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

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, 2, 3
This margin accounts for microscopic disease extension beyond the visible tumor 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

For non-MRI guided treatments, historical series used generic CTV-PTV margins ranging from 10-15 mm 1, 3
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, and institution-specific setup uncertainties 2

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 should be used for treatment planning 1, 2
Manual PTV adjustments should not be performed 1

For planning organ at risk volume (PRV), margins should be applied to critical serial organs near the treatment area 2
PRV margin calculations should be based on each institution's data of setup margins and other uncertainties 2