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.
