Characteristics Of Ioning Radiation In Tissue

CHARACTERISTICS OF IONIZING RADIATION IN TISSUE

Learning objectives

• At the end of this session the student should be able to:
• Apply knowledge of molecular biology in relation to radiation sciences
• Describe the properties of x-rays in living tissues.
• Understand the quantities and units of radiation
• Cellular response to ionizing radiation

Describe the biological effects of ionizing radiation Describe the mechanism of functioning of radiation dose measuring device Thermoluminescent dosimeter (TLD)

OVERVIEW OF STRUCTURE OF DNA

DNA….

DNA….

DNA, or deoxyribonucleic acid, is a complex molecule that carries genetic information in living organisms.

Double helix: The twisted ladder-like shape of DNA, which is composed of two strands of nucleotides wound around each other.

Nucleotide: The basic building block of DNA, consisting of a sugar molecule, a phosphate group, and a nitrogenous base.

Nucleoside: Consists of sugar and nitrogenous base Sugar-phosphate backbone: The alternating sugar and phosphate molecules that make up the outer edges of the DNA double helix.

DNA…

Nitrogenous base: The chemical component of the nucleotide that provides the genetic code for the DNA molecule.

Adenine (A): A nitrogenous base that pairs with thymine (T) in the DNA molecule.

…..

• Thymine (T): A nitrogenous base that pairs with adenine (A) in the DNA molecule.
• Guanine (G): A nitrogenous base that pairs with cytosine (C) in the DNA molecule.

Cytosine (C): A nitrogenous base that pairs with guanine (G) in the DNA molecule.

Hydrogen bond: The weak bond that holds together the nitrogenous base pairs in the DNA molecule.

Base pair: The complementary pairing of nitrogenous bases in the DNA molecule (A-T and C-G).

5′ and 3′ ends: The two ends of the DNA molecule that are defined by the position of the sugar molecule.

Cellular response to ionizing radiation

Ionizing radiation such as x-rays can affect the atoms in living things, so it poses a health risk by damaging tissue and DNA in genes.

Ionizing radiations like x-rays and gamma rays have sufficient energy to affect the atoms in living cells and thereby damage their genetic material (DNA). Fortunately, the cells in our bodies are extremely efficient at repairing this damage.

….

Radiation can kill the cells by inhibiting their ability to divide, its effects in human beings occur primarily in tissues with high cell turnover or renewal rates characterized by a large amount of proliferative activity.

However, if the damage is not repaired correctly, a cell may die or eventually become cancerous.

Characteristics Of Ioning Radiation In Tissue

Radiation can kill cells by two distinct mechanisms. The first is apoptosis (=falling off), also called programmed cell death or interphase death (facilitated by apoptotic genes) .It is an active process that involve DNA fragmentation and cell shrinkage and fragmentation. Cells undergoing apoptosis as an immediate consequence of radiation damage usually die in interphase within a few hours of irradiation.e.g low doses of radiation can induce apoptosis in lymphocytes, spermatogonia (stem cells) and oocytes.

…

Second mechanism is called necrosis (Greek word Nekrosis= death) that involves loss of cell membrane integrity, leakage of cell contents, enzymatic digestion of cells with multiple death (autolysis)

CELL CYCLE

…..

Quantities of ionizing radiation

i)Radioactivity /Radiation intensity (directly measurable): Represents rate of radioactive decay and is measured in Becquerel (Bq) as per SI unit.

• i.e. 1 Bq= 1 decay per second.
• Other unit used is Curie (Ci) where 1Ci=3.7×10^10 Bq (for Radium) and

1Bq= 2.7×10^ -11 Ci

Quantities of ionizing radiation………

Radiation dose quantities:

ii) Absorbed dose/Exposure dose (directly measurable): Is the amount of energy deposited per unit mass (C/kg) in a person.

• It is measured in gray (Gy) as per SI unit.

Other unit used is rad where 1Gy=100 rad

……

iii) Equivalent dose (protection quantity): This indicates effects on individual human organs and tissues.

• It is measured in sievert (Sv) as per SI unit.

iv) Effective dose (protection quantity):

This indicates effects on the whole body by combining effects on individual organs and tissues.

It is measured in sievert (Sv) as per SI unit.

BIOLOGICAL EFFECTS OF IONIZING RADIATION

• There are two general types of biological effects from ionizing radiation namely:
• Deterministic effects and

Stochastic effects

Deterministic (non stochastic) effect

Deterministic effects are those effects whose severity in the exposed individual is dependent on dose; these effects are commonly regarded as having a threshold. Deterministic effects (tissue reactions) are symptoms caused by deaths or degeneration of a number of cells constituting organs and tissues.

Since in most organs and tissues there is a continuous process of loss and replacement of cells, a slight increase in the rate of loss due to cell killing can be compensated for by an increase in the replacement rate. If the radiation exposure is higher, there may be some reduction in function of that particular tissue.

Early deterministic somatic effects occur within a short period of time after exposure to ionizing radiation.

These effects include nausea, fatigue, erythema, epilation, and blood and intestinal disorders.

Acute radiation syndrome (ARS)

Acute radiation syndrome (ARS) refers to health effects that are caused by being exposed to high amounts of ionizing radiation in a short period of time.

ARS can manifest as;

Hematopoietic syndrome; Bone marrow syndrome, causes the number of red blood cells, white blood cells, and platelets to decrease.

Gastrointestinal syndrome; (nausea and vomiting, rectal bleeding, diarrhea).

Cerebrovascular syndrome; (headache, fever, dizziness, confusion (disorientation) Skin reddening and tenderness Hair loss (alopecia) Infertility, Congenital malformations and brain maldevelopments in the fetus).

Syndrome is the medical term that defines a collection of symptoms.

Skin reddening

EFFECTS TO FOETUS

Exposure to 0.1 Gy at an early stage of pregnancy (preimplantation period) may lead to miscarriage.

Radiation-induced congenital abnormalities can occur approximately 10 days to 12 weeks after conception.

Radiation exposure in the second and third trimesters can cause malformations, growth retardation, congenital defects and a predisposition to the development of childhood cancer

Stochastic (health) effect

A stochastic effect is one in which the probability of occurrence increases with increasing dose but the severity in affected individuals does not depend on the dose (induction of cancer, radiation carcinogenesis and genetic effects).

There is no threshold dose for effects that are truly stochastic, because these effects arise in single cells and it is assumed that there is always some small probability of the event occurring even at very small doses.

Thermoluminescent dosimeter

Thermoluminescent dosimeter (TLD): a passive dosimeter that produces a radiation-induced signal, which is stored in the device crystal.

The crystal is made of Lithium floride TLDs absorb radiation, store energy, and release it as light when heated, with the light emitted proportional to the radiation dose.

TLDs ensure radiation doses remain within safe limits, helping protect workers' health and comply with regulatory standards.

Disadvantages of TLDs

• Each dose cannot be read out more than once.

The readout process effectively “zeroes” the TLD.

Advantages of TLDs

• TLDs can measure a greater range of doses compared to film badges.
• Doses from TLDs may be easily obtained.
• TLDs can be read on site instead of being sent away for development.

TLDs are easily reusable.