Oxygen Effect/ LET and RBE

Reoxygenation

Cells are more sensitize to radiation in the presence of oxygen

Out of all pharmologic radiosensitizers,

oxygen has been determined to be the most effective and practical

Oxygen enchancement ratio (OER)

Ratio of the dose needed to kill a certain number of hypoxic cells to the dose needed to kill the same number of oxygenated cells

OER equation

D hypoxic/ D oxygen

Once a free radical has reacted with DNA it is called a

DNA radical

Oxygen fixation hypothesis

In the absence of O2, certain protein components like sulfhydryl groups bind to the DNA radical to help facilitate repair

However, if O2 is in the cellular environment, it will bind to the DNA radical instead to form RO2, which makes the damage permanent (fixes/freezes repair) by blocking the action of other helpful repair molecules

DNA radicals are more highly-reactive and susceptible to binding of

O2

DNA that has been damaged via the direct method of interaction is

NOT susceptible to oxygen fixation

DNA that has been damaged via the indirect method of interaction is

HIGHLY SUSCEPTIBLE to oxygen fixation

(predominantly low LET)

In order for a cell to be affected by the block that oxygen creates to DNA damage repair,

Repair must be possible in the first place

(not doable with single event killing)

Areas of single-event killing on cell survival curve are

Less affected or completely unaffected to the presence of oxygen

Areas of cell survival curve dominated by sublethal damage repair (shoulder) or double event killing (quadratic component) are most affected by

Oxygen

Oxygen needs to be delivered before the cell

starts to repair itself

Oxygen must be present

1) at the exact moment of irradiation

2) within microseconds (10-6s) of the irradiation

OER range for low LET

2.5 (low doses) -3.5 (high doses)

OER on graph

Oxygenated cell populations are significantly more radiosensitive

Hypoxic cell populations are more radioresistant

The green box on high-dose assay curve represented by low-dose assay graph

Why does the area of the curve in the green box have a lower OER?

This is the lowest-dose region of the curve, where cell survival is incredibly good

If cells are going to die here, it is most likely due to single event killing, which is unaffected by oxygen fixation

Higher dose needed typically under

hypoxic conditions

OER value for alpha particles

Nonexistent or 1

OER for neutrons

1.6

OER is diminished or nonexistent for

1) Radiation that damages DNA via direct interactions (high LET)

2) Radiation that is dominated by single-event killing (high LET)

Oxygen concentration needed to observe an increase in radiosensitivity of a cell population

5% needed for a dramatic increase in cell killing

Beyond 5% cell killing

remains the same even if the concentration of oxygen is increased

Very small amounts of oxygen produce big effects, but once all DNA damage has been made permanent by oxygen,

increasing the oxygen concentration does not result in more cell killing

Different tissues in the body exist at different levels of

hypoxia

Oxygen status effects radiosensitivity of those tissues

Tumors composed of varying levels of

hypoxic cells

42 types of human tumors contain up to

50% hypoxic cells, making it harder to treat

average of 14% hypoxic cells

Chronic hypoxia

Centers of large tumors tend to be chronically hypoxic/anoxic due to distance from capillary beds

Cells between oxygenated and anoxic layer may be hypoxic enough to be radioresistant but oxygenated enough to promote tumor growth

Acute hypoxia

Tumors can become acutely hypoxic due to blockage of a blood vessel

TRANSIENT OR INTERMITTENT

Fractionation to increase radiosensitivity of chronically hypoxic tumors

Delivering a dose of radiation kills a layer of oxygenated cells, allowing the hypoxic cells underneath to receive oxygen again (reoxygenation)

Note: fractionation ALLOWS necrotic cells to reoxygenate (making it more effective to kill tumors cells)

Layers of cells in tumor

Oxygenated (outside)

Hypoxic (middle)

Anoxic (inner)

Three types of cells tumors consist of

Oxygenated

Acutely hypoxic

Chronically hypoxic

Acutely hypoxic cells reoxygenate in a matter of

hours

Chronically hypoxic cells reoxygenate over

Days

Distance between survival curves represent number of

hypoxic cells remaining in tumor

Low LET components

Create free electrons in a medium

Electrons incredibly light and fast

High LET components

Create recoil protons in a medium, and also more direct ionization

Protons and alpha particles more heavy and lumbering

Linear energy transfer (LET)

Average energy deposited in a given material over a given distance

How is LET reported?

keV/um

keV/micrometers

Advantages of utilizing concept of LET

1) there are clear LET patterns that can be seen in cell survival curves

2) allows us to speak in broad terms about different types of radiation

general framework for describing the biological effects of radiation

Disadvantages of utilizing the concept of LET in radiobiology

1) different ways to calculate LET

-track average (breaking up sections in equal distance)

-energy average (amount of energy deposit)

Note: the two calculation methods can give significantly different results in certain situations

2) not the most accurate framework for radiobiology

Relative biological effectiveness (RBE)

Used to compare the different degrees of biological damage from different types of radiation

MUCH MORE COMPLETE PARAMETER THAN LET

Measure of killing efficiency

Takes radiation we are testing and compares to the gold standard

Gold standard in RBE evaluations

250 kV x-rays (D250)

RBE equation/ratio

The ratio of the dose of 250 kV x-rays to the dose of the test radiation that results in EQUIVALENT BIOLOGICAL EFFECT

D250/Dr

D250: gold standard (250 kV x-rays)

Dr: dose of test radiation

Two mandatory experimental conditions for calculating RBE

1) Must be using the SAME CELLS

2) Must be using the SAME BIOLOGICAL ENDPOINT

What changes in RBE?

DOSE

Example of RBE

10 Gy/ 6.6 Gy

=1.5

so test radiation is 1.5x more effective

LD50

Important biological endpoint for relative biological effectiveness (RBE) experiments

-cell survival/death is a common and useful biological endpoint used for RBE

The radiation dose required to kill HALF of the study population

Meaning of RBE as it relates to biological impact of different radiation types

MEASURES CELL KILLING EFFICIENCY

Example: RBE of 1.5 for neutrons means that the neutrons were 1.5 times more efficient at killing cells than 250 kV x-rays (if RBE less than 1, usually means it is worse)

RBE is influenced by

1) LET

-higher LET, more RBE

2) Dose

-higher dose, more RBE

3) number of fractions

-more fractionations, more RBE

4) dose rate

5) type of biological system

Once LET exceeds 10 keV/micrometer, RBE

increases dramatically

As LET approaches 100 keV/micrometer, RBE

decreases dramatically

Peak of 100 keV/micrometer LET

Average distance between ionizing events is about the same width as the DNA double helix

Ionizations are perfectly spaced to create double strand breaks

-killing effiency is good

Before the peak

Probability of two DSB is low

After peak

Many DSB are produced, but extra energy is wasted

OER and RBE at 100 keV/micrometer

RBE rapidly increases

OER rapidly decreases

Radiation weighting factors (Wr)

Practical application of RBE

takes into account

-RBE

-type of effect produced (cancer and hereditary effects)

-considerations of lower doses and dose rates

Wr for low LET

1

Wr for high LET

5-20

Equivalent dose (Sv) equation

Absorbed dose (Gy) x Weighting factor

100 rem= 1 Sv

Equivalent dose: Dosimeters

-deposition of energy into crystals is true absorbed dose (Gy)

-once the dose is determined, that value is multiplied by a radiation weighting factor appropriate to the radiation type

-the application of a weighting factor of any kind transforms the unit Gy (absorbed dose) into the unit Sv (equivalent dose)