Radiation Effects in Body Tissues

Radiation Effects in Body Tissues

RADIATION SCIENCE

CHAPTER 3

• Cell chemical composition
• Cell differentiation
• Radiation damage at cellular level
• Patient age and gender in radiation effects
• Radiation in Tissue
• When ionizing radiation (X-rays, gamma rays, particles)
• interacts with tissue, its biological effects depend on;
• chemical composition of cells
• their level of differentiation
• mechanisms of damage at the cellular level
• Levels Of Structural Organization of a Human Body About cell
• A cell is the basic structural and functional unit of
• living organisms. So when you define cell properties you
• are in fact defining the properties of life.
• The activity of an organism depends on both the
• individual and the collective activities of its cells.
• Continuity of life has a cellular basis. ✓ Cell Chemical Composition
• Inorganic compounds
• o Are compounds which lack carbon and have a simple
• structure.o Includes;
• Water, salts, acids and bases.
• o Water is the most abundant and important inorganic
• compound in living material. It makes up 60–80% of
• the volume of most living cells.
• o Their molecules have only a few atoms and cannot be
• used by cells to perform complicated biological
• functions
• Organic compounds (~20–30%)
• o Are relatively large carbon-based molecules and
• have unique characteristics that allow them to carry
• out complex biological functions.
• o Includes;
• carbohydrates, lipids, proteins, nucleic acids, and

adenosine triphosphate (ATP).✓ Cell Propagation

• Cells propagate (increase in number) through division.❖There are two types of human cells in the body;
• Germ cells
• ii. Somatic cells
• Germ cells
• These are special cells that are involved in sexual
• reproduction.
• These are; – Sperms in males
• Eggs (ova) in females
• Division of germ cells is called meiosis and involves two
• fissions of the nucleus giving rise to four sex cells, each
• possessing half the number of chromosomes of the
• original germ cell.Somatic cells
• These are all other cells in the body that are not
• involved in reproduction.
• Division of somatic cells is called mitosis and results in
• two genetically identical daughter cells.
• When a somatic cell divides, two cells are produced each
• carrying a chromosome complement identical to that of
• the original cell. New cells themselves may undergo further division and
• the process continues producing a large number of
• progenies.
• 𝐍𝐁; Mitosis results in identical cells. This alone is not
• going to result in different body cells with different
• functions.
• To achieve different cells performing different
• functions in the body, cells also undergo cell
• differentiation.✓ Cell Differentiation
• Definition
• Cell differentiation is the biological process
• unspecialized cell (like a stem cell) develops into a
• specialized cell type with distinct structures and
• functions (e.g., muscle cell, nerve cell, red blood cell).
• Somatic cells are classified as:
• Stem cells
• Are special human cells that are able to
• develop into many different cell types.
• ii. Transit cells
• Which are cells in movement to another
• population and have properties intermediate
• between stem and mature cells.
• iii. Mature cells,
• Which are fully differentiated and do not exhibit
• mitotic activity.
• Examples of Differentiated Cells
• Erythrocytes (RBCs): Specialized to transport oxygen
• (contain hemoglobin, lack nucleus).
• Neurons: Specialized for transmitting impulses (long
• axons, dendrites).
• Muscle cells: Specialized for contraction (contain actin
• and myosin).
• Epithelial cells: Specialized for protection, secretion, and
• absorption.
• Importance of Cell Differentiation
• Creates diversity of cell types in the body (over 200 in
• humans).
• Enables formation of tissues and organs.
• Crucial for growth, repair, and healing.
• Malfunction of differentiation can lead to cancers and
• developmental disorders.✓ Cell Cycle
• Cell cycle is a series of events that a cell passes
• through from the time it was produced to its death.
• It is the growth and division of a single cell into
• daughter cells and duplication (replication).
• In prokaryotic cells, the cell cycle occurs
• termed binary fission but in eukaryotic cells, cell cycle
• can be divided into two periods namely;
• a)Interphase
• b)Mitosis
• Interphase
• These is the first phase that prepare a cell for division.
• During this period a cell grows, accumulate nutrients
• needed for mitosis and duplicate its DNA.
• Interphase consists of three stages namely
• G1 (Pre -synthetic phase)
• S (DNA synthesis phase)
• G2 (Pre mitotic phase)
• Mitotic phase
• Mitotic phase involves division of cell to produce two
• daughter cells.❖Mitosis involves four stages
• Prophase
• Metaphase
• Anaphase
• Telophase
• Time between successive divisions (mitoses) is called cell
• cycle time.
• Cell cycle time for mammalian cells is of the order of 10 –
• 20 hours:
• S phase is usually in the range of 6 – 8 hours.
• M phase is less than 1 hour.

G2 is in the range of 2 – 4 hours.

G1 is in the range of 1 – 8 hours.❖Cell cycle time for stem cells in certain tissues is up to 10 days.

• In general, cells are most radio-sensitive in the M and G2
• phases, and most radio-resistant in the late S phase.
• Cell cycle time of malignant cells is shorter than that of
• some normal tissue cells, but during regeneration after
• injury normal cells can proliferate faster.✓ Radiosensitivity
• Radiosensitivity refers to how sensitive a cell is to
• radiation damage
• o Cells are more radiosensitive if they are:
• 1.Actively dividing (high mitotic rate).
• 2.Undifferentiated (immature).

3.Have a long-life span (young age).

• Highly Radiosensitive Cells

• o Stem cells, bone marrow cells, lymphocytes,
• spermatogonia, basal cells of skin, intestinal crypt
• cells.
• Moderately Radiosensitive Cells
• o Endothelial cells, fibroblasts, salivary gland cells,

growing cartilage and bone cells.

• Radioresistant Cells

• o Nerve cells, muscle cells, fully differentiated
• connective tissue.✓ Radiation Damage at Cellular Level

Basic Concept

• When ionizing radiation (X-rays, γ-rays, particles) passes
• through a cell, it interacts with atoms and molecules,
• causing ionization and excitation.
• This leads to molecular changes, especially in water and
• DNA.
• The biological effects depend on the type of radiation,
• dose, dose rate, and radiosensitivity of the cell.
• Review of types of radiation
• Radiation is classified into two main types:
• 1.Non-ionizing radiation (cannot ionize matter).
• 2.Ionizing radiation (can ionize matter).
• Ionizing radiation contains two major categories
• 1.Directly ionizing radiation (charged particles).
• eg. electrons, protons, alpha particles, heavy ions.
• 2.Indirectly ionizing radiation (neutral particles).
• photons (x rays, gamma rays), neutrons.Linear Energy Transfer
• When ionizing radiations traverse through matter, they
• lose energy gradually through various interaction
• processes along the length of their path.
• In radiobiology linear energy transfer (LET) is defined as
• the amount of energy that an ionizing particle transfers
• to the material per unit distance.
• LET focuses attention on the linear rate of energy
• absorption by the absorbing medium as the radiation
• traverses the medium.
• LET essentially indicates the quality of different types of
• radiation and is important because the biological effect
• of a radiation (its relative biological effectiveness, RBE)
• depends on its average LET.
• The SI unit of LET is kiloelectron volt per micrometer.
• Types of Radiation Based on Linear Energy Transfer
• There are two types
• 1.Low LET radiation
• 2.High LET radiation
• 1.High LET radiation
• This is a type of ionizing radiation that deposit a large
• amount of energy in a small distance.
• High LET characteristics include; high dense, limited
• penetration, more effective in causing DNA damage, High
• biological effectiveness
• eg. Neutrons, alpha particles and beta particles.
• 2.Low LET radiation
• This is a type of ionizing radiation that deposit less
• amount of energy along the track ie. have inadequate
• ionizing events.
• Characteristics of low LET radiation include; less dense
• iionization, more penetrative, indirect DNA damage, low
• biological effectiveness.
• eg X-ray and gamma rays
• High vs low LET Radiations
• High LET radiations are more destructive to biological
• material than low LET radiations.
• The localized DNA damage produced
• produced by high LET radiations is difficult to repair than
• the diffuse DNA damage caused by the sparse ionization
• from low LET radiations.High LET radiations result in lower cell survival per
• absorbed dose than low LET radiations.
• The high LET radiation is aimed at effectively killing of
• tumor cells while minimizing dose to normal tissues to
• prevent toxicity.
• Mechanisms/ Pathways of Cell Damage
• Direct Action
• Radiation directly strikes critical targets (mainly DNA).
• Causes ionization or breakage of molecular bonds.
• Indirect Action
• Radiation ionizes water molecules (which make up ~70–
• 80% of the cell).
• Produces free radicals (e.g., hydroxyl radical •OH,
• hydrogen radical •H).
• These reactive species damage DNA, proteins, and
• membranes.
• This is the most common pathway for X-ray and γ-ray
• damage.3. Types of Cellular Damage
• 1.DNA Damage
• o Strand breaks (single or double).
• o Base modifications.
• o Chromosome aberrations (deletions, translocations,
• dicentrics).
• o Can lead to mutation, apoptosis, or carcinogenesis.
• 2.Membrane and Organelle Damage
• o Lipid peroxidation damages cell membranes.
• o Mitochondrial dysfunction affects energy production.
• o Lysosomal rupture may release digestive enzymes.3.Protein Damage
• o Denaturation or cross-linking of proteins.
• o Enzyme inactivation → metabolic disturbances.
• Cellular Outcomes of Radiation Damage
• a)Repair: Cell successfully repairs DNA (enzymatic repair
• mechanisms).
• b)Misrepair:
• If the DNA is not repaired correctly, mutations may
• persist
• This altered cell can potentially lead to development of
• cancer over time.
• The risk of cancer is a long-term risk, probabilistic
• effect.
• c)Cell death:
• If the damage is too extensive to be repaired, the cell
• can die.
• Apoptosis (programmed cell death, active process of
• cellular self-destruction).
• Mitotic death (cell fails to complete the cell division
• cycle, leading to death).

NB; Extensive cell death in a tissue can impair its function and potentially lead to organ failure.d)Senescence: Cell survives but loses ability to divide.

Factors Influencing Cellular Radiation Damage

• Cell type: Rapidly dividing, undifferentiated cells (e.g.,
• bone marrow, GI lining, gonads) are more radiosensitive.
• ii. Cell cycle stage: Cells are most sensitive in G2/M phase,
• more resistant in late S phase.
• iii. Oxygen effect: Presence of oxygen enhances free radical
• formation (oxygen enhancement ratio).
• iv. Radiation quality: High LET (linear energy transfer)
• radiation (e.g., α-particles) causes more direct damage
• than low LET (X-rays).✓ Patient Age and Gender in Radiation Effects
• Age and Radiation Effects
• Children and Infants
• More radiosensitive than adults.
• Reasons:
• o Higher proportion of dividing/undifferentiated cells
• (active growth).
• o Longer life expectancy → more time for late effects
• (e.g., cancer, genetic mutations) to manifest.
• o Organs are smaller → a given dose affects a larger
• fraction of the body.
• Clinical significance:
• o Extra care in pediatric radiology.
• o Use of ALARA principle (As Low As Reasonably
• Achievable).
• o Preference for non-ionizing imaging (ultrasound,
• MRI) when possible.
• Adults
• Radiosensitivity decreases with age because:
• o Cells are more differentiated.
• o Slower mitotic activity.
• However:
• o Certain tissues (e.g., gonads, thyroid, breast) remain
• sensitive.
• Elderly
• Less radiosensitive overall.• Shorter life expectancy → reduced probability of longterm (stochastic) effects like cancer.
• More prone to deterministic effects (tissue reactions) if
• exposed to high doses due to reduced repair capacity.
• Gender and Radiation Effects
• Female Sensitivity
• On average, women have slightly higher lifetime risk of
• radiation-induced cancer than men.
• o Example: breast and ovarian tissues are highly
• radiosensitive.
• o Thyroid cancer is more common in women.
• Pregnancy considerations:
• o Embryo/fetus is extremely radiosensitive, especially
• during organogenesis (weeks 2–8).
• o Risks include congenital malformations, growth
• retardation, mental impairment, and childhood
• cancers.
• o Radiation protection guidelines require pregnancy
• screening before certain exams.
• Male Sensitivity
• Testes (especially spermatogonia) are highly
• radiosensitive.
• High doses can cause temporary or permanent sterility.
• Male gonadal exposure also carries hereditary risks
• (genetic mutations passed to offspring).3. Combined Age and Gender Considerations
• Group Sensitivity to

Radiation

Key Clinical

• Implications
• Infants/Children Very high Prefer nonionizing
• imaging;
• minimize
• dose strictly
• Women of
• childbearing
• age
• High concern
• (due to breast,
• ovaries,
• pregnancy risk)
• Screen for
• pregnancy;
• shield
• gonads;
• justify every
• exam
• Pregnant
• women
• Extremely high
• concern (fetal
• exposure)
• Use
• alternative
• imaging
• when
• possible;
• follow strict
• dose limits
• Men
• (reproductive
• age)
• Moderate
• (testicular
• radiosensitivity)
• Protect
• gonads with
• lead
• shielding
• Elderly Lower long-term
• risk
• Late
• stochastic
• risks less significant,
• but tissue
• tolerance
• lower
• Summary
• Age: Younger = more radiosensitive, higher cancer risk,
• longer time for effects to appear.
• Gender: Females slightly more radiosensitive overall;
• reproductive organs (testes/ovaries) need special
• protection.
• Clinical practice: Always apply justification, optimization
• (ALARA), and protection strategies tailored to patient age

and gender.