Principles of Radiation Therapy

Radiobiology & Goals - Zap Zone Science

• Mechanism: DNA damage primarily via indirect action (water radiolysis producing free radicals like OH•) and direct action on DNA.
• Cell Cycle Sensitivity: Most sensitive phases are M & G2; most resistant is late S phase.
• The 4 R's of Radiobiology:
  • Repair: Of sublethal cellular damage.
  • Repopulation: Proliferation of cells between fractions.
  • Redistribution: Cells progress through cell cycle, potentially into more sensitive phases.
  • Reoxygenation: Hypoxic tumor cells become oxygenated and more radiosensitive.
• Goals: Curative (e.g., eradicate localized tumor); Palliative (e.g., relieve pain, bleeding).
• Therapeutic Ratio: Aim to maximize Tumor Control Probability (TCP) while minimizing Normal Tissue Complication Probability (NTCP). $TR = TCP / NTCP$.

⭐ Oxygen Enhancement Ratio (OER) is the ratio of radiation doses required to produce the same biological effect under hypoxic versus aerated conditions; for X-rays, OER is typically 2.5-3.5.

Radiation Types & Tech - Beam Team Tactics

• Radiation Types:
  • Photons: X-rays (from Linear Accelerators - Linacs), Gamma rays (from Cobalt-60, radioactive isotopes like $^{192}\text{Ir}$, $^{137}\text{Cs}$, $^{125}\text{I}$).
  • Particles: Electrons (Linacs), Protons, Carbon ions (hadron therapy).
• External Beam Radiotherapy (EBRT): Source distant.
  • Techniques:
    • 3D Conformal RT (3D-CRT): Beams match tumor shape.
    • Intensity Modulated RT (IMRT): Varying beam intensity; spares organs at risk (OARs).
    • Volumetric Modulated Arc Therapy (VMAT): Rapid IMRT delivery.
    • Image-Guided RT (IGRT): Ensures accurate targeting.
    • Stereotactic Radiosurgery (SRS): Single high dose (e.g., Gamma Knife for brain mets).
    • Stereotactic Body RT (SBRT/SABR): 1-5 high-dose fractions for extracranial sites.
• Brachytherapy: Radioactive source placed in/near tumor.
  • Types: Intracavitary, Interstitial, Surface.
  • Dose Rates: LDR (Low), HDR (High), PDR (Pulsed).

⭐ IMRT significantly reduces xerostomia in head & neck cancers by sparing parotid glands.

Planning & Precision - Target Lock Strategy

• Goal: Maximize tumor dose; minimize normal tissue (OAR) toxicity. Precision is key.
• Planning Workflow:

• Target Volumes (ICRU):
  • GTV (Gross Tumor Volume): Visible tumor extent.
  • CTV (Clinical Target Volume): GTV + microscopic disease.
  • PTV (Planning Target Volume): CTV + margin for setup errors & organ motion. $PTV = CTV + IM + SM$.
  • 📌 Mnemonic: Go Catch Patients.

⭐ PTV ensures the CTV receives the prescribed dose despite uncertainties. Its size directly impacts toxicity.

H&N Toxicities & Care - Damage Control Duty

• Acute (≤90 days)
  • Mucositis: Pain, dysphagia. WHO/RTOG grades. Rx: Analgesia, hygiene, sucralfate.
  • Dermatitis: Erythema, dry/moist desquamation. Rx: Emollients, topical steroids.
  • Xerostomia: Salivary gland damage. Rx: Pilocarpine, saliva substitutes.
  • Dysgeusia/Ageusia: Altered/lost taste.
  • Laryngeal edema: Hoarseness.
• Late (>90 days)
  • Xerostomia (often permanent).
  • Osteoradionecrosis (ORN): Mandible common. Risk: Dose >60 Gy, post-RT extraction. 📌 PENTOCLO (Pentoxifylline, Tocopherol, Clodronate).
  • Soft tissue fibrosis: Trismus, neck stiffness.
  • Hypothyroidism: Monitor TSH.
  • Laryngeal chondronecrosis/stenosis.
  • Radiation-induced secondary malignancy (RISM).
• General Care
  • Nutritional support (PEG/Ryles).
  • Pre-RT dental assessment & prophylaxis.
  • Speech & swallowing rehabilitation.
  • Regular, long-term follow-up.

⭐ Osteoradionecrosis (ORN) risk significantly increases with radiation doses >60 Gy to the bone, especially mandible. Dental prophylaxis pre-radiotherapy is crucial for prevention.

High‑Yield Points - ⚡ Biggest Takeaways

• Fractionation is key: spares normal tissue, ↑ tumor kill.
• Teletherapy (External Beam RT) most common; Brachytherapy for localized boosts.
• Radiosensitivity varies: Squamous cell carcinomas are generally radiosensitive.
• IMRT enables precise dose delivery, sparing critical structures like salivary glands.
• Acute effects: Mucositis, dermatitis, xerostomia.
• Late effects: Osteoradionecrosis (ORN), fibrosis, hypothyroidism.
• Concurrent Chemoradiation (CCRT) is standard for advanced H&N cancers, improving outcomes.