What are the recommended contouring guidelines for radiotherapy planning?

Recommended Contouring Guidelines for Radiotherapy Planning

The recommended contouring guidelines for radiotherapy planning should follow disease-specific protocols that define appropriate clinical target volumes (CTVs) and planning target volumes (PTVs) based on anatomical landmarks, with standardized margins that account for microscopic disease extension and setup uncertainties.

General Principles for Contouring

• CT scans with slice thickness of 2-3 mm are recommended for accurate tumor delineation and generation of high-quality digitally reconstructed radiographs 1
• Intravenous contrast should be used to improve delineation of centrally located tumors and lymph nodes 1
• 4D-CT scanning is strongly preferred for treatment planning in thoracic malignancies to account for respiratory motion 1
• FDG-PET reduces the risk of missing pathologic lymph nodes and should be incorporated in the target volume definition process when appropriate 1
• PET scans for radiotherapy planning should be acquired in the treatment position and coregistered with planning CT using rigid registration methods 1

Target Volume Definitions

• Gross Tumor Volume (GTV): Visible and palpable tumor based on clinical examination, imaging, and pathologic findings 1
• Clinical Target Volume (CTV): GTV plus margin for microscopic disease extension 1
• Planning Target Volume (PTV): CTV plus margin for setup uncertainties and internal motion 1
• Internal Target Volume (ITV): Used when accounting for respiratory motion, especially in thoracic malignancies 1

Standard Margin Recommendations

• A 5 mm CTV margin from GTV is typically recommended for lymph nodes based on EORTC guidelines 2
• For non-stereotactic treatments, CTV to PTV margins typically range from 5-10 mm depending on immobilization and image guidance techniques 1
• For SBRT treatments, smaller CTV-PTV margins (typically 3-5 mm) may be used with advanced image guidance 2
• Manual PTV adjustments should not be performed as the PTV accounts for setup errors and breathing motion 1

Disease-Specific Contouring Guidelines

Lung Cancer

• Recommended CT settings for tumor delineation: for lung parenchyma, window width 1,600 and level 600; for mediastinum, window width 400 and level 20 1
• Elective irradiation of mediastinal lymph nodes is not recommended for non-small cell lung cancer 1
• Selective nodal irradiation based on CT and FDG-PET is the recommended standard 1
• Planning organ at risk volume (PRV) margins should be applied around critical serial organs 1

Anal Cancer

• All elective nodal regions should be routinely contoured for all disease stages, with possible exception of inguinal and high pelvic nodes for select early-stage T1N0 disease 3
• A 20 mm CTV margin for the primary tumor, 10-20 mm CTV margin for involved nodes, and 7 mm CTV margin for elective pelvic nodal groups are recommended 3
• A 5-10 mm PTV margin is suggested while respecting anatomical boundaries 3

Testicular Cancer

• For modified dog-leg fields, the superior border should be placed at the bottom of vertebral body T11 1
• The inferior border should be placed at the top of the acetabulum 1
• The aorta and inferior vena cava should be contoured from the bottom of T11 vertebra inferiorly, and ipsilateral iliac arteries and veins down to the top of the acetabulum 1
• A 1.2-1.9 cm margin on vascular structures for the CTV is recommended 1
• The PTV is established by uniformly expanding the CTV by 0.5 cm in all directions 1

Malignant Pleural Mesothelioma

• For postoperative radiotherapy after extrapleural pneumonectomy, the entire ipsilateral pleural bed should be encompassed 1
• A CTV can encompass this region with a margin at the mediastinal/pleural interface of 0.5 cm 1
• Total dose of 45-54 Gy in 1.8-2.0 Gy fractions is recommended 1

Advanced Techniques and Considerations

• IMRT or volumetric modulated arc therapy is recommended for complex treatment sites to spare organs at risk and reduce toxicity 1
• When using 4D-CT scans, an internal target volume (ITV) approach can be used to account for respiratory motion 2
• Advanced dose calculation algorithms (type B) are recommended for thoracic radiotherapy for more accurate computation of dose distributions 1
• Dose prescriptions and reporting should follow the international ICRU standards 1

Quality Assurance in Contouring

• Interobserver variability in contouring can be significant, particularly for organs with poor visibility or without distinctive boundaries 4
• Automated contouring tools using atlas-based or deep learning approaches can reduce contouring time while maintaining accuracy when properly reviewed and adjusted 5
• Consensus guidelines and high-resolution atlases should be used whenever available to standardize contouring practices 3, 6

Common Pitfalls and Recommendations

• Failure to account for respiratory motion can lead to systematic errors in thoracic radiotherapy 1
• Inadequate margins for microscopic disease extension may result in marginal recurrences 2
• Inconsistent window/level settings during contouring can lead to significant variations in target delineation 1
• Lack of standardized contouring guidelines can result in significant interobserver variability 4, 6
• Overreliance on automated contouring tools without proper review and adjustment can lead to inaccurate target volumes 5

By following these evidence-based contouring guidelines, radiation oncologists can ensure consistent and accurate target delineation, which is essential for high-quality radiotherapy planning and delivery.