Radiobiology-1

Radiobiology I

Direct and Indirect action

• The biologic effects of radiation result principally

from damage to DNA which is the critical target

• The atoms of the target itself may be ionized or

excited, thus initiating the chain of events that

leads to a biologic change – called direct action

• The atoms may interact with other atoms or

molecule in the cell to produce free radicals that

are able to diffuse far enough to reach and damage

the critical target.

Direct and Indirect action

• Critical Target Theory

• Direct action -occurs when the radiation is absorbed by a molecule known to be critical to maintain life of the cell e.g. DNA.This may initiate a series of events that lead to changes that may be lethal to the cell

• This is the dominant process for high LET radiation

Indirect action

• Indirect action --occurs when radiation interacts with other molecules in the cell,most importantly water.The products of these interactions may then go on to interact with the DNA

• This is the dominant process for low LET radiation

• About two thirds of the biologic damage by x-rays is caused by indirect action

How does the ionising radiation

damage the cells?

• Indirect action: Electrons

produce free radicals which

break chemical bonds and

produce chemical changes

• Direct Action: Photon ejects an

electron which produce a

biological damage on the DNA

Free radicals

• A free radical is a molecule or

atom,which is not combined to anything

(ie.free) and carries an unpaired

electron in its outer shell,i.e.its looking

for something to interact with, or in

purely scientific terms, it is in a state

associated with a high degree of

chemical reactivity.

Free Radical…..

For simplicity let us consider what happens if radiation interacts with water molecule (80% of a cell is composed of water.

If the water molecule is ionised

• H2O =H2 O++e- ; H2 O is the water

molecule

• H2O+ is an ion radical.

Free Radical…..

• Ion meaning it is electrically charged,

because it has lost an electron and a

radical because it has an unpaired

electron in the outer shell,making it very

reactive.

• Ion radicals have a short life,usually no

more than 10-10 seconds, before they

decay to form free radicals

• Free radicals are not charged, but do have an unpaired electron in the outer shell. The water ion radical can for example do the following:

• H2 O+ +H2O =H3O

+ +OH*

• H2 O+, H3O

+ are the ion radicals

• OH*is a highly reactive hydroxyl radical, with 9 electrons, therefore one is unpaired.

• Hydroxyl radicals (OH*),are highly reactive and can go on to react with DNA.It is estimated that 2/3 of the

x-ray to react with DNA.

Indirect action – The Process

Incident X-ray photon

Fast electron (e-)

Ion Radical

Free Radical

Chemical changes from the

breakage of bonds

Biologic effects

In summary the Indirect action is as follows

Incident x-ray photon

Fast electron (e-)-occurs in 10-15 seconds

Ion radical -live about 10-10 seconds

Free radical -live about 10-5 seconds

Chemical changes from the breakage of bonds

Biological effect -may be expressed in hours, days, months, years or not at all, depending on the consequences of the bonds

broken.

The time scale

• The Physics of the absorption process is over in 10-15 second;

• The chemistry takes longer because the lifetime of the DNA radicals is about 10-3 to 10-5 second;

• The biology takes days to months for cell killing, years for carcinogensis, and generations for heritable damage.

Single & Double strand break

• When cells are irradiated with x-rays, many breaks of single strand occur.

• These single-strand breaks are of little biologic consequence as far as cell killing is concerned as they are repaired readily using the opposite strand as a template

Double strand break

• If both the strands of the DNA are broken and the

breaks are well separated, repair again occurs

readily, because the two breaks are handled

separately.

• If the breaks in the two strands are opposite one

another, or separated by only a few base pairs this

may lead to a double-strand break. That is the

piece of chromatin snaps into two pieces

Single & Double strand break

• A double strand break is believed to be the

most important lesion produced in

chromosomes by radiation.

• The yield in irradiated cells is about 0.04

times that of single strand breaks.

• The double strand break is induced linearly

with dose

What is cell death?

• For differentiated cells that do not

proliferate, such as nerve, muscle, or

secretory cells, death can be defined as loss

of specific function.

• For proliferating cell such as stem cell in

hematopoietic system or the intestinal

epithelium, loss of capacity for sustained

proliferation loss of reproductive integrity

THESE DAMAGES CAN LEAD

TO

• Slowdown in the cell synthesizing copies of its DNA, so

that there is a delay in one cell dividing into two cells

• Delays (to allow repair) as the cell progresses towards its

next cell division (delay in cell cycle progression)

• Decrease in the overall rate of cell proliferation (increase in cell number) of a population of cells

• Death of the cell

• Mutation of the cell

• Changes in the cell which will make it cancer-like (called cell transformation)

THE TYPES OF DAMAGE TO THE

DNA INCLUDE

• DNA Single Strand Breaks

• DNA Double Strand Breaks

• Sugar Damage

• Base Damage

• Local Denaturation (Separation of the 2 strands)

• DNA-DNA Cross-links

• DNA-Protein Cross-links

DIFFERENT TYPES OF CELL

DEATH

• General Description

• INTREPHASE DEATH: Death before the next cell division, or death of a cell that does not divide

• REPRODUCTIVE DEATH: Death of the cell (and its daughter cells) after one or more cell divisions

• Specific Description

• NECROSIS: Death of a contiguous (touching) field of cells

Does not require energy; the contents of the cells leak into the surrounding tissue and blood supply

• CELL LYSIS: The cell simply bursts open, releasing its contents

• APOPTOSIS (or Programmed Cell Death): This type of death is under genetic control (specific genes must be present and active or inactive). It requires energy, and when the cells die, DNA fragments of specific sizes, and the contents of the cells, are encapsulated in membranes as small vesicles.

CHROMOSOME ABERRATIONS

• Types, Dose and Dose Rate Dependence

• Ionizing radiation exposure results in many different types of aberrations, with the type depending on where the cell is in relation to its next division (position in its cell cycle).

• The most commonly measured types of aberrations are ring and dicentric aberrations, which can be used for biological dosimetryafter an acute whole-body exposure above 10 – 25cGy (within a defined period after the exposure)

• There are many other types of aberrations that can occur, and if they (like the ring and dicentric aberrations) are obvious upon microscopic observation, the cell with those aberrations would likely have died.

• Certain kinds of chromosome aberrations, as well as genetic mutations of the DNA in the chromosomes, can be associated with causing cancer.

THE RELATIVE RADIOSE�SITIVITY OF

THE CELLS I� THE BODY

• Fully differentiated, functional and non-dividing cells (e.g, nerve

cells, muscle cells) are RADIORESISTANT

• Partially differentiated cells that can be called upon to divide again

(e.g., liver cells, glandular cells) are somewhat less radioresistant

• Cells which can divide but lend support to the other cells in a tissue

(e.g. endothelial cells lining the blood vessels, fibroblasts of the

connective tissue) are intermediate in radiosensitivity

RADIATION AND CANCER

• Cancer resulting from exposure of cells to ionizing radiation is a “stochastic” or probability phenomena

• The outcome is either yes or no, and there is no threshold of dose below which ionizing radiation cannot induce cancer

• The types of cancers due to exposure of a large number of persons to ionizing radiation include both blood cancers and solid tumors

• The relationship may be either linear or linear-quadratic, depending on the type of cancer (e.g., for blood cancers, the incidence increases in a linear quadratic manner with dose, while for solid tumors, the increase is linear with dose, and fractionation does not decrease the risk

Cell survival curve

• The cell survival curve is plotted on a semi

log plot

• For low doses for sparsely ionizing

radiations, the survival curve starts out

straight on the semi log plot with a finite

initial slope i.e. the surviving fraction is an

exponential function of dose

Linear quadratic theory

• Cell is inactivated by DSB

• Single hit, single lesion [random process],

governed by Poisson statistics

• S = exp (-αD)

• S - surviving fraction of cells

• α - average probability per unit dose that this will

occur

• D - dose delivered


• 2 separate ionising events,

• probability of one interaction causing one lesionis linearly proportional to dose, as is mean probability of second particle doing the same

• Mean probability of both events is βD2

• β - mean probability per unit dose squared that such complementary events will occur


• In general cell survival is described by:

S = exp (-αD -βD2)

• ν α damage (irrepairable)

• ν β damage (repairable)

NB of special interest is when αD = -βD2

i.e. the curviness of cell survival curve

α/β ratio

• High α/β [straighter curve], characteristic of cell

with little repair capability e.g. tumour cells [from

5 - 20 Gy]

• Low α/β [more curved], characteristic of high

repair potential e.g.late responding normal tissue

[1-4 Gy]

• ν This difference in cell surv ival curves provides

rationale for fractionated radiation therapy

treatment and explains radiobiological advantage

MAMMALIAN CELL SURVIVAL

CURVE

• Survival vs. Dose

Shoulder Region

Shows accumulation of SUBLETHAL DAMAGE

– The larger the shoulder region, the more dose will initially be needed to kill the same proportion of cells

• Beyond the Shoulder Region

– The Do Dose, or the inverse of the slope of the curve, indicates the relative radiosensitivity. The smaller the Do dose, the greater the radiosensitivity

Cell Survival

S=e-αD+βD2

αD=βD2

D=α/ β

MAMMALIAN CELL SURVIVAL

CURVEThe Effect of Lowering the Dose Rate

Cell cycle and Radiosensitivity

• Cells are most sensitive at or close to mitosis

• Resistance is usually greatest in the latter part of S

phase

• If G1 phase has an appreciable length a resistant

period is evident early in G1, followed by a

sensitive period toward the end of G1

• G2 phase is usually sensitive, perhaps as sensitive

as M phase

The stages of the mitotic Cycle

S (DNS Synthetic phase)

G1G2

M

Cell survival curve for various

Phases of Cell cycle