Radiation Biology Flashcards

Introduction to Radiation Biology

• Radiation Biology Definition: The study of the effects of radiation on living tissue.
• Ionizing Radiation: X-rays are a form of ionizing radiation, which results in ionization when striking patient tissues. All ionizing radiations can produce biologic changes in living tissue.

What is Radiation?

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

Ionizing Radiation

• Ionization: Atoms lose or gain electrons, altering their chemical state.
• Definition: Capable of changing the chemical state of matter, causing biological damage, and potentially harmful to human health.
• Examples: Alpha, beta, and gamma radiation.

Non-ionizing Radiation

• Definition: Bounces off or passes through matter without displacing electrons.
• Harmful Effects: Unclear whether harmful to human health.
• Examples: Visible light and radio waves.

Types of Radiation

1. Alpha Particle:

   • Composition: Two protons and two neutrons.
   • Characteristics: Heaviest type of radiation particle with a large charge.
   • Penetration: Doesn't travel far; cannot penetrate a sheet of paper or the surface of the skin.
   • Hazards: Harmful if inhaled or ingested.
   • Sources: Naturally occurring radioactive materials like uranium and thorium.

2. Beta Particle:

   • Composition: An electron not attached to an atom.
   • Characteristics: Small mass and negative charge.
   • Penetration: Travels farther than alpha particles; can be stopped with minimal shielding.
   • Effects: Can enter the body but not pass all the way through
   • Sources: Tritium (produced by cosmic radiation) and Carbon-14 (used in carbon-dating).

3. Neutron:

   • Composition: Particle with no charge in the nucleus of an atom.
   • Characteristics: Does not interact well with materials, travels a long way.
   • Shielding: Requires large quantities of water or light atom materials to stop.
   • Sources: Nuclear fission in a nuclear reactor (splitting of uranium atoms).

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

   • Characteristics: More energy than sunlight, no mass or charge.
   • Penetration: Can penetrate through the body.
   • Uses: Medical treatments.
   • Energy Levels: Vary from low (dental x-rays) to very high (sterilizing medical equipment).
   • Shielding: Dense materials like concrete and lead.

Radioisotopes

• Definition: Radioactive isotopes of an element.
• Isotopes: Same number of protons but differing numbers of neutrons.
• Characteristics: Unstable combination of neutrons and protons, or excess energy in their nucleus (nuclei are unstable and spontaneously emitting excess energy).
• Example: Hydrogen isotopes (hydrogen-3/tritium is radioactive).

Radioisotope Occurrence

• Occur naturally or by artificially altering the atom.
• Produced in nuclear reactors.
• Example: Uranium (uranium-238 is most abundant, uranium-235 is more radioactive).

Radioactive Decay

• Process: Unstable nucleus regains stability by shedding excess particles and energy in the form of radiation.
• Half-life: Unique time period for each radioisotope to undergo radioactive decay (time it takes for half of the unstable atoms to decay).
• Medical Value: Used in diagnosis and treatment of disease.

Sources of Radiation

1. Natural Background Radiation:

   • Sun.
   • Earth.
   • Atmosphere.

2. Artificial Radiation:

   • Man-made.
   • Medical/dental x-rays.
   • Nuclear sources.
   • Consumer products/activities.

Pathways of Radiation

• Radioactive materials reach people via various routes.
• Examples:
  • Airborne radioactive material falling on pasture, eaten by cows, present in milk, consumed by people.
  • Inhalation of radioactive material.
  • Radioactive material in water, exposure through fish consumption or swimming.

Radiation Effects

• Radiation causes ionization of atoms, which affects:
  • Molecules.
  • Cells.
  • Tissues.
  • Organs.
  • The whole body.

Types of Radiation Injury

1. Direct Effects:

   • Radiation interacts directly with atoms of the DNA molecule or other critical cellular components.
   • May affect the ability of a cell to reproduce and survive.
   • Infrequent due to the small proportion of critical components in the cell.
   • Direct interaction with an active cell results in cell death or mutation; less effect on dormant cells.

2. Indirect Effects:

   • Ionizing radiation breaks bonds that hold the water molecule together, producing toxic substances (radicals like hydroxyl OH, superoxide anion $O_2^{-}$, etc.).
   • These radicals lead to cell destruction.
   • Frequent occurrence because body cells are mostly water (70-80%).

Free Radical Formation

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

Sequence of Radiation Injury

1. Prodromal Period:

   • Symptoms: Nausea, vomiting, loss of appetite, possible diarrhea (dose-dependent).
   • Timing: Minutes to days following exposure; symptoms last minutes to days.

2. Latent Period:

   • Time between exposure and appearance of radiation damage.
   • Varies depending on:
     • Total dose of radiation received.
     • Rate of dose delivery.
   • More radiation + faster dose rate = shorter latent period.

3. Period of Injury:

   • Cell injuries:
     • Cell death.
     • Changes in cell/tissue/organ function.
     • Breaking or clumping of chromosomes.
     • Formation of giant cells.
     • Abnormal or stopped cell division.

4. Recovery Period:

   • Not all injuries are permanent.
   • Most low-level radiation damage is repaired within the body's cells.
   • Scatter radiation clears in 24-48 hours.
   • Repeated exposure doesn't allow for adjustment.

Factors Influencing Health Effects

• Dose Rate: How fast the dose is received. A dose received over time is less severe than the same dose received all at once. High dose rates don't allow time for repair.
• Dose Location: Impact less severe if only part of the body receives the dose.
• Sensitivity: Fetus is most vulnerable. Infants, children, elderly, pregnant women, and immunocompromised individuals are more vulnerable. Rapidly dividing cells are more susceptible.

Cumulative Effect

• Additive effects of radiation exposure; unrepaired damage builds up.
• Can lead to: Cancer, cataract formation, birth defects.

Short-Term and Long-Term Effects

• Short Term: High doses over short periods kill cells, leading to death, skin burns, hair loss, sterility, cataracts.
• Long Term: Low doses over extended periods cause chronic effects that may not be observed for years.

Mutations

• Germ cells (sperm and ova): Genetic abnormalities in offspring.
• Somatic cells: Diseases including cancer (carcinogenesis).
• Oncogenes affect cancer incidence.

Somatic and Genetic Effects

• Somatic Effects:
  • Occur in all body cells except reproductive cells.
  • Not passed to future generations.
  • Only affect the individual exposed.
  • Primary consequence is cancer.
• Genetic Effects:
  • Occur in reproductive cells.
  • Passed to future generations.
  • Do not affect the exposed individual.
  • Genetic damage cannot be repaired.

Mitotic Cycle Radiosensitivity

• Cells most sensitive at or close to M (mitosis).
• G2 phase as sensitive as M phase.
• Resistance greatest in the later part of S phase.
• G1 phase has resistant period early, sensitive period late.

Mechanisms of Cell Death After Irradiation

1. Mitotic Death:

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

2. Apoptosis:

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

3. Bystander Effect:

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

DNA Damage

• Single-Strand Breaks:
  • Little biologic consequence because repaired readily using the opposite strand as a template.
  • Often repairable.
• Double-Strand Breaks:
  • Most important lesions produced in chromosomes by radiation.
  • Interaction of two double-strand breaks may result in cell killing.
  • Lethal.

Uses of Radiation

1. Diagnostic Radiology:

   • Diagnostic.

2. Radiotherapy:

   • Treatment.

3. Nuclear Medicine:

   • Diagnostic.
   • Treatment.
   • Lab tests.

Diagnostic Radiology Goal

• Produce an anatomical or functional patient image (using x-rays) which is clinically useful while delivering as low a radiation dose as possible.

Mammography

• X-ray picture of the breast.
• Uses a small dose of ionizing radiation.
• Detects early signs of breast cancer.
• Best test for early detection (up to three years before physical detection).

Nuclear Medicine

• Uses radioactive isotopes.
• Radioisotope tracer is taken orally, injected, or inhaled.
• Circulates through the body or taken up by certain tissues.
• Distribution tracked by emitted radiation.
• Captured by imaging techniques.
• Radioisotopes have short half-lives, decay before causing damage.

Radionuclide Angiogram

• Tells how well the heart pumps and how much blood is pumped with each heartbeat.
• Radioactive tracer injected into arm vein.
• Tracer “tags