Radiotherapy: Principles, Effects, and Applications

Posted on May 5, 2024 in Biology

Radiotherapy: Principles and Applications

Introduction to Radiation Therapy

Radiotherapy, also known as radiation oncology, utilizes ionizing radiation to treat cancer. This form of treatment has largely replaced more invasive procedures like amputations and excisions. Ionizing radiation, including X-rays and gamma rays, possesses sufficient energy to ionize matter and disrupt cellular processes.

Types of Radiation

Radiation used in therapy can be categorized based on its origin:

  • Photons: X-rays and gamma rays generated by linear accelerators.
  • Corpuscular particles: Alpha particles, beta particles, protons, neutrons, pions, and muons, which can be artificially accelerated.

Physical Principles of Radiation

Laws Governing Radiation Interaction

  • Grotthus-Draper Law: Only absorbed radiation produces an effect.
  • Bunsen-Roscoe Law (Reciprocity Law): The product of intensity and time determines the biological effect.
  • Inverse Square Law: Radiation intensity decreases with the square of the distance from the source.
  • Lambert’s Cosine Law: Maximum radiation intensity is achieved when the beam is perpendicular to the surface.

Linear Energy Transfer (LET)

LET describes the amount of energy deposited by radiation per unit length of travel through a medium. High-LET radiation deposits energy rapidly, resulting in a high dose over a small area, while low-LET radiation deposits energy more slowly, leading to a lower dose over a larger area.

Biological Effects of Radiation

Cellular Injuries

  • Sublethal damage: Cells can repair this type of injury.
  • Lethal damage: Irreversible cell death.
  • Potentially lethal damage: Repair is possible under certain conditions.

Benefits of High-Energy Beams

  • Increased penetration depth
  • Reduced skin exposure
  • Lower scatter
  • More uniform dose distribution
  • Similar absorption in bone and soft tissue

Principles of Dosimetry

Dosimetry ensures that the treatment volume receives a homogeneous dose while minimizing exposure to surrounding healthy tissue. Fractionated doses are often used to enhance treatment efficacy and reduce side effects.

Tumor-Tissue Differential Effect

Fractionation offers several advantages:

  • Reoxygenation of hypoxic tumor cells, increasing their radiosensitivity.
  • Redistribution of tumor cells into radiosensitive phases of the cell cycle.
  • Enhanced repair of sublethal damage in healthy tissue.
  • Repopulation of healthy tissue.

Cellular Responses to Radiation

  • Interphase death: Cell lysis, common in radiosensitive cells.
  • Mitotic delay: Delayed entry into mitosis, leading to a decrease in the mitotic index.
  • Reproductive failure: Loss of cell division capacity, resulting in radiological cell death.

Fractionation Effects

  • Repair: More efficient in normal tissues.
  • Repopulation: Renewal of irradiated tissue by healthy tissue.
  • Redistribution: Tumor cells move into radiosensitive phases.
  • Reoxygenation: Hypoxic cells become oxygenated and more radiosensitive.

Treatment Planning and Delivery

Beam Modification

  • Collimators: Shape the radiation beam.
  • Wedge filters: Compensate for irregular body contours.

Treatment Types

  • Radical radiotherapy: Aims to eliminate all tumor cells.
  • Adjuvant radiotherapy: Complements other treatments, such as surgery.
  • Palliative radiotherapy: Relieves symptoms and improves quality of life.
  • Conditional radiotherapy: Treats advanced locoregional disease.

Preoperative and Postoperative Radiotherapy

Preoperative radiotherapy aims to shrink the tumor and reduce the risk of spread during surgery. Postoperative radiotherapy targets any remaining cancer cells after surgery.

Radiobiology: Effects of Radiation on Living Tissues

Relative Biological Effectiveness (RBE)

RBE compares the biological effectiveness of different types of radiation. High-LET radiation generally has a higher RBE.

Effects on DNA

  • Base damage: Loss or alteration of a DNA base, leading to mutations.
  • Single-strand breaks: Breakage of one DNA strand, which can be repaired.
  • Double-strand breaks: Breakage of both DNA strands, more difficult to repair and potentially lethal to the cell.

Radiotherapy plays a crucial role in cancer treatment, offering a range of options for various types and stages of the disease. Understanding the principles and effects of radiation is essential for optimizing treatment plans and minimizing side effects.