Stereotactic Radiosurgery

Stereotactic Radiosurgery - Precision Zap Basics

• Definition: Non-invasive radiation therapy delivering a single high dose (or 2-5 high-dose fractions) of precisely targeted ionizing radiation to well-defined intracranial or extracranial lesions.
• Core Principles:
  • Stereotaxy: Sub-millimeter accurate lesion localization using 3D coordinate systems (frame-based or frameless image guidance).
  • Conformality: Radiation dose is meticulously shaped to closely match the 3D contour of the target volume.
  • Steep Dose Gradient: Achieves a rapid decrease in radiation dose at the periphery of the target, minimizing exposure to adjacent healthy tissue.
• Goal: Achieve complete biological ablation of the target tissue while preserving the integrity and function of surrounding normal structures.
• Key Feature: Delivers a high Biological Effective Dose (BED) due to the large dose per fraction.

⭐ SRS is characterized by its extreme precision, allowing for the treatment of lesions, especially those that are small and well-circumscribed, even when located near critical or eloquent structures where surgery might be high risk.

Stereotactic Radiosurgery - Zap Tech Parade

• Definition: High-dose, precisely focused radiation (1-5 fractions). Ablative doses, steep dose fall-off.
• Key Technologies:
  • Gamma Knife (GK): Multiple Cobalt-60 sources (e.g., 192, 201). Intracranial. Highly conformal.

    ⭐ Gamma Knife typically uses 192 or 201 Cobalt-60 sources, converging on the target to deliver a high dose with rapid fall-off.

  • LINAC-based SRS: Linear accelerators.
    • Techniques: Conformal arcs, Dynamic Conformal Arcs (DCA), IMRS.
    • Versatile: Intracranial & extracranial (SBRT).
  • CyberKnife: Robotic arm-mounted LINAC.
    • Real-time image guidance (X-ray/fiducials). Tracks tumor motion.
• Immobilization: Critical for sub-mm accuracy.
  • Invasive: Stereotactic frames.
  • Non-invasive: Masks, vacuum bags.
• Targeting: CT/MRI/PET fusion for delineation.

Stereotactic Radiosurgery - Target Practice Time

• Key Targets & Doses (Single Fraction):
  • Brain Metastases: Most common (≤ 4-5 lesions).
    • Dose: 15-24 Gy (size-dependent: <2cm: 20-24 Gy; 2-3cm: 18 Gy; >3cm: 15-16 Gy).
  • Arteriovenous Malformations (AVMs):
    • Goal: Obliteration. Dose: 16-25 Gy to nidus.
    • 📌 AVM: "Rule of 2s" - 2 yrs, ~80% obliteration, ~20 Gy.
  • Benign Tumors:
    • Acoustic Neuroma: 12-13 Gy (hearing preservation).
    • Meningioma: 12-16 Gy. Pituitary Adenoma: 12-25 Gy.
  • Functional Disorders:
    • Trigeminal Neuralgia: 70-90 Gy to nerve root.
    • Movement disorders (e.g., Essential Tremor).

⭐ SRS is typically for lesions < 3-4 cm; larger lesions may require fractionated SRT or surgery due to normal tissue toxicity risk.

Stereotactic Radiosurgery - Zap Zone Dangers

• Contraindications:
  • Large tumors (> 3-4 cm / > 20-30 cc)
  • Multiple lesions (> 5-10)
  • Poorly defined targets on imaging
  • Prior overlapping high-dose RT
• Complications:
  • Acute: Edema (common; steroids), headache, nausea.
  • Late: Radiation Necrosis (RN) (months-years, mimics recurrence), cranial neuropathies.
    • RN risk ↑ with dose & volume.
• SRS vs. Alternatives:
  • SRT: 2-5 fractions; for larger/eloquent area tumors, ↓ toxicity.
  • Conventional RT: Lower dose/fraction, longer course, less precise.
  • Surgery: Invasive, immediate debulking, histology; SRS for inaccessible/high-risk cases.

    ⭐ Radiation necrosis is a key delayed complication of SRS, often presenting months to years post-treatment and can be challenging to differentiate from tumor recurrence.

High‑Yield Points - ⚡ Biggest Takeaways

• SRS delivers a single, high ablative dose of radiation with sub-millimeter accuracy.
• Employs rigid immobilization (frame-based/frameless) for precise targeting of intracranial lesions.
• Key indications: small brain metastases, acoustic neuromas, arteriovenous malformations (AVMs), and trigeminal neuralgia.
• Technologies include Gamma Knife (cobalt-60), CyberKnife (robotic LINAC), and specialized LINACs.
• Characterized by a sharp dose gradient, maximally sparing adjacent normal tissues.
• Distinguished from SRT (Stereotactic Radiotherapy) which uses 2-5 fractions.