Fractionation and Dose-Rate Effect

Clinical advantages of dose fractionation

To help kill the cancer cells while avoiding healthy tissues

Four R's of Radiobiology

Repair, Reassortment, Repopulation, Reoxygenation

Which of the four R's is meant for protecting helping tissue?

Repair and repopulation

Which of the four R's is for damaging tumor cells?

Reoxygenation and Reassortment

Quick overview of repair

DNA damage is fixed promptly, protecting healthy tissue

Quick overview of reassortment

Cells move through the cell cycle between fractions, moving to more sensitive spots of the cell cycle

Quick overview of repopulation

Protecting healthy tissue, cell population increases due to cell division if time between fractions is long enough

Quick overview of reoxygenation

Damages tumor cells, cells are more sensitive to radiation in the presence of oxygen, helps to sensitize tumor cells

Single strand breaks are primarily caused by

Low LET

Double strand breaks are more likely with

High LET

Double strand breaks with low LET radiation are possible but more likely at

High doses

Lethal damage

Not repairable!!

-inevitably it leads to cell death

-abberations, rings, dicentric, anaphase

-caused by two double strand breaks interacting with one another

-cell is unable to divide properly in mitosis and is no longer variable

Potentially lethal damage (PLD)

Damaage that would be lethal under normal conditions

Can be repaired under SPECIAL conditions (such as a modified environment)

Sublethal (SLD)

Can be normally repaired

Sometimes becomes lethal is an additional single SLD is added

Dq on multitarget model

Conditions required for potentially lethal damage repair to occur in a cell

In order for PLD repair to occur, post-irradiation conditions must be SUBOPTIMAL for growth

What does a suboptimal environment trigger?

Triggers checkpoint mechanisms that prevent the cell from moving through the cell cycle

The resulting delay in mitosis seems to allow cells to repair radiation damage

Sublethal damage repair conditions

If a lone double strand break can be repaired before another radiation event causes a second strand break in the vicinity

DNA double strand breaks that can be repaired before that break has time to interact with another double strand break

Sublethal damage can be repaired in a matter of hours under normal cellular conditions BUT

If additional double strand breaks occur before the repair is complete, the damage has a higher chance of becoming a lethal aberration

If we want to harness the power of sublethal damage repair, we must

Find a way to lower the probability of a second double strand break

Dose

the amount of energy radiation deposits in any given unit mass via ionization

Two ways that a radiation dose can be manipulated to encourage sublethal damage repair

1) We can lower the total dose

OR

2) We can use the same dose but slow down the rate of interaction

How fractionation can be used to increase the radioresistance of a cell population

By dividing a large radiation dose into several smaller doses separated by a time interval

-the plateau that occurs on the graph indicates the time interval where all sublethal damage has been repaired and there is no advantage to increasing time between fractions

-the cells in this graph were maintained at room temp between dose fractions to prevent them from moving through the cell cycle

Single vs. fractionated dose and cell survival

Single event killing and sublethal damage repair

Single event killing is more devastating in relation to sublethal damage repair

(if one radiation event creates both DSB's, there is no way for the cell to repair the first break before the second one occurs)

What type of DNA damage is most likely to occur at LOW doses and LOW LET?

Single strand breaks

Almost all radiation interactions under these conditions result in single strand breaks, which are easily repaired by the cell

Cell survival at low doses of low LET

Cell survival is generally high due to single strand breaks and sublethal damage repair

If cell death does occur, it is likely due to single radiation event causing both DSB, which leaves no time for repair (linear component)

Type of DNA damage that is most likely to occur at HIGH doses of LOW LET

Dominant mechanism: single strand breaks

HOWEVER, higher statistical chance of double strand breaks now compared to at low doses because there are more radiation events happening in general

The odds of different radiation event creating a second double strand break is much higher (quadratic component)

High doses of low LET overview

Dominant damage: single strand breaks

Sublethal dose repair: less common

Chances of another DSB from another radiation event: more likely

What does the shoulder of a survival curve represent?

The ability of a cell population to repair sublethal damage quickly!

Linear component of linear-quadratic curve

Dominates in the early low-dose regions of the curve

-probability of DSB is low with low LET radiation at low doses

-if DSB occurs, it will likely represent sublethal damage

-lethal damage: predominantly the result of single events

How is the linear portion reflected?

High cell survival region of the curve, the shoulder of the curve is contaned within this region

Quadratic component of linear-quadratic curve

Dominates in high-dose regions of the curve

-there is significantly a higher number of radiation events

-more radiation events= higher number of interactions

-becomes more likely that two interactions will cause two DSB

How is the quadratic portion reflected?

Reflected in comparatively low cell survival in this region

Where does the shoulder of the cell survival curve end?

Ends at the same point where the quadratic portion of the curve takes over.

Cell-survival curve: linear quadratic model

Cell populations that are very efficient at repairing sublethal damage will have

broad shoulders

D = α/β

Represents the end of the shoulder and the beginning of the quadratic portion of the curve

The shift in where double events will dominate more of the low LET curve, making it much harder to repair sublethal damage

D = α/β in relation to sublethal damage

If we choose a dose below D = α/β point, we can harness the power of sublethal damage repair

D = α/β in relation to fractionation

If we know where D = α/β point is for a given tissue, we can use that to determine the dose needed for each fraction

By breaking up the total dose into smaller doses that eliminate the quadratic portion of the curve, we can give those normal tissues a chance at sublethal repair every time we deliver a fraction

How does fractionation effectively make a cell population more radioresistant?

By repeating the shoulder of the curver over and over

What is the dominant mechanism of DNA damage from high LET?

Double strand breaks

At which doses do double strand breaks overtake at high LET?

At all dose levels

What event also dominates at high LET?

Single-event killing due to high LET radiation being so densely ionizing

With single event killing, what is the chance of sublethal damage repair?

Zero chance

Since both DSB occur at the same time

High LET cell survivial curves do not have a

Shoulder

No D = α/β

High vs low LET cell survival curve

High LET

-no shoulder

-no D = α/β

-single event killing takes over at all dose levels

-no quadratic to calculate

Low LET

-has shoulder

-has linear and quadratic

High LET fractionation effectiveness

Ineffective at increasing the radioresistance of a cell population because there is no sublethal damage repair to take advantage of, and therefore no shoulder to repeat

Fractionating the dose almost makes no difference in cell survival for low-energy neutrons in this graph

Low LET fractionation effectiveness

Effective at increasing radioresistance because there is sublethal damage repair that can be taken advantage of

Radiosensitivity changes with position in

Cell cycle

Cells are more radiosensitive in

M phase and G2

Cells are more radioresistant in

S phase

Fractionation and reassortment

radiation will naturally produce a rough synchronization of in vivo cell populations by disproportionately killing cells in sensitive phases of the cell cycle

We can purposefully plan the timing between radiation fractions to be delivered when large numbers of cells are in the sensitive cycle phases

If we allow more cells to cycle into sensitive phases like G2 and M, we can kill more of them with the same dose

Fractionation and Repopulation

Longer periods between fractions give cells more time to progress through full cell cycle, including mitosis

Everytime a group of cells progress through M, it divides and doubles the original number of cells

Given enough time between fractions: repopulation

Cell survival increases due to multiplication in the M phase

Repair overall

Lower the probability of double strand breaks, cells can repair sublethal damage in hours

Reassortment overall

If we allow cells to cycle into sensitive phases like G2 or M, we can kill more of them at the same dose

Repopulation overall

If cells are given enough time to progress through the entire cell cycle, the population will increase via normal cell division

What does the curve look like with the four R's for proliferating cell lines?

If non-proliferating cells, what does it look like on a curve

Straight line, experience some repair because repair always happens on some level with low LET

If total cell cycle time is long/ non proliferating, which R's do not apply?

Reassortment

Repopulation

Repair: Fractionation Overall

We can protect healthy tissue by harnessing the power of sublethal repair

A dose should be carefully chosen to allow for a repeaat of cell survival curve shoulder

How many hours is needed between fractons for repair to happen?

Up to 2 hours

Reassortment: Fractionation Overall

Radiosensitizes tumor cells by choosing fraction timing that allows cells to cycle into sensitive phases

Repopulation: Fractionation overall

Must choose fraction timing that allows normal tissue to repopulate, even if that means some repopulation of tumor cells

Dose rate

Dose per unit time

Examples of common dose rates in radiation science

10 Gy/min

1 mR/hr

5rem/year

Dose fractionation

Administering discrete doses of radiation

There is a beginning and an end to administration

Dose rate effect

Irradiation is continous!!!

Lowering the rate of radiation interactions in tissue is an important way to prevent too many DSB in close proximity (lethal effects)

If you lower the dose rate and extend the total exposure time

It is equivalent to delivering an infinite number of infintively small fractions

Dose rate is fractionation's

fraternal twin

On a graph low dose rates are more

radioresistant on a graph, more to the right and transverse

more broad shoulders

On a graph high dose rates are more

radiosensitive, demonstrated to the left with less of a shoulder

Common applications influenced by principles of dose-rate effect

Occupation dose limits

Regulatory limits on device output

Shielding devices

Patient dose reduction programs (image gently/wisely)

ALARA

INVERSE SQUARE LAW (distance increases, dose decreases)

Inverse dose-rate effect

Lower dose rates trigger checkpoint genes in HELA CELLS, resulting in a blocked G2

G2 near mitosis is very sensitive

Under constant low-dose rate irradiation, many cells die here that would actually survive at higher dose rates

Which cell population is affected by inverse dose rate effect?

HELA CELLS

Repair: Dose rate effect

Low dose rates promote sublethal repair

Reassortment: Dose-rate effect

Lowering the dose rate cana trrigger a reassortment sensitivity in some cells

Repopulation: Dose-rate effect

Incredibly low dose rates allow depleted cells to repopulate

High LET and dose-rate

DNA damaged by single events represent a non-repairable component that is NOT AFFECTED BY DOSE RATE