Radiobio part 1 Flashcards
6 Rs of Radiobiology (ordered according to time)
Repair
Redistribution
Reoxygenation
Repopulation
Radiosensitivity
Remote bystander effect
Heirarchical or flexible - what are the 3 types of cells
- Stem cells - has unlimited proliferation potential
- Functional - fully differentiated e.g granulocytes - incapable of further division and die after finite life span.
- Maturing partially differentiating cells - e.g. granuloblasts
Hierarchical Tissues
Have all 3 populations with stem cells constantly giving rise to maturing cells which eventually differentiate to become functional cells.
Rapid turnover & high rate of cell loss
Early responding tissues - bone marrow, epidermis & intestinal epithelium
(fails when precrusor cell pool fails to generate enough differentiated cells e.g. bone marrow)
Response to radiation (cell death after irradiation occurs mostly as cells attempt to divide).
- damage becomes evident quicker.
- cells on the road to differentiation are more radioresistant - stem cells are more radiosensitive
stem cell populationis killed
Functional subunits
In some tissues FSU are discrete e.g. kidneys - nephrons, liver - lobule
In others non clear anatomical demarcation ie skin, mucosa, spinal cord
survival of structurally defined fsu depends on the survival of one or more clonogenic cell within them
FSUs may be arranged in serial, parallel or combination.
An FSU is the largest tissue volume or unit of cells that can be regerated from a single surviving clonogenic cell.
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 cell cycle if required i.e. no strict hierarchy
E.g. late responding tissues - liver, thyroid, dermis
Low turnover rate & respond slowly to irradiation damage
Response to radiation:
- 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
- late damage may be repaired to some extent but is not fully reversible.
Serial Organs
FSUs structured serially (like links in chain - each one critical to organ function).
Damage of 1 fsu can lead to loss of function of whole organ
sensitive to hot spot/high point doses
E.g. spinal cord, oesophagus, rectum
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
higher functional reserve capacity
risk of complication related to total dose rather than hot spots
CFV (critically functioning volume)
HIGH CFV:
- e.g. liver & kidney
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
- said to have parallel like behaviour
LOW CFV
said to have serial like behaviour
clonogenic cells
cells with capacity for sustained cell division (at least 7 generations)
Clonogenic tumour cell - capacity to generate new tumour
Clonogenic normal tissue cell - can regenerate functional tissue following cytotoxic insult
Radiobiologically cell is killed if it is rendered unable to divide & cause further growth
Clonogenic survival assay
used to measure clonogenicity in vivo and in vitro.
= GOLD standard measure for radiotherapy sensitivity in the lab.
Clonogenic assay in vivo
(living organism)
Irradiate cells ex vivo & inoculate a known number into a mouse - measuring how many colonies form in lung/spleen
Clonogenic assay In vitro
(lab dish/test tube)
more relevant to clinical radiotherapy than in vivo assays of cell growth or proliferation
take a precise no. of tumour cells, plate & irradiate with set doses, measure survival and colony formation based on Radiation dose
measures reproductive intergrity of the clonogenic stem cells in tissue
Cell survival Curves
Describes relationship between radiation dose & proportion of cells that survive
Mitotic death is dominant mechanism of death following irradiation
Radiobiologically, death = unable to divide and cause further growth . (if cell fully differentiated, death could be defined as loss of function)
A dose 100 Gy is necessary to destroy cell function in non proliferating systems. In contrast the mean lethal dose for loss of proliferative capacity is usually less than 2Gy.
Process of cell survival curve
Specimen taken from tumour or 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, 1is control & then they are compared
Under normal conditions all cells should survive - they don’t - use plating efficiency
Plating Efficiency
Describes the % of cells seeded that grow into colonies
PE = colonies observed / number of cells plated
(x 100)
Surviving fraction of irradiated plate is calculated
Colonies counted / (cells seeded x {plating efficiency/100})
Shape of cell survival curve
Dose plotted on a linear scale and fraction on a log scale (allows measurement over huge range 0.001 to 1)
not a straight line because at low doses cells are able to regenerate
Initial linear slope - low dose region, less DNA damage, cells are able to repair
Shoulder region - higher dose, bend in curve
Exponential region as dose continues to increase ++ DNA damage overwhelms cells ability to repair
All mammalian cells, normal or malignant exhibit similar XR survival curves
Factors affecting cell cycle curves
- Type of radiation, dose rate, dose delivered
- Type of tissue (acute or late responding) - a/b ratio
Rs of radiobiology
Combination with other therapies
Multi target model for cell survival curve
used for many years
Described in terms of initial slope (D1), the final slope (D0) and n/Dq to represent the width of the shoulder
Width of shoulder is determined by:
- n (extrapolation number). If large (10-12)= broad shoulder, if small (1.5-2)= narrow shoulder
- Dq quasithreshold dose (almost threshold dose/sublethal dose). Defined as dose at which the straight portion of the survival cure, extrapolated backwards cuts the dose axis through a survival fraction of unity. There is no true threshold dose as there is no dose below which radiation causes no effect.
Radiosensitive cells are characterised by curves with a steep DO (or narrow shoulder). i.e. if High LET there is no shoulder and Dq is very small
Linear Quadratic Model
now model of choice
assumes 2 components to cell killing by radiation. One that is proportional to the dose ( linear) and one that is proportional to the square of the dose (quadratic)
A = gradient of the shoulder (low dose). Cell death increases linearly with dose. Directly proportional
B = the gradient of the exponential (high dose) - Cell death increases in proportion to the square of the dose. (directly proportional to square of dose)
E = AD + BD^2.
E= effect, D = radiation dose
S = fraction of cells surviving dose (D)
A & B are constnats
S = e^-ad - BD^2
Linear Quadratic Equation
Linear component = unrepairable cell kill
Quadratic component = repairable but cumulative cell kill
Cell kill is the result of single lethal hits, plus accumulated damage from 2 independent sublethal hits
E= aD - lethal non repairable linear term (low dose region, cells that can’t repair themselves after 1 radiation hit). Important for HIGH LET RADIATION
- Apoptotic and mitotic cell death are dominant
E = BD^2 - sublethal repairable quadratic term. Corresponds to cells that stop dividing after more than 1 radiation hit but can repair damage caused by radiation. LOW LET RADIATION. Mitotic death is dominant.
LQ model best describes data in range of 1-6 Gy and should not be used outside this range
A/B ratio
dose at which death due to single lethal lesion = death due to accumulation of sublethal lesions. ie. aD =bD^2
indicates sensitivity to changes in dose fractionation
can be used to predict response to different fractionation schedules
Determines bendiness of survival curve
Larger e.g. 10Gy for early responding tissues and smaller i.e. 2Gy for late responding tissues
most tumours have a high ratio, some (e.g. melanoma, sarcoma & prostate) have a low ratio.
a= sensitivity to low doses of radiation
b = sensitivity to high doses of radiation
Factors determining tumour growth
cell cycle time
growth fraction
cell loss fraction
cell cycle time
quantitative assessment of the constituent parts of the cell cycle
1) Mitotic index
counts proportion of cells in mitosis
if assume all cells in population are dividing & all have the same mitotic cycle
= length of mitosis / total length of mitotic cycle/cell cycle.
2) Labelling index
cell population is first ‘flash labelled’
fraction of cells that take up titrated thymidine (ie. the fraction of cells in S) (proportion of cells that are labelled)
Both tells us the ratio of mitosis and DNA synthesis as fractions of the Tc. But not absolute duration of any part of the cycle.
Growth Fraction
= fraction of cells in active cell cycle
= p/ p + q
p = proliferating cells
q = quiscient cells
Because not all tumour cells are in the growth fraction, the actual measured tumour volume doubling time is usually longer than cell cycle time.
In normal adult tissues show no net growth i.e. GF is balanced by net loss
For tumours cell cycle time roughly 1-5 days. Tumour doubling time is roughly 40-100 days.
Potential doublign time = Cell cycle time (tc) / GF
(defined as cell doubling time without any cell loss)
Cell Loss
Cell loss factor = fraction of cells produced by cell division that are lost from the tumour. Tends to be large for carcinomas and small for sarcomas.
Cell loss factor = 1 - Tpot (potential doubling time) / Td (actual tumour doubling time)
