Radiobiology Lecture 5: Radiation Effects

Radiation Effects on Human Health

• Radiation effects can be categorized into three types: sub-acute, acute, and chronic
• Each type has different severity and characteristics

Sub-Acute Radiation Effects

• Occur weeks to months after exposure
• Include:
  • Hematopoietic Syndrome: damage to bone marrow, leading to infections, anemia, and bleeding
  • Gastrointestinal Syndrome: damage to the lining of the gastrointestinal tract, leading to nausea, vomiting, diarrhea, and severe dehydration

Acute Radiation Effects (ARS)

• Occur within hours to days after exposure
• Severity depends on the dose received:
  • Mild Exposure (1-2 Gy): nausea, vomiting, and fatigue
  • Moderate Exposure (2-6 Gy): severe nausea and vomiting, hair loss, and damage to the bone marrow leading to infections and bleeding
  • Severe Exposure (6-10 Gy): severe gastrointestinal symptoms, significant bone marrow damage, and a high risk of infections and bleeding
  • Very High Exposure (above 10 Gy): severe damage to internal organs and tissues, often resulting in death within days to weeks

Chronic Radiation Effects

• Emerge months to years after exposure
• Include:
  • Cancer: increased risk of various types of cancer due to DNA damage and mutations
  • Cataracts: radiation-induced damage to the lens of the eye, leading to clouding and vision impairment
  • Cardiovascular Diseases: increased risk of heart disease and stroke due to damage to blood vessels and heart tissues
  • Fibrosis: tissue scarring and loss of function in organs such as the lungs and liver

Tumor Response to Radiation

• Radiation therapy targets cancer cells by damaging their DNA, disrupting their ability to replicate and survive
• Key factors:
  • DNA Damage and Repair: radiation causes breaks in DNA strands, and cancer cells are less efficient at repairing DNA than normal cells
  • Cell Cycle Sensitivity: cells are most sensitive to radiation during certain phases of the cell cycle
  • Hypoxia: tumor cells in low-oxygen environments are more resistant to radiation

Therapeutic Index (Combined Radiation and Drug Treatments)

• Enhanced Efficacy: drugs can enhance the effect of radiation on tumor cells
• Radio Protectors: some agents can protect normal tissues from radiation damage
• Radio Sensitizers: drugs that make cancer cells more sensitive to radiation
• Synergistic Effects: the combination of radiation and drugs can have a greater effect than either treatment alone

Tumor Control Probability (TCP)

• Likelihood that a given treatment regimen will completely eradicate a tumor
• Factors affecting TCP:
  • Dose-Response Relationship: TCP increases with higher doses of radiation
  • Heterogeneity: tumor cells can vary widely in their sensitivity to radiation
  • Hypoxia: hypoxic tumor cells are more resistant to radiation
  • Biomarkers: identifying biomarkers that predict tumor response to radiation can help personalize treatment

Normal Tissue Complication Probability (NTCP)

• Estimates the risk of damage to healthy tissues from radiation therapy
• Factors affecting NTCP:
  • Dose Constraints: limits are set on the maximum dose that normal tissues can receive
  • Volume Effects: the amount of normal tissue exposed to radiation affects NTCP
  • Fractionation: spreading the total radiation dose over several sessions reduces the risk to normal tissues
  • Individual Sensitivity: genetic differences can affect how patients react to radiation