What are the recommended margins for expanding the Gross Tumor Volume (GTV) to Clinical Target Volume (CTV) and Planning Target Volume (PTV) for Stereotactic Body Radiation Therapy (SBRT) to lymph nodes of the paraortics?

Margins for CTV and PTV from GTV for SBRT to Paraortic Lymph Nodes

For stereotactic body radiation therapy (SBRT) to paraortic lymph nodes, a 5 mm GTV to CTV margin and a 3 mm CTV to PTV margin are recommended based on current evidence. 1

GTV to CTV Margin Recommendations

• A 5 mm expansion from GTV to CTV is recommended for lymph nodes, based on European Organisation for Research and Treatment of Cancer (EORTC) guidelines 1
• This margin accounts for microscopic disease extension beyond the visible tumor 1
• Some studies have found that extracapsular extension (ECE) of lymph node metastases is ≤3 mm in 95% of nodes, supporting the adequacy of a 5 mm margin 2
• Larger lymph nodes (≥20 mm) may warrant more generous margins due to increased risk and extent of extracapsular extension 2
• In some SBRT protocols, no CTV margin has been used, but this approach deviates from International Commission on Radiation Units and Measurements (ICRU) guidelines 1

CTV to PTV Margin Recommendations

• A 3 mm isotropic CTV to PTV margin has been validated as adequate for SBRT to lymph nodes when using MRI-guided adaptive radiotherapy 3
• For non-MRI guided treatments, historical series used generic CTV-PTV margins ranging from 10-15 mm 1
• With modern image guidance and motion management techniques, these margins can be reduced 1
• The specific CTV-PTV margin should account for:
  • Method of immobilization 1
  • Assessment of tumor motion 1
  • Methods for on-treatment setup verification/repositioning (e.g., cone beam CT) 1
  • Institution-specific setup uncertainties 1

Anatomical Considerations for Paraortic Lymph Nodes

• For paraortic lymph node regions, the CTV should extend:
  • 1.8 cm to the left of the aorta 4
  • 1.4 cm to the right of the inferior vena cava (IVC) 4
  • 7 mm posterior to the aorta/IVC or to the vertebral body 4
  • 4 mm anterior to the aorta/IVC 4
  • Superior border at the T11/T12 vertebral interface 4
  • Inferior border at the bifurcation of the aorta/IVC 4

Treatment Planning Considerations

• When using 4D-CT scans, an internal target volume (ITV) approach can be used to account for respiratory motion 1
• Dose calculations should be performed on the CT reconstruction phase representing the time-averaged mean position of the tumor 1
• When gating or tracking techniques are used, the CTV-PTV margin should account for the gating window 1
• Type B dose calculation algorithms (those that consider changes in lateral electron transport) should be used for treatment planning 1
• Manual PTV adjustments should not be performed 1

Special Considerations

• For planning organ at risk volume (PRV), margins should be applied to critical serial organs near the treatment area 1
• PRV margin calculations should be based on each institution's data of setup margins and other uncertainties 1
• Adaptive planning strategies that account for daily anatomy variations may improve target coverage and normal tissue sparing 3
• MRI-guided radiotherapy may allow for reduced margins due to improved soft tissue visualization and daily adaptive planning 3