What are the guidelines for sarcoma contouring in radiation therapy?

Sarcoma Contouring Guidelines for Radiation Therapy

For preoperative radiotherapy of soft tissue sarcomas, the GTV should be defined by T1 contrast-enhanced MRI fused with planning CT, and the CTV should include the GTV plus 3-cm longitudinal margins and 1.5-cm radial margins, modified by anatomical barriers. 1

Gross Tumor Volume (GTV) Definition

The GTV is the gross tumor as defined by T1 contrast-enhanced MRI images. 1

• Fusion of MRI and CT images is strongly recommended to accurately delineate the GTV, as MRI provides superior soft tissue contrast compared to CT alone 1
• The GTV represents the visible tumor extent on imaging without additional margins 1
• For cases where MRI-CT fusion is performed, attention must be paid to potential positional differences between the two imaging modalities, which can introduce variability particularly in upper extremity cases 2

Clinical Target Volume (CTV) Definition

For high-grade large soft tissue sarcomas, the CTV typically includes the GTV plus specific directional margins that account for microscopic disease extension. 1

Longitudinal Margins

• Add 3-cm margins in the longitudinal (superior-inferior) directions from the GTV 1
• If the 3-cm margin would extend beyond the anatomical compartment, the field can be shortened to include the end of the compartment rather than extending beyond it 1
• This compartment-based modification prevents unnecessary irradiation of uninvolved tissue while maintaining oncologic adequacy 1

Radial Margins

• Apply 1.5-cm radial margins from the lesion in all directions 1
• This radial margin should include any portion of the tumor not confined by natural anatomical barriers 1
• Intact fascial barriers, bone, or skin surface serve as natural boundaries that can limit the radial CTV expansion 1
• When the tumor abuts these structures, the CTV margin can be reduced at that interface 1

Technical Considerations for Contouring

Image Guidance Requirements

• Daily image-guided radiation therapy (IGRT) is necessary for soft tissue sarcomas, as daily repositioning errors are substantial 3
• Without daily IGRT, a CTV-to-PTV margin of 1.5 cm would be required to account for setup variations 3
• With daily IGRT using bone anatomy matching, estimated CTV-to-PTV margins can be reduced to approximately 10-13 mm in different directions 3

Multi-Modality Imaging Integration

• Incorporating multi-modality imaging (FDG-PET, CT, and T1-weighted MRI) provides improved contouring accuracy compared to single or dual-modality approaches 4
• Pre-contrast T1-weighted MRI is particularly valuable for defining tumor extent in soft tissue sarcomas 4

Inter-Observer Variability Considerations

Despite standardized guidelines, some variability in contouring exists, particularly for CTV delineation in anatomically complex regions. 2

• GTV agreement among experienced sarcoma radiation oncologists is almost perfect (kappa=0.86-0.9) when using standardized guidelines 2
• CTV agreement is almost perfect for lower extremity cases (kappa=0.85) but shows more variation in upper extremity cases (kappa=0.77), likely related to positional differences between planning CT and diagnostic MRI 2
• The mean GTV volumes can vary by approximately 5-15% among experienced observers even with standardized protocols 2
• CTV volumes show greater variability, with ranges from 1145-1911 mL in lower extremity cases and 637-1246 mL in upper extremity cases among different observers 2

Common Pitfalls and How to Avoid Them

• Pitfall: Extending CTV margins uniformly without considering anatomical compartments

  • Solution: Limit longitudinal margins at compartment boundaries rather than extending beyond natural anatomical barriers 1

• Pitfall: Applying full radial margins at intact fascial barriers or bone

  • Solution: Reduce radial margins where tumor is confined by intact fascia, periosteum, or skin surface 1

• Pitfall: Relying solely on CT imaging for GTV definition

  • Solution: Always use T1 contrast-enhanced MRI fused with planning CT for accurate tumor delineation 1

• Pitfall: Ignoring positional differences between MRI and CT

  • Solution: Carefully verify anatomical landmarks during image fusion, particularly for upper extremity cases where positioning variability is greater 2

• Pitfall: Using inadequate PTV margins without daily IGRT

  • Solution: Implement daily IGRT with bone-based matching, or use 1.5-cm CTV-to-PTV margins if daily IGRT is unavailable 3