Radiation Oncology Basics

Intro to Radiotherapy - Zap That Tumor!

• Principle: Uses ionizing radiation (photons or particles) to induce lethal DNA damage in cancer cells, minimizing harm to normal tissue.
• Intents: Curative (definitive), Adjuvant (post-op), Neoadjuvant (pre-op), Palliative (symptom relief).
• Radiation Types:
  • Photons (X-rays, γ-rays): Most common; deep tissue penetration.
  • Particles (electrons, protons, neutrons): Electrons for superficial lesions (e.g., skin, keloids).
• Units:
  • Absorbed Dose: Gray (Gy). $1 \text{ Gy} = 1 \text{ J/kg}$.
  • Equivalent Dose: Sievert (Sv) (accounts for biological effectiveness).
• Mechanism: Direct action (DNA strand breaks) & Indirect action (free radical formation from water radiolysis).

⭐ Indirect action, via hydroxyl radicals (•OH), accounts for ~2/3 of DNA damage by X-rays.

Radiobiology - Cells Under Fire

• The 4 R's (Fractionated Radiotherapy):
  • Repair: Sublethal damage repair (SLDR), esp. in normal tissues.
  • Redistribution: Surviving cells progress to more sensitive cycle phases.
  • Repopulation: Cell proliferation between fractions (normal & tumor).
  • Reoxygenation: Improved oxygenation of hypoxic tumor cells, ↑radiosensitivity.
• Cell Cycle Sensitivity:
  • Most Sensitive: M (Mitosis), G2 phases.
  • Most Resistant: Late S phase (higher repair).
  • 📌 Order (Sensitive→Resistant): M > G2 > G1 > S.
• Oxygen Effect:
  • $O_2$ presence significantly ↑ radiation damage (free radical fixation).
  • Oxygen Enhancement Ratio (OER): Ratio of doses (hypoxic / aerobic) for isoeffect.
  • OER (X-rays/γ-rays): Typically 2.5-3.5.

  ⭐ Hypoxic tumor cells are a major cause of radioresistance; OER for X-rays is typically 2.5-3.5.

Delivery Techniques - Precision Strikes

• External Beam RT (EBRT): Radiation from external source.
• 2D RT: Basic, X-ray guided, large margins.
• 3D-CRT: CT-planned, conforms to tumor.
• IMRT (Intensity-Modulated RT): Highly conformal, varied beam intensity. Spares OARs. 📌 IMRT: I M Really Targeting.
• VMAT (Volumetric Modulated Arc Therapy): Rotational IMRT; faster delivery.
• IGRT (Image-Guided RT): On-board imaging (e.g., CBCT) for accuracy.
• Stereotactic RT:
  • SRS (Radiosurgery): Single high dose, intracranial.
  • SBRT/SABR (Ablative): 1-5 fractions, high dose, extracranial.
• Particle Therapy: Protons (Bragg peak), Carbon ions (↑RBE).
• Brachytherapy: Internal source, rapid dose fall-off due to inverse square law ($I \propto 1/d^2$).

  ⭐ Brachytherapy's efficacy hinges on this rapid dose fall-off: high tumor dose, sharp gradient, spares normal tissue.

• Types: Intracavitary, Interstitial, Surface.
• Dose Rates: LDR (0.4-2 Gy/hr), HDR (>12 Gy/hr), PDR.

Toxicity & Management - The Aftermath

• Acute Toxicity (within 90 days of RT):
  • Skin: Erythema, dry/moist desquamation.
  • Mucositis (oral, GI), esophagitis.
  • Systemic: Fatigue, nausea/vomiting.
  • Bone marrow suppression (site-dependent).
• Late Toxicity (> 90 days post-RT, often irreversible):
  • Fibrosis, necrosis (e.g., osteoradionecrosis).
  • Organ-specific: Pneumonitis, proctitis, cystitis, myelopathy, xerostomia.
  • Secondary malignancies (long-term risk).
  • Endocrine dysfunction.
• Management Principles:
  • Grading: Common Terminology Criteria for Adverse Events (CTCAE) / RTOG scales.
  • Supportive care: Analgesia, antiemetics, hydration, nutrition.
  • Specific agents: Amifostine (radioprotector), sucralfate, topical steroids.
  • Radiation Recall: Inflammatory reaction in previously irradiated field upon new chemotherapy.

⭐ Lhermitte's sign: Electric shock-like sensation down the spine on neck flexion, indicates transient radiation myelopathy (usually resolves spontaneously within months).

High‑Yield Points - ⚡ Biggest Takeaways

• Radiosensitivity: High in lymphoid, bone marrow, gonads; low in nerve, muscle.
• Teletherapy (external) & brachytherapy (internal) are main radiation delivery methods.
• Fractionation exploits the 4 R's: Repair, Repopulation, Reoxygenation, Redistribution.
• Gray (Gy) is the SI unit of absorbed dose; 1 Gy = 100 rads.
• Acute effects (mucositis, dermatitis) within 90 days; late effects (fibrosis) after.
• OER is ~3 for X-rays; hypoxic cells radioresistant.
• Goal: Max tumor kill, min normal tissue toxicity.