What is the typical treatment protocol with Stereotactic Radiosurgery (SRT) for localized tumors?

Stereotactic Radiosurgery (SRT) Treatment Protocol for Localized Tumors

For localized tumors, stereotactic radiosurgery (SRS) should be delivered as a single high-dose treatment (18-24 Gy) for small lesions, while stereotactic radiotherapy (SRT) using 3-5 fractions is recommended for larger tumors or those near critical structures. 1

Patient Selection and Evaluation

• Ideal candidates:

  • Patients with limited (1-3) metastatic lesions
  • Small, deep lesions (<3 cm)
  • Good performance status
  • Stable systemic disease or reasonable systemic treatment options

• Pre-treatment imaging:

  • High-resolution contrast-enhanced MRI (gold standard)
  • Consider advanced imaging (MR spectroscopy, MR perfusion, or PET) to differentiate tumor from radiation necrosis 2

Treatment Planning Parameters

Single-Fraction SRS

• Dose range: 18-24 Gy in a single fraction 1
• Planning target volume: 0-2 mm margin
• Ideal for:
  • Small lesions (<2-3 cm)
  • Deep-seated tumors
  • Tumors distant from critical structures

Fractionated SRT

• Common fractionation schemes: 1
  • 20 Gy in 2 fractions
  • 24-27 Gy in 3 fractions
  • 25-40 Gy in 5 fractions
• Ideal for:
  • Larger tumors (>3 cm)
  • Tumors adjacent to critical structures
  • Tumors with mass effect

Treatment Delivery Systems

• Gamma Knife: Uses cobalt-60 sources, typically for single-fraction treatments
• Linear accelerator (LINAC): Can deliver both single-fraction and fractionated treatments
• CyberKnife: Frameless system allowing for both single-fraction and fractionated delivery

Specific Protocols by Tumor Type

Brain Metastases

• Limited (1-3) metastases: SRS alone (18-24 Gy) with close MRI surveillance 1
• Multiple (>4) metastases:
  • Consider SRS for up to 10 lesions in selected patients
  • Alternative: whole brain radiation therapy (WBRT) with 30 Gy in 10 fractions or 37.5 Gy in 15 fractions 1

Meningiomas (WHO Grade I)

• Primary treatment: SRS with margin dose of 12-16 Gy 1
• Post-surgical residual: SRS to residual tumor
• Recurrent disease: Consider repeat SRS if good response to initial treatment 2

Vestibular Schwannomas

• Dose range: 11-14 Gy in single fraction 1
• For larger tumors: Fractionated SRT using up to 10 fractions 1

Follow-up Protocol

• Imaging schedule:

  • MRI every 3 months for the first year
  • Then every 6-12 months thereafter 1, 2

• Response assessment:

  • Be aware that post-treatment changes may mimic progression within first 3 months 2
  • Consider advanced imaging to differentiate tumor progression from radiation necrosis

Management of Recurrence

• Local recurrence options:

  1. Repeat SRS if previous good response lasting >6 months
  2. Surgery for symptomatic lesions
  3. Consider alternative systemic therapy 1

• Distant brain recurrence:

  • 1-3 new lesions: Consider repeat SRS

  • 3 new lesions: Consider WBRT or systemic therapy 1

Potential Complications and Management

• Radiation necrosis:

  • Incidence: 5-10% depending on dose and volume
  • Management: Steroids, bevacizumab for refractory cases

• Edema:

  • Prophylactic steroids for large lesions or those near critical structures
  • Taper steroids gradually post-treatment

Key Pitfalls to Avoid

1. Inadequate imaging for planning: Always obtain high-quality, thin-slice MRI
2. Overlooking systemic disease status: Coordinate with medical/radiation oncology
3. Treating too many lesions with SRS: Consider WBRT for widespread metastatic disease
4. Misinterpreting post-treatment changes: Treatment effect can mimic progression

SRS/SRT has revolutionized the treatment of localized brain tumors, offering excellent local control with minimal toxicity compared to conventional approaches. The treatment protocol should be tailored based on tumor size, location, histology, and patient factors, with close follow-up to monitor response and manage potential complications.