Are closer margins recommended for Stereotactic Body Radiation Therapy (SBRT) to lymph nodes?

Margins for Stereotactic Body Radiation Therapy (SBRT) to Lymph Nodes

For SBRT to lymph nodes, a 5 mm expansion from GTV to CTV is recommended, with an additional institution-specific CTV to PTV margin based on setup accuracy and motion management. 1

GTV to CTV Margin Recommendations

• A 5 mm expansion from GTV to CTV is the standard recommendation for lymph nodes according to European Organisation for Research and Treatment of Cancer (EORTC) guidelines 2, 1
• This margin accounts for microscopic disease extension beyond the visible tumor boundary 1
• While some SBRT protocols have used no CTV margin, this approach deviates from International Commission on Radiation Units and Measurements (ICRU) guidelines 1
• The CTV should be adjusted according to normal anatomical boundaries when appropriate 2

CTV to PTV Margin Considerations

• The specific CTV-PTV margin should be determined based on:

  • Method of immobilization used 1
  • Assessment of tumor motion (particularly for abdominal/paraortic nodes) 1
  • Methods for on-treatment setup verification/repositioning (e.g., cone beam CT) 1
  • Institution-specific setup uncertainties 1

• With modern image guidance techniques, CTV-PTV margins can be reduced compared to historical margins of 10-15 mm 1, 3

• For MRI-guided radiotherapy systems, margins as small as 3 mm have been shown to be adequate for pelvic and para-aortic lymph nodes 3

• When using 4D-CT scans, an internal target volume (ITV) approach can account for respiratory motion 2, 1

Technical Considerations

• Dose calculations should be performed using advanced (type B) algorithms for more accurate computation 2
• Manual PTV adjustments should not be performed as the PTV accounts for setup errors and breathing motion 2
• For critical serial organs near the treatment area, planning organ at risk volume (PRV) margins should be applied 2, 1
• When treating abdominal lymph nodes, hypofractionated SBRT has shown good clinical results with minimal toxicity 4

Clinical Outcomes and Implementation

• SBRT for lymph node metastases has demonstrated good local control rates:
  • 77.8% freedom from local progression at 24 months for abdominal nodes 4
  • 82.1% local control at 3 years in a prospective trial 5
• Treatment is generally well-tolerated with minimal toxicity 4, 5
• For MRI-guided adaptive treatments, "adapt to shape" planning strategies that account for daily anatomy provide superior target coverage compared to position-only adaptations 3

Practical Implementation

• For planning:

  • Use CT slice thickness of 2-3 mm for high-resolution treatment planning 2
  • Consider intravenous contrast to improve delineation of lymph nodes 2
  • Use of FDG-PET reduces the risk of missing pathologic lymph nodes 2
  • For respiratory motion management, 4D-CT planning is strongly preferred 2

• Common pitfalls to avoid:

  • Failing to account for respiratory motion, particularly for abdominal/paraortic nodes 2, 1
  • Using type A dose calculation algorithms, which are less accurate for thoracic/abdominal treatments 2
  • Manually adjusting the PTV, which compromises the purpose of accounting for uncertainties 2