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RadTech: RadBio Ch.3


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When were injuries first reported after the discovery of X-rays?

What caused the delay?
As early as 11 years after the discovery of radiation

This delay is because of the latency period in tumor development
Give two ways to study the responses to radiation.
In Vivo - in the living organism

In Vitro - in glassware or test-tube
Why is In Vitro tissue culture extremely useful for studying the response on a single cell line?
This is because vasculature and other physiological factors present in the living organism do not contribute to the response
What are asynchronous populations of cells?
Populations in all four phases of the cell cycle
What are synchronous populations of cells?
populations in a given phase of the cell cycle at a given time

require techniques that place the cells there
Which type of population are dividing populations of cells in the body?
Synchronous or asynchronous?
Dividing populations of cells in the body are asynchronous and thus study using this method more nearly simulates the in vivo situation.
Who constructed the first cell survival curves?
The first cell survival curves were constructed by Puck and Marcus.
What were the first cell survival curves constructed from?
HeLa cells (derived from human carcinoma of the cervix) in tissue cultures.
Who else constructed cell survival curves?

What did they do?
Withers, McCulloch and Till

developed techniques to construct cell survival curves using in-vivo systems, such as the skin and hemopoietic systems.
What are the possible fates of Irradiated Cells?
1. It can be delayed from going through division - termed Division Delay

2. It can die before it divides - termed Interphase death

3. It can die when attempting mitosis - termed Reproductive Failure
Where does Division delay occur?
Division delay occurs in both nonlethally and lethally damaged cells.
Where do Interphase death and reproductive failure occur?
Interphase death and reproductive failure (by definition) occurs in only lethally damaged cells.
What is the Mitotic Index?
The ratio of the number of cells in mitosis to the total number of cells in the population
What is Division delay?
Irradiation of the population disturbs the Mitotic Index.

Cells in mitosis at the time of irradiation complete division.

But those about to enter division are delayed in G2.

The mitotic index decreases for a period of time. (during this delay).
What is Mitotic Overshoot?

What causes it?
An increased number of cells in the M phase

If the dose is low enough the cells recover from Division delay and proceed through M phase.

The overshoot is due to the contribution from those cells progressing through the cell cycle normally PLUS those that were delayed
What doses affect the Mitotic Index?
Low doses (0.5Gy) produced a negligible effect. However Division delay can be induced in human kidney cells at doses as low as 0.1 to 0.5Gy.

As dose increased (0.83 to 3.0Gy) the response became more pronounced.

Both the length and the magnitude of the delay increased.

The mitotic overshoot also increased in magnitude with these doses.
What happens to the Mitotic Index at higher doses?

After higher doses the mitotic index fell below the pre-irradiated value and remained there

At these doses the cells divided but died after division (reproductive Failure).
What happens to the Mitiotic index at doses of 10Gy.

The length and magnitude of the delay were greatly increased.

NO mitotic overshoot occurred

The cells were not able to overcome the block imposed by the radiation and died before division - interphase death
What is Division Delay dependent on?
Division Delay is a DOSE DEPENDENT phenomenon.
What is the underlying cause of mitotic delay?
Three theories:

1. A chemical involved in division is altered by irradiation.

2. Proteins necessary for mitosis are not synthesized.

3. DNA synthesis does not progress at the same rate following irradiation.

These are guesses but are ALL a function of Dose
What does the mitotic overshoot demonstrate with regard to a cell's capacity?
The mitotic overshoot reflects the ability of the cells to overcome the radiation induced block and proceed through mitosis along with unaffected cells
In which part of the cell cycle does Division Delay occur?
Division delay only occurs with cells in G2 and cells at the beginning of DNA synthesis
Define Interphase Death?
This is cellular death before entering mitosis.

Also called non-mitotic or non-division death
In which types of cells can Interphase death occur?
Interphase death can occur in rapidly dividing cells (stem cells) as well as those that do not divide (adult nerve).

It has been observed in oocytes, erythroblasts and cancer cells.
Which cell contradicts the types of cells where Interphase death occurs?
The prototypical cell that is known to die via this mechanism after low doses of radiation is the small mature lymphocyte.
What general relationship is there between, dose, cell types and Interphase death?
In general, rapidly dividing, undifferentiated (radiosensitive) cells exhibit interphase death at lower doses
eg. Mouse spermatogonia at 0.25Gy.

While non-dividing fully differentiated cells require higher doses, (more radioresistant).
eg. Parotid cells at about 9.0Gy.

The one exception is the lymphocyte - it exhibits interphase death at doses as low as 0.50Gy.
Is Interphase death a degenerative process?
Morphologically, interphase death is not a degenerative process.

In interphase death the cell condenses and breaks up into pieces, but the cytoplasmic organelles remain intact. These pieces are then phagocytosed by other cells

This is not a unique effect of irradiation
What is Apoptosis?
Apoptosis is programmed self-destruction of cells

It can occur spontaneously in healthy as well as diseased tissues.
What may the mechanisms of Interphase Death be?
It may be due to changes in the plasma membrane, with accompanying imbalances in extracellular and intracellular salts (Na, K and Ca).
What is Reproductive Failure?
Reproductive failure is defined as the inability of the cell to undergo repeated divisions after irradiation

all cells that can not repeatedly divide and produce a large number of progeny are considered non-survivors or “dead”.

Even though technically they are still alive (metabolizing or capable of limited number of divisions).
A cell survival curve describes the relationship between radiation dose and proportion of cells that SURVIVE.

a semilogarithmic curve.
X-axis is linear with dose in rads.
Y-axis is logarithmic with surviving fraction
What can be said about the proportion of cells killed and the absolute number of cells killed for EQUAL incremental doses?
The same dose always kills the same proportion

the absolute number of cells killed will vary

This is a logarithmic relationship between dose and surviving fraction
Do equal increases of dose produce equal decreases in the surviving fraction in the broad shoulder of mammalian cell curves?

In the shoulder equal increase in dose DOES NOT produce equal decrease in surviving fraction. (otherwise it would be straight).
Are doses of less than 2Gy efficient at producing cell death?
doses less than 2Gy are inefficient in producing cell death.

This implies that for mammalian cells damage must first accumulate before the cells die.
What mechanisms are suggested to explain the shape of the cell Survival Curve?
(the small straight portion right at the very beginning of the shoulder and the final straight line portion)
1. This small initial exponential region must be due to single hit killing.

Just by random chance some cells are killed by one hit to a critical area

2. The final exponential region is the accumulation of a sufficient number of sublethal hits as to cause death
What is the “Target Theory” model?
It states there are “n” number of targets in a cell which must be hit to kill the cell.

If one target is not hit, the cell will survive and repair the damage.

As the dose increases more sublethal damage accumulates and the cell dies.

This produces the final exponential region
How many cell targets are left when the final exponential part of the curve is reached?
The final exponential shape of the survival curve is obtained when all cells have sustained “n-1” hits and the population now has only one target in each cell that must be hit to be killed
What parameters are used to describe the mammalian shouldered Cell Survival Curve?
Four parameters are used to describe these more complex curves.
1. “n”, or extrapolation number

2. The second “Dq” or quasi-threshold dose

These first two parameters refer to the shoulder of the curve.

3. “Do” which refers to the final or terminal slope

4. “1Do”, is used to describe the initial exponential slope.
What is the extrapolation number?
The extrapolation number “n” (also called the target number in the past), is described by extrapolating the final exponential region back to the y-axis

It, in theory represents the number of “targets” that must be hit in each cell to cause death
What is the “n” range for mammalian cells?
The “n” for mammalian cells ranges from 2 to 10
What is defined by Dq⬝?
The “Dq” defines the width of the shoulder region of the curve.

This is the dose at which the extrapolation of the terminal portion of the curve intercepts the dose axis (y-axis) at 1.0 or 100%.

Dq only provides information on the size of the shoulder NOT its shape.
What is defined by Do⬝?
Do is determined from the final exponential region of the curve
Itis the reciprocal of the slope ( 1/slope ).

Do is the dose that inactivates (kills) all but 37% of the population

It is an expression of RADIOSENSITIVITY of a population
What is the Do dose range for mammalian cells?
Do doses for mammalian cells vary between 1 and 2Gy
What is defined by 1Do⬝?
1Do describes that small initial exponential region falling right before the shoulder of the curve.

This is a result of single hit killing
What is the linear quadratic model?
A purely mathematical representation of a cell populations outcome to radiation interactions
What are the parameters of the linear quadratic model?

What do they represent?
The parameters are alpha and beta. (not the particulate radiations)

Alpha which is proportional to the dose.

Beta which is proportional to the dose squared
What is the formula for the linear quadratic model?
SF = alphaD + BetaDsquared
What does Alpha represent?

What does Beta represent?
Alpha represents cells killed by single hit interactions

Beta is in reference to the accumulation of multiple hits or lesions
In what dose region is the linear quadratic model most useful?
This model provides a good empirical description of cell survival especially in the low dose region
What three things can happen to a population of cells after irradiation?
1. Some cells will receive NO damage.

2. Some cells will accumulate enough damage to be lethal and will die in the next division.

3. Some cells will accumulate a degree of damage that is not lethal, which given enough time can be repaired, thus forming the shoulder.
Can damage from sparsely ionizing radiations be repaired?
This is seen in both normal tissues and malignant tumors.
Define the different types of repair.
SLD (sub lethal damage repair).

PLD (potentially lethal damage repair).
What is sub lethal damage repair?
When a dose of radiation is divided into two equal doses (split dose) separated by various intervals of time, the surviving fraction of cells is larger than if the total dose is given all at one time.
What factor increases survival in sub lethal damage repair?
Survival increased with time between doses up to 2hours when survival reached a plateau

This increase in survival was due to repair of sublethal damage
What happens to the survival rate in split dose studies?
The first dose reduces survival to the terminal exponential portion of the curve

If sufficient time elapses between the two doses the surviving cells respond as if they were never irradiated
What type of radiation is only applies to sublethal damage repair?
It is very important to note that sublethal damage repair applies ONLY to x-rays and gamma rays.

And is nonexistent for neutrons.
What do large and narrow shoulders represent with respect to sublethal damage repair?
Cells with large shoulders (broad) exhibit a large amount of SLDR.

Cells with narrow shoulders (stem cells) show very little SLDR

For the linear quadratic model.
Narrow shoulders have large alpha/beta ratio.
Large shoulders have small alpha/beta ratios
What is the range of repair half times in vivo for normal tissue?
Repair half times in vivo for normal tissues vary from 0.5hours to 1.5hours.
Where is Sublethal damage and its repair of critical importance?
Sublethal damage and its repair are critical factors in the sparing effect in normal tissues during fractionated radiotherapy
What is Potentially lethal damage repair?
This type of repair has to do with the conditions the cell is placed post irradiation exposure
What factor affects Potentially lethal damage repair?
Placing a cell, post exposure, in suboptimal growth conditions by depleting its nutrient supply
How is depleting a cell's nutrient supply likely to affect it?
Their survival actually INCREASES

This happens because placing the cells in suboptimal conditions delays (slows) their entry into mitosis

This extra time permits DNA repair and survival increases
What ype of radiation is most likely to allow Potentially lethal damage repair?
Sparsely ionizing radiations

NO repair of PLD has been seen after exposure to high LET radiations

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