15/11/2025
Radiobiology:
The Science Behind How Radiation Shapes Life at the Cellular Level
What Actually Happens When Radiation Hits a Cell?
Radiation doesn’t kill cells directly —
it transfers energy to atomic electrons, triggering a cascade of molecular events.
1️⃣ Direct Action
• Radiation deposits energy directly onto DNA.
• Causes double-strand breaks, the most lethal lesion.
• More common with high LET radiation (alpha, neutrons).
2️⃣ Indirect Action
• Radiation ionizes water, generating reactive species:
•OH, H•, e⁻aq
• These free radicals damage DNA, proteins & membranes.
• Dominant mechanism for low LET radiation (gamma, beta).
Dose–Response: The Foundation of Radiobiology
• Deterministic effects: Threshold; severity ↑ with dose
(e.g., marrow suppression, skin erythema).
• Stochastic effects: No threshold; probability ↑ with dose
(e.g., cancer, heritable mutations).
Cell Survival Curves
Low LET radiation:
• Classic linear–quadratic (α/β) model
• Shoulder region → sublethal damage repair
• α/β ratio determines tissue sensitivity
High LET radiation:
• Straight line → minimal repair → high relative biologic effectiveness (RBE)
Key Concepts Every NM Professional Should Know
• RBE (Relative Biological Effectiveness):
Higher for high-LET particles like α (40–100× damage).
• OER (Oxygen Enhancement Ratio):
Hypoxic cells are 2–3× more radio-resistant.
• Radiosensitivity:
Cells are most radiosensitive in G2/M phase, least in late S phase.
Big Picture
Radiobiology is not about radiation quantity —
It’s about how cells interpret and survive the molecular chaos created by ionization.
This is the science that makes imaging safe, therapy effective, and personalized medicine possible.
