What is the recommended margin from Clinical Target Volume (CTV) to Planning Target Volume (PTV)?

CTV to PTV Margin Recommendations

For non-stereotactic radiotherapy treatments, a uniform 5 mm CTV to PTV margin is recommended when daily image guidance (cone-beam CT or kV imaging) is used, while 10-15 mm margins are appropriate for treatments without advanced image guidance. 1, 2

Standard Margin Framework

The Planning Target Volume (PTV) accounts for two primary sources of uncertainty: patient setup variations and internal organ motion. 1

General Recommendations by Treatment Context

For conventional radiotherapy with modern image guidance:

• 5-10 mm CTV to PTV margin when daily cone-beam CT or kV imaging is performed 2, 3
• 10-15 mm CTV to PTV margin for historical series or treatments with less frequent image verification 1

For stereotactic body radiotherapy (SBRT):

• 3-5 mm CTV to PTV margin with advanced image guidance and motion management 2, 4

For MRI-guided radiotherapy:

• 3 mm PTV margin for intracranial treatments with daily MRI guidance, which provides 98% coverage of the CTV in 95% of fractions in 95% of patients 5

Site-Specific Margin Recommendations

Thoracic Malignancies (Lung Cancer)

The margin depends critically on whether respiratory motion is accounted for separately:

With 4D-CT and Internal Target Volume (ITV) approach:

• First create an ITV from the CTV accounting for respiratory motion across all breathing phases 1
• Then apply 5 mm margin from ITV to PTV for setup uncertainties only 1

Without motion management:

• 10-15 mm CTV to PTV margin to account for both setup error and respiratory motion 1

Malignant Pleural Mesothelioma

For IMPRINT technique (intensity-modulated pleural radiation):

• 5 mm uniform margin from ITV to PTV 1
• PTV outer defined as CTV outer plus 10 mm external expansion into chest wall 1
• PTV inner defined as CTV inner with 6 mm expansion into lung parenchyma 1

For post-pneumonectomy adjuvant radiation:

• 5 mm uniform margin from CTV to PTV 1

Head and Neck Cancer

With daily image guidance:

• 3-5 mm CTV to PTV margin when daily cone-beam CT or kV imaging is performed 2, 3
• Population-based margins of 5.1-6.6 mm (anterior-posterior) and 3.7-5.7 mm (superior-inferior) for skull/cervical fields 6

Without daily image guidance:

• 5-10 mm CTV to PTV margin 3

Prostate Cancer

With image-guided radiotherapy:

• 7.3 mm (right-left), 7.0 mm (superior-inferior), 9.0 mm (anterior-posterior) based on cone-beam CT verification studies 7

Critical Organs at Risk

Planning organ at risk volume (PRV) margins for serial organs:

• 3-5 mm margin around spinal cord to account for positioning uncertainties 1
• PRV margins should be applied to all critical serial organs (spinal cord, main bronchi, large blood vessels) in centrally located tumors 1

Technical Considerations That Modify Margins

Factors That Allow Margin Reduction:

• Daily image guidance with cone-beam CT or kV imaging 2, 3
• Rigid immobilization devices (thermoplastic masks for head/neck) 6, 7
• 4D-CT planning with ITV approach for respiratory motion 1
• Gating or breath-hold techniques 1

Factors That Require Larger Margins:

• Infrequent setup verification 3
• Significant respiratory motion without motion management 1
• Poor patient immobilization 6
• Institution-specific setup uncertainties exceeding standard values 1

Common Pitfalls to Avoid

Do not manually adjust PTV margins after they have been systematically calculated, as the PTV is designed to account for setup errors and breathing motion through standardized expansion 1, 2

Do not use symmetric margins without considering anatomical boundaries—margins can be trimmed at natural barriers such as bone without invasion or air spaces 3

Do not apply the same margins across all treatment sites—margins must be tailored to the specific anatomical location, immobilization method, and image guidance frequency 1, 2

Do not forget to account for respiratory motion separately in thoracic treatments—use 4D-CT to create an ITV first, then apply setup margins to create the PTV 1

Mathematical Basis for Margin Selection

The recommended 5 mm margin corresponds to approximately 1.65 standard deviations of setup uncertainty, which ensures that any point on the CTV surface is within the PTV 95% of the time 8

For complete CTV enclosure within the PTV 95% of the time, a margin of 2.8 standard deviations would be required, but this is unnecessarily conservative and increases normal tissue dose 8