Radiobiology Fundamentals

Radiobiology Fundamentals - Radiation's Tiny Targets

• Primary Target: DNA.

• Damage Mechanisms:

  • Direct: Radiation ionizes DNA. High-LET (alpha, neutrons).
  • Indirect: Radiation creates water free radicals (e.g., $OH^{\cdot}$) from $H_2O$ damaging DNA. Low-LET (X-rays, gamma).

• DNA Lesions: Base damage, SSBs, DSBs. DSBs most lethal.

• Oxygen Effect: $O_2$ enhances damage (OER ~2.5-3.5 for X-rays).

• Cell Cycle: Sensitive: M, G2. Least: late S. (📌 G₂/M > G₁ > $S_e$ > $S_l$)

⭐ Indirect action via water radiolysis ($OH^{\cdot}$ radicals) is dominant for DNA damage by X-rays & gamma rays.

Radiobiology Fundamentals - Survive, Repair, or Perish

• Cellular Response: DNA damage is primary lesion, dictating cell fate.
  • Repair: Sublethal Damage (SLD), Potentially Lethal Damage (PLD).
  • Death: Apoptosis (programmed), Mitotic Catastrophe (replication-linked).
• Cell Survival: Linear-Quadratic Model $S = e^{-(\alpha D + \beta D^2)}$ quantifies reproductive survival.
  • $\alpha$ component: irreparable, single-hit lethal events.
  • $\beta$ component: repairable, multi-hit sublethal events.
• Cell Cycle Sensitivity: 📌 "Go Sally Go, Make Children!" (G1→S→G2→M).

  ⭐ Cells in M (Mitosis) and G2 phases are generally the most radiosensitive, while cells in the late S (synthesis) phase are the most radioresistant.

Radiobiology Fundamentals - Dose Makes the Poison

• Radiation damage: Direct (DNA target) & Indirect (free radicals from H₂O).
• Cell cycle sensitivity: M & G2 phases most sensitive; S phase most resistant.
• Deterministic Effects (Non-Stochastic):
  • Occur above a threshold dose.
  • Severity increases with dose ↑.
  • Examples: Skin erythema, sterility, organ failure.
  • LD50/60 (human, whole body): ~3-4 Gy.
• Stochastic Effects:
  • No threshold (probabilistic).
  • Probability increases with dose ↑; severity is independent of dose.
  • Examples: Carcinogenesis, heritable genetic effects.

⭐ Cataract formation is a classic example of a deterministic (non-stochastic) effect, with a threshold dose of approximately 2 Gy for acute exposure or 5 Gy for prolonged/fractionated exposure to the lens of the eye.

Radiobiology Fundamentals - Dialing Radiation's Impact

• Linear Energy Transfer (LET): Energy deposited per unit path length. Low LET (X-rays, γ-rays) → indirect damage (free radicals). High LET (α-particles, neutrons) → direct damage.
• Relative Biological Effectiveness (RBE): Ratio of doses for same biological effect ($RBE = D_{250kVp} / D_{test}$). RBE ↑ with ↑ LET (to ~100 keV/µm).
• Oxygen Effect: O2 sensitizes cells to radiation, especially low LET.

  ⭐ The Oxygen Enhancement Ratio (OER) for low-LET radiation (X-rays, gamma rays) is typically 2.5 to 3.5, indicating that hypoxic cells are significantly more radioresistant.

• Cell Cycle Sensitivity: Most sensitive: M & G2 phases. Most resistant: Late S phase.
• Law of Bergonié & Tribondeau: Radiosensitivity is proportional to mitotic activity & inversely to differentiation.
• 4 R's of Radiobiology (Fractionation): Repair, Reassortment, Repopulation, Reoxygenation. 📌 RRRR.

Radiobiology Fundamentals - Radiotherapy's Pillars

• Therapeutic Ratio: Goal is ↑ Tumor Control Probability (TCP) & ↓ Normal Tissue Complication Probability (NTCP).
• The 5 Rs of Radiobiology (📌 RRRRR):
  • Repair: Of sublethal damage in normal & tumor cells.
  • Repopulation: Cell division between fractions.
  • Redistribution: Cells progress to sensitive cycle phases (G2/M).
  • Reoxygenation: Hypoxic tumor cells become oxygenated, ↑sensitive.
  • Radiosensitivity: Intrinsic differences between cells.

⭐ The 5 Rs of Radiobiology (Repair, Repopulation, Redistribution, Reoxygenation, and Radiosensitivity) explain the biological basis for the efficacy of fractionated radiotherapy.

High-Yield Points - ⚡ Biggest Takeaways

• Indirect action (via water radiolysis) is dominant for X-rays/gamma rays; direct action is significant for high-LET.
• Cellular radiosensitivity is highest in M/G2 phases; lowest (most resistant) in late S phase.
• Oxygen effect (OER ~2.5-3.5 for low-LET) enhances damage; High-LET radiation has ↑RBE, ↓OER.
• Deterministic effects have a threshold dose, severity is dose-dependent; Stochastic effects are probabilistic, no proven threshold.
• The 4 R's of Radiobiology: Repair, Reoxygenation, Repopulation, Redistribution explain fractionated radiotherapy success.