Radiation Biology Flashcards

Radiation Biology

Introduction

Komar University of Science and Technology, 2009. Lecture 10 focuses on Radiation Biology, presented by Nika Atta.

Learning Objectives

• Understand different types of radiation.
• Distinguish between direct and indirect effects of radiation on cells.
• Explain the use of radiation in medicine.
• Explain long-term effects of excessive radiation exposure.

What is Radiation Biology?

• Study of effects of radiation on living tissue.
• X-rays are ionizing radiation.
• X-rays striking tissue results in ionization.
• Ionizing radiations can produce biologic changes in living tissue.

What is Radiation?

• Radiation (radiant energy) is energy in waves or particles moving through space.
• Warmth from sunlight is radiant energy from the sun.
• Electromagnetic radiation: radiation in electromagnetic waves (gamma rays, ultraviolet light, and radio waves).
• Particulate radiation: radiation in form of particles (alpha and beta particles).

Ionizing Radiation

• Atoms have equal protons and electrons but can lose or gain electrons (ionization).
• Ionizing radiation changes chemical state of matter causing biological damage, harmful to health.
• Examples: alpha, beta, and gamma radiation.

Non-ionizing Radiation

• Non-ionizing radiation bounces off or passes through matter without displacing electrons.
• Harmful effects on human health are currently unclear.
• Examples: visible light and radio waves.

Types of Radiation

1. Alpha Particle

• Two protons and two neutrons.
• Heaviest type of radiation particle with a large charge.
• Travels short distances.
• Cannot penetrate a sheet of paper or the surface of skin.
• Hazardous if inhaled or ingested.
• Emitted by naturally occurring radioactive materials like uranium and thorium.

2. Beta Particle

• Electron not attached to an atom.
• Small mass and negative charge.
• Travels farther than alpha particles.
• Stopped with minimal shielding.
• Can enter the body but not pass through completely.
• Tritium (produced by cosmic radiation) and Carbon-14 (used in carbon-dating) emit beta particles.

3. Neutron

• Particle with no charge in the nucleus of an atom.
• Interacts weakly with materials leading to long travel distances.
• Stopped by large quantities of water or materials with light atoms.
• Commonly seen when uranium atoms split (fission) in a nuclear reactor.
• Essential for nuclear power generation.

4. Electromagnetic Radiation (X-rays and Gamma Rays)

• More energy than sunlight with no mass or charge.
• Penetrates through the body.
• Widely used in medical treatments.
• Energy levels range from low (dental x-rays) to high (sterilization of medical equipment).
• Shielding with dense materials like concrete and lead is necessary.

Radioisotopes

What are Radioisotopes?

• Radioactive isotopes of an element.
• Same number of protons but different numbers of neutrons.
• Unstable combination of neutrons and protons or excess energy.
• Hydrogen isotopes: hydrogen-3 (tritium) is radioactive, others are stable.

How do Radioisotopes Occur?

• Unstable nucleus occurs naturally or artificially altered.
• Nuclear reactors used to produce radioisotopes.
• Uranium is a common naturally-occurring radioisotope; uranium-235 is more radioactive than uranium-238.

Radioactive Decay

• Unstable nucleus regains stability by emitting excess particles and energy (radiation).
• Radioactive decay is unique for each radioisotope, measured by half-life.
• Half-life: time for half of unstable atoms to undergo radioactive decay.
• Radioisotopes valuable in medicine for diagnosis and treatment despite radiation being harmful.

Sources of Radiation

1. Natural Background Radiation

• Comes from the sun, earth, and atmosphere.

2. Artificial Radiation

• Man-made: medical/dental x-rays, nuclear sources, consumer products/activities.

Pathways of Radiation

• Radiation and radioactive materials reach people through various routes.
• Examples: radioactive material in air falling on pasture, consumed by cows, present in milk, exposure to people drinking milk or inhaling radioactive material.
• Radioactive material in water affecting fish and people consuming fish or swimming in the water.

Effects of Radiation

Radiation causes ionization affecting:

• Atoms
• Molecules
• Cells
• Tissues
• Organs
• The Whole Body

Types of Radiation Injury

Direct Effects

• Radiation interacts directly with atoms of DNA molecule or critical cellular component.
• Affects cell's ability to reproduce and survive.
• Infrequent due to the small size of critical components within the cell.
• Direct interaction with active cell can cause death or mutation; interaction with dormant cell has less effect.

Indirect Effects

• Ionizing radiation breaks bonds in water molecules producing toxic radicals (hydroxyl OH, superoxide anion O_2^-, etc.).
• Radicals destroy the cell.
• Occurs frequently because cells contain 70-80% water.

Free Radical Formation

• X-ray photons interact with water in cells, resulting in ionization and free radical formation.
• Free radicals also formed by UV light, air pollution, inflammation, metabolism, and smoking.

Sequence of Radiation Injury

1. Prodromal Period
2. Latent Period
3. The Period of Injury
4. The Recovery Period

1. Prodromal Period

• Classic symptoms: nausea, vomiting, loss of appetite, and diarrhea.
• Occurs minutes to days following exposure, lasting minutes to days.

2. Latent Period

• Time between exposure and appearance of damage.
• Duration depends on total dose and rate of radiation.
• More radiation + faster dose rate = shorter latent period.

3. Period of Injury

• Cell injuries: cell death, changes in cell/tissue/organ function, chromosome damage, giant cell formation, abnormal cell division.

4. Recovery Period

• Not all injuries are permanent; low-level damage is often repaired.
• Scatter radiation in cells can be eliminated in 24-48 hours.
• Repeated exposure does not allow for adjustment.

Factors that Influence Health Effects

Dose Rate

• If dose received over a long period, the effect is less severe than a single, large dose.
• A high dose rate doesn't allow for cellular repair.

Location of Dose

• Partial body exposure is less severe than whole-body exposure.

Sensitivity to Radiation

• Developing fetus is most vulnerable.
• Infants, children, elderly, pregnant women, and immunocompromised individuals are more vulnerable.
• Rapidly dividing cells are more easily damaged.

Cumulative Effect

• Effects are additive; unrepaired damage builds up in tissues.
• Repeated exposure can lead to cancer, cataracts, and birth defects.

Short Term and Long Term Effects

Short Term Effects

• High doses over short periods kill cells, leading to death, skin burns, hair loss, sterility, and cataracts.

Long Term Effects

• Low doses over extended periods produce chronic effects which may take years to manifest.

Mutations

• In germ cells (sperm and ova), abnormalities can be passed to offspring.
• In somatic cells, can lead to diseases including cancer (carcinogenesis).
• Oncogenes affect cancer incidence.

Somatic and Genetic Effects

Somatic Effects

• Occur in all cells except reproductive cells.
• Not passed to future generations; affect only the individual.
• Primary consequence is cancer.

Genetic Effects

• Occur in reproductive cells; passed to future generations.
• Do not affect exposed individual but are passed via mutations in offspring.
• Genetic damage cannot be repaired.

Mitotic Cycle and Radiosensitivity

• Cells are most sensitive at or close to M (mitosis) and G2 phase.
• Resistance is greatest in the later part of S phase due to DNA repair.
• G1 phase has resistant and sensitive periods.

Mechanisms of Cell Death After Irradiation

1. Mitotic Death

• Cells die attempting to divide due to chromosome anomalies.
• Most common mechanism.

2. Apoptosis

• Programmed cell death.
• Results in cell separation in apoptotic bodies.

3. Bystander Effect

• Cells directly affected by radiation release cytotoxic molecules causing death in neighboring cells.

DNA Damage

Single-Strand Breaks

• Little biologic consequence because they are easily repaired.

Double-Strand Breaks

• Most important lesions produced by radiation.
• Interaction of two double-strand breaks may result in cell killing.
• Double breaks are lethal.

Uses of Radiation

1. Diagnostic radiology (Diagnostic)
2. Radiotherapy (Treatment)
3. Nuclear medicine (Diagnostic, Treatment, Lab tests)

Diagnostic Radiology

• Goal is to produce anatomical or functional patient image with lowest possible radiation dose.

Mammography

• X-ray picture of the breast, exposing to a small dose of ionizing radiation.
• Used to look for early signs of breast cancer; can detect up to three years before it can be felt.

Nuclear Medicine

• Uses radioactive isotopes as tracers; taken orally, injected, or inhaled.
• Radioisotope circulates or is taken up by certain tissues and tracked via emitted radiation.
• Emitted radiation captured by imaging techniques.
• Radioisotopes typically have short half-lives and decay before causing damage.

Radionuclide Angiogram

• Assesses heart pumping efficiency and blood flow.
• Radioactive tracer is injected to show heart chambers in motion.
• Gamma camera records heart muscle; inadequate blood supply (ischemia) absorbs less tracer for fainter image.

Medical Treatment Radiation

• Radioisotopes treat illnesses, particularly cancerous tumors.
• Caesium-137 and Cobalt-60 shrink tumors.
• Cobalt-60 sterilizes medical instruments.

Radiotherapy

• Treats cancer and abnormal tissue growth (e.g., hyperthyroidism).
• Tumor is bombarded with ionizing radiation.
• Disrupts atomic and molecular structure of targeted tissue.
• Breaks double-stranded DNA, killing cancer cells and preventing replication.
• Effective in slowing cancer progression or prompting regression despite side effects.

Indications for Radiation Therapy

• Treats solid tumors (breast, cervix, larynx, lung, pancreas, prostate).
• Treats leukemia and lymphoma.
• Palliative radiation reduces symptoms (pain) from cancer spread to bones.

Radiation Sterilization

• Kills germs and neutralizes harmful organisms.
• Ionizing radiation inactivates microorganisms efficiently.
• Sterilizes surgical instruments (syringes, gloves, clothing) using gamma emitting radionuclides (Cobalt-60 and Caesium-137).
• Safe and cost-effective for single-use medical devices.

Radiation Health Effects

• Ionizing radiation damages genetic material (DNA).
• DNA is primary target; radiation breaks bonds.
• Cells can repair damage but incorrect repair can lead to cell death or cancer.

Outcomes of Cell Damage

1. Cell repairs itself and returns to normal.
2. Cell damage is incorrectly repaired, leading to change and potential cancer.
3. Cell damage is too extensive, leading to cell death.