Fractionation in Radiotherapy

Fractionation Fundamentals - Dose Delivery Dance

• Definition: Dividing total radiotherapy dose into multiple smaller, daily fractions over weeks.
• Aim: Enhance therapeutic ratio: maximize tumor damage, minimize normal tissue injury.
• The 4 R's of Radiobiology:
  • Repair: Normal tissues repair sublethal damage (SLD) more efficiently.
  • Reoxygenation: Hypoxic tumor regions gain O₂, ↑ radiosensitivity.
  • Redistribution: Cells enter radiosensitive cell cycle phases (G2/M).
  • Repopulation: Normal cells regenerate; tumor cells too if overall treatment time is protracted.
• Standard: 1.8-2 Gy/fraction, daily, 5x/week.

⭐ Fractionation primarily exploits differential repair in normal tissues and reoxygenation in tumors, improving therapeutic outcomes.

The 5 R's of Radiobiology - Cell Survival Saga

The biological basis for fractionation's success. 📌 Key principles:

• Repair: Normal tissues repair Sublethal Damage (SLD) more efficiently than tumors between fractions. Allows normal tissue sparing.
• Repopulation: Surviving normal and tumor cells divide between fractions. Can be detrimental if tumor repopulation is fast; prolonging treatment time can ↓ local control.
• Reoxygenation: Hypoxic (radioresistant) tumor core cells gain oxygen access as outer cells die, becoming more radiosensitive.
• Redistribution (Reassortment): Surviving cells progress through the cell cycle. Fractionation catches more cells in sensitive phases (G2/M).
• Radiosensitivity: Intrinsic differences in radiation sensitivity among cells and tumor types. Fractionation exploits these differences.

⭐ Reoxygenation is critical: hypoxic cells can be 2-3 times more radioresistant than normoxic cells. Fractionation helps overcome this tumor radioresistance mechanism.

Scheduling Strategies - Time-Dose Tactics

Fractionation exploits differences in repair capacity (one of the 5 R's) between tumor and normal tissues. Based on Linear-Quadratic (LQ) model: $E = n(\alpha d + \beta d^2)$. Biologically Effective Dose (BED) = $D(1 + d/(\alpha/\beta))$ helps compare schedules.

• α/β ratio: Tumors ≈ 10 Gy (early effects); Late-responding tissues ≈ 3 Gy.

LQ Model & BED - Radiobiologic Math

• Linear-Quadratic (LQ) Model: Predicts cell kill from radiation.
  • Total biological effect: $E = n(\alpha d + \beta d^2)$.
    • $n$: number of fractions, $d$: dose/fraction.
  • $\alpha$ component: irreparable DNA damage (linear cell kill).
  • $\beta$ component: repairable/misrepaired DNA damage (quadratic cell kill).
  • $\alpha/\beta$ ratio: Measures fractionation sensitivity.
    • Tumors & acute effects: ~10 Gy (less sensitive to changes in $d$).
    • Late effects: ~3 Gy (more sensitive to changes in $d$; sparing with lower $d$).
• Biologically Effective Dose (BED): Total biological impact of a radiotherapy schedule.
  • Formula: $BED = D(1 + d/(\alpha/\beta))$.
    • $D$: total physical dose ($nd$).
  • Allows isoeffective dose calculations for different fractionation regimens.

  ⭐ For late-responding tissues (low $\alpha/\beta$), BED increases significantly with larger fraction sizes ($d$), potentially increasing late toxicity.

High‑Yield Points - ⚡ Biggest Takeaways

• Fractionation improves therapeutic ratio by exploiting the 4 R's: Repair, Repopulation, Reoxygenation, Redistribution.
• BED compares schedules using total dose, dose/fraction, and α/β ratio.
• Late-responding tissues have low α/β ratios (e.g., ~3 Gy), spared by smaller fractions.
• Tumors & early-responding tissues usually have high α/β ratios (e.g., ~10 Gy).
• Hyperfractionation (smaller dose/fraction) aims to ↓ late toxicity.
• Hypofractionation (larger dose/fraction) for palliative care or specific tumors_._