Basics of Radiation

Radiation is the emission or transmission of energy in the form of waves or particles. It occurs naturally and is also produced artificially. In medicine, radiation is widely used for diagnostic imaging and cancer treatment. Below is a detailed overview of the basics of radiation:

Types of Radiation

Radiation can be classified into ionizing and non-ionizing radiation based on its ability to ionize atoms (i.e., remove electrons from atoms).

1. Ionizing Radiation

Ionizing radiation has enough energy to remove tightly bound electrons from atoms, creating ions. It can damage DNA and tissues, which is why it is both useful (in cancer therapy) and harmful (causing radiation injury).

• Examples:
  • X-rays: Used in diagnostic imaging (e.g., X-ray radiographs, CT scans).
  • Gamma rays: Used in cancer therapy (e.g., gamma knife radiosurgery) and produced by radioactive isotopes like cobalt-60.
  • Alpha particles: Heavy, highly ionizing, and short-ranged radiation from the decay of certain radioactive materials.
  • Beta particles: High-energy, fast-moving electrons or positrons emitted from the nucleus during radioactive decay.
  • Neutrons: Uncharged particles produced in nuclear reactions, used in neutron therapy.

2. Non-Ionizing Radiation

Non-ionizing radiation lacks the energy to ionize atoms but can still excite molecules and atoms, causing them to vibrate, leading to heat generation or chemical changes.

• Examples:
  • Ultraviolet (UV) radiation: Causes skin tanning, sunburn, and DNA damage (e.g., skin cancer).
  • Microwaves: Used in cooking and telecommunication.
  • Radio waves: Used for communication and broadcasting.
  • Infrared radiation: Associated with heat generation (e.g., thermal imaging, heat therapy).

Properties of Radiation

1. Energy: Measured in electron volts (eV). Ionizing radiation typically has energy above 10 eV.

2. Penetration Ability: Different types of radiation penetrate matter to different degrees:

   • Alpha particles are stopped by a sheet of paper or the outer layer of skin.
   • Beta particles can be stopped by plastic or a few millimeters of aluminum.
   • Gamma rays and X-rays penetrate deeply into tissues and require dense materials like lead or concrete for shielding.

3. Half-Life: The time it takes for half the atoms of a radioactive material to decay. This concept is important in radiotherapy and nuclear medicine.

4. Activity: Measured in becquerels (Bq) or curies (Ci), it refers to the rate at which a radioactive material decays.

Radiation in Medicine

1. Diagnostic Use of Radiation

• X-rays: A form of ionizing radiation that creates images by passing through tissues and being absorbed at different rates. Used in:

  • Radiography: For bones and chest imaging.
  • Fluoroscopy: Real-time imaging of moving internal structures.
  • CT (Computed Tomography): Detailed cross-sectional images of the body.

• Nuclear Medicine: Uses radioactive isotopes (radionuclides) to diagnose conditions by tracking the distribution of radiation in the body.

  • Examples:
    • PET (Positron Emission Tomography): Detects metabolic activity using a radiotracer like fluorodeoxyglucose (FDG).
    • SPECT (Single Photon Emission Computed Tomography): Similar to PET, but uses gamma radiation for functional imaging.

2. Therapeutic Use of Radiation

• Radiotherapy: The use of ionizing radiation to kill cancer cells. Radiation damages the DNA of cancer cells, preventing their replication and causing cell death.

• Types of Radiotherapy:

  • External Beam Radiotherapy (EBRT): Delivers radiation from an external source, targeting tumors with X-rays or gamma rays.
  • Brachytherapy: Involves placing radioactive sources inside or near the tumor, delivering high doses directly to the cancerous tissue.
  • Stereotactic Radiosurgery (SRS): Uses focused beams of high-energy radiation (e.g., gamma knife, cyberknife) for treating brain and other localized cancers.
  • Proton Therapy: Uses protons instead of photons, offering precise delivery with less damage to surrounding healthy tissues.

Radiation Units and Measurement

1. Absorbed Dose: The amount of radiation absorbed by tissues, measured in grays (Gy).

   • 1 Gray = 1 joule of radiation energy absorbed per kilogram of tissue.

2. Equivalent Dose: Takes into account the type of radiation and its biological effect. It is measured in sieverts (Sv) and used to assess the risk of radiation exposure.

   • For example, alpha particles cause more damage than beta particles, so their equivalent dose will be higher even if the absorbed dose is the same.

3. Effective Dose: Adjusts the equivalent dose based on the sensitivity of the tissues or organs exposed to radiation. It helps estimate the overall risk to the body.

Biological Effects of Radiation

• Deterministic Effects: These effects occur when the radiation dose exceeds a certain threshold and can lead to tissue damage. Severity increases with dose.

  • Examples: Skin burns, hair loss, radiation sickness, cataracts.

• Stochastic Effects: These effects have no threshold and occur randomly. The probability increases with dose, but the severity does not depend on dose.

  • Examples: Cancer, genetic mutations. Ionizing radiation can induce DNA mutations that may lead to cancer years after exposure.

Radiation Protection

To minimize the harmful effects of radiation exposure, certain principles of radiation protection are applied:

1. ALARA Principle (As Low As Reasonably Achievable): This principle aims to minimize radiation exposure while still achieving the desired medical benefit.

2. Time, Distance, and Shielding:

   • Time: Reducing the time spent near radiation sources lowers exposure.
   • Distance: Increasing distance from the radiation source reduces exposure, as intensity decreases with the square of the distance.
   • Shielding: Using protective materials such as lead aprons or walls can block or reduce radiation.

3. Radiation Monitoring: Workers exposed to radiation (e.g., healthcare professionals in radiology) use personal dosimeters to track cumulative exposure.

4. Regulations and Limits: International bodies such as the International Commission on Radiological Protection (ICRP) set recommended limits on radiation exposure for occupational and public safety.

• Occupational Limits: For radiation workers, the annual dose limit is usually around 20 mSv.
• Public Limits: For the general public, the annual exposure limit is set at 1 mSv above background radiation levels.

Natural and Artificial Radiation Sources

• Natural Radiation:

  • Cosmic radiation from space.
  • Terrestrial radiation from radioactive materials in the earth (e.g., radon gas).
  • Internal radiation from radioactive isotopes within the body (e.g., potassium-40).

• Artificial Radiation:

• Medical imaging (X-rays, CT scans).
• Nuclear medicine procedures (e.g., PET scans).
• Industrial and research applications (e.g., nuclear power plants).