External Radiotherapy: High-energy gamma rays or X-rays are directed at a tumor from a source outside the body. To minimize damage to healthy surrounding tissue, the beam is often rotated around the patient, ensuring the tumor receives a high cumulative dose while healthy cells receive only a fraction.
Internal Radiotherapy (Brachytherapy): Radioactive 'seeds' or liquids are placed directly inside or near the tumor. This allows for a very high, localized dose of radiation. Beta emitters are often used here because their limited range (a few millimeters) ensures the radiation stays concentrated within the target area.
Alpha Therapy: Because alpha particles have extremely high ionizing power but very low penetration, they are used in targeted therapies where the isotope is chemically bound to a molecule that specifically seeks out cancer cells. Once attached, the alpha emission destroys the cell with minimal impact on neighbors.
Biological Effectiveness: The success of therapy depends on the cell's inability to repair DNA damage before the next division cycle. Cancer cells often have poorer repair mechanisms than healthy cells, which is why fractional dosing (multiple small treatments) is used.
| Feature | Diagnostic Tracers | Therapeutic Radiotherapy |
|---|---|---|
| Primary Goal | Information/Visualization | Cell Destruction/Treatment |
| Radiation Type | Gamma (High penetration) | Alpha/Beta (High ionization) or Gamma |
| Half-Life | Very Short (Hours) | Short to Medium (Days/Weeks) |
| Location | Distributed through system | Highly localized to target |
| Patient Risk | Minimal (Low dose) | Controlled (High localized dose) |
Half-Life Logic: When asked to select an isotope, always justify the half-life based on the procedure duration. If a scan takes 30 minutes, a 6-hour half-life is appropriate; a 50-year half-life would be a safety disaster, and a 2-second half-life would be useless.
Type Selection: Always check if the radiation needs to leave the body. If it does (imaging), choose Gamma. If it needs to stay in a small spot (therapy), choose Alpha or Beta.
Safety Calculations: Remember that the total dose is a function of the activity of the source and the time of exposure. In exam problems, look for the 'ALARA' principle (As Low As Reasonably Achievable) as a guiding framework for safety answers.
Common Units: Be comfortable converting between activity (Becquerels, ) and time. Ensure you understand that decay per second.
The 'Glow' Myth: A common misconception is that patients become 'radioactive' and glow after an X-ray. In reality, X-rays pass through the body instantly; only patients who ingest or are injected with radioactive tracers remain radioactive until the isotope decays or is excreted.
Alpha Safety: Students often think alpha is 'safe' because it can be stopped by paper. While true externally, alpha radiation is the most dangerous if internalized (swallowed or injected) because it deposits all its energy in a very small volume of tissue.
Half-Life vs. Shelf Life: Do not confuse the physical half-life of the isotope with the biological half-life (the time it takes for the body to naturally excrete the substance). Both contribute to how long a patient remains radioactive.
