together for different stages of treatment?
Yes eg external beam then brachy
you CANT add BEDs with different α/β ratios
How is BED effected by α/β ratio and dose per fraction?
α/β ratio - inversely related to fractionation sensitivity- smaller ratio provides larger fractionation sensitivity and higher BEDs
As you increase dose per fraction the BED goes up
How do you use BED to work out how to compensate for delays?
BED = nd ( 1 + d/α/β) - k x (T- Tdelay)
k = dose per day to compensate for repopulation k = 0.9 for H+N cancer k = 0.1 for prostate cancer
T = overall treatment time Tdelay = time to onset for repopulation 21-28 days
What is the dose rate for EBRT?
Usually around 1-5Gy/min. Therefore this is within the uppre range of high dose rate radiotherapy.
So exposure usually 1-3 mins and during this time no repair occurs
Which mechanism of cells recovering is the fastest?
Intracellular repair- starts at around 1 hr
Repopulation takes days to weeks
With a low dose rate what does the survival curve look like?
Radiosensitivity of cells decreases and so shouldered cell survival curve becomes straigher and flatter
What are the advantages of LDR brachy?
Allows max tissue recovery over shortest overall time.
However tolerance of early tissues reduced as the rely on repopulation more than repair.
Spare late responding tissues due to repair
Disadvantage - inadequate time for reoxygenation
At what dose rate would repopulation occur during brachy?
<1cGy/min
What is LDR, MDR and HDR brachy?
LDR = 0.4 - 2 Gy/hr MDR = 2-12 Gy/hr HDR = >12 Gy /hr
What is pulsed brachy and what are its advantages/disadvantages?
High dose rate, multiple times
Worse for late responding tissues
More accurate with todays planning systems
What is the disease modifying factor
Ratio of isoeffective radiation doses in the absence and presence of radiosensitisers
Also called sensitiser enhancement ratio
How do you convert BED to EQD2?
EQD2 = BED / (1 + 2 /α/β)
Name some radiosensitising chemotherapy agents?
Inhibit repair of radiation damage
- nucleoside analogues
- cisplatin
- bleomycin
- doxorubicin
Directly damage DNA - radiation SSBs may be converted to DSBs
- platinium adducts
Block cells in most radiosensitive phase - G2/M - taxanes
Apoptosis enhanced- taxanes and most drugs
Stop repopulation; block cells in most radiosensitive phase; stop DNA repair/turn ssDNA damage into dsDNA damage; kill outer layers so allowing oxygen into inner layers; cause additional cell death so activating immune system
What are the 5 Rs of radiobiology?
Order according to time
- Repair – DNA repair after exposure to radiation (<1 hour).
- Redistribution – Of cells into various phases of the cell-cycle after radiation exposure
(Several hours). - Reoxygenation - Increase in O2 from decreased tumour bulk and increased blood
supply after radiotherapy (hours to days). - Repopulation – Increase in growth of tumours after radiotherapy (several days).
- Radiosensitivity – intrinsic radiosensitivity that any cell has – can be exploited to improve
radiotherapy effects.
What are stochastic vs deterministic effects?
Stochastic effects have increased PROBABILITY of outcome with increasing dose with no threshold dose. Outcome is discrete - all or none.
Deterministic effects have increased SEVERITY of outcome with increasing dose with a threshold minimum dose. Outcome is continuous.
What are the radiation tolerances of blood vessels?
Radiation tolerances is capillaries > arteries > veins – capillaries radiotolerant
What is the RBE of standard radiation types?
250 kV X rays - 1.0
MV X rays - 1.0
Electrons - 1.0
Protons - 1.1 (1.5 at bragg peak)
Carbon ions - 1.5-5 - higher value is at the bragg peak distance
Fast neutrons - 4-5
