Radiation Effects on Normal Tissues

Radiation Effects on Normal Tissues - Hit & Harm Basics

• Damage Types:
  • Stochastic: Probability ↑ with dose, no threshold (e.g., carcinogenesis, genetic effects).
  • Deterministic (Tissue Reactions): Severity ↑ with dose, has a threshold dose (e.g., fibrosis, necrosis, skin erythema).
• Target Theory: DNA is the primary cellular target.
  • Direct Action: Radiation directly ionizes DNA or other critical macromolecules.
  • Indirect Action: Radiation ionizes water → free radicals (e.g., OH•) damage DNA; predominant for X-rays & gamma rays (low-LET).
• Cellular Radiosensitivity (Law of Bergonié & Tribondeau): Cells are more radiosensitive if:
  • Actively dividing (high mitotic rate)
  • Undifferentiated (primitive)
  • Long mitotic future
  • Cell cycle phase: M > G2 > G1 > S (late S phase is most resistant).
• Tissue Radiosensitivity Hierarchy: (📌 VIM > DIM > RPM > FPM)
  • VIM (Vegetative Intermitotic cells): Most sensitive (e.g., bone marrow, intestinal crypts, gonads).
  • FPM (Fixed Post-mitotic cells): Most resistant (e.g., adult nerve tissue, muscle).

⭐ The Oxygen Enhancement Ratio (OER) for low-LET radiation (X-rays, gamma rays) is typically 2.5-3.5, meaning hypoxic cells are 2.5-3.5 times more resistant to radiation damage than well-oxygenated cells.

Radiation Effects on Normal Tissues - Tissue Trauma Time

• Acute (Early) Effects:

  • Onset: Days-weeks (< 90 days / < 6 months post-RT).
  • Pathogenesis: Mitotic death of rapidly proliferating cells (high α/β ratio).
  • Tissues: Skin, GI mucosa, bone marrow.
  • Manifestations: Dermatitis, mucositis, marrow suppression.
  • Usually heal; may lead to consequential late effects.

• Chronic (Late) Effects:

  • Onset: Months-years (> 90 days / > 6 months post-RT).
  • Pathogenesis: Vascular/stromal damage, fibrosis; affects slowly proliferating cells (low α/β ratio).
  • Tissues: Lung, kidney, CNS, heart.
  • Manifestations: Fibrosis, necrosis, telangiectasia, organ failure.
  • Often progressive, irreversible.

⭐ Late effects are more sensitive to fraction size (low α/β ratio tissues) than early effects (high α/β ratio tissues).

Radiation Effects on Normal Tissues - Organ System Alerts

• TD 5/5: Dose for 5% severe complication risk in 5 yrs (focal RT, specific volumes).
• Early Responders (High Sensitivity): Bone marrow, GI mucosa, skin, gonads (rapid turnover).
• Late Responders (Lower Sensitivity): Lung, kidney, CNS, liver (slow turnover).

Key Organ TD 5/5 & Effects (Typical Values):

• Bone Marrow (segmental): ~25-30 Gy (aplasia).
• GI (Small Intestine): ~45 Gy (mucositis, ulceration).
• Lungs (whole/partial): ~17 Gy (pneumonitis → fibrosis).
• Kidneys (whole/partial): ~20 Gy (nephropathy).
• CNS (Brain/Spinal Cord): ~50 Gy (necrosis/myelopathy).
• Lens: Cataracts. Threshold ~2 Gy (single), 5 Gy (fractionated).
• Gonads (Sterility): Ovary >2-3 Gy; Testes (permanent) >5-6 Gy.
• Skin (local): Erythema 2-6 Gy; Moist Desquamation >20 Gy; Necrosis >35 Gy.

Acute Radiation Syndromes (ARS) - Whole Body Irradiation:

⭐ Lhermitte's sign (electric shock on neck flexion) can indicate transient radiation myelopathy post-spinal cord RT.

High‑Yield Points - ⚡ Biggest Takeaways

• Lymphocytes are most radiosensitive; nerve/muscle cells are most radioresistant.
• Early effects (mucositis, dermatitis) are deterministic with a clear threshold dose.
• Late effects include fibrosis, necrosis, cataracts (threshold), and secondary malignancies (stochastic).
• Highly radiosensitive tissues: Bone marrow, gonads, lymphoid tissue, intestinal epithelium.
• Highly radioresistant tissues: Mature bone, muscle, brain, spinal cord.
• TD 5/5: Tolerance dose for 5% severe complication risk in 5 years.
• Fractionation spares normal tissues by allowing repair of sublethal damage between doses.