Radiotherapy Planning Guidelines

General Principles for Contouring

The American Society of Clinical Oncology recommends CT scans with slice thickness of 2-3 mm for accurate tumor delineation and generation of high-quality digitally reconstructed radiographs 1
The use of intravenous contrast is recommended 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, 2
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

The International Association for the Study of Lung Cancer defines Gross Tumor Volume (GTV) as visible and palpable tumor based on clinical examination, imaging, and pathologic findings 3
Clinical Target Volume (CTV) is defined as GTV plus margin for microscopic disease extension 3
Planning Target Volume (PTV) is defined as CTV plus margin for setup uncertainties and internal motion 3
Internal Target Volume (ITV) is used when accounting for respiratory motion, especially in thoracic malignancies 3

The European Organisation for Research and Treatment of Cancer (EORTC) recommends a 5 mm CTV margin from GTV for lymph nodes 4
For non-stereotactic treatments, CTV to PTV margins typically range from 5-10 mm depending on immobilization and image guidance techniques 3
For SBRT treatments, smaller CTV-PTV margins (typically 3-5 mm) may be used with advanced image guidance 4
Manual PTV adjustments should not be performed as the PTV accounts for setup errors and breathing motion 1

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

For modified dog-leg fields, the superior border should be placed at the bottom of vertebral body T11 5, 6
The inferior border should be placed at the top of the acetabulum 5, 6
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 5, 6
A 1.2-1.9 cm margin on vascular structures for the CTV is recommended 5, 6
The PTV is established by uniformly expanding the CTV by 0.5 cm in all directions 5, 6

For postoperative radiotherapy after extrapleural pneumonectomy, the entire ipsilateral pleural bed should be encompassed 3
A CTV can encompass this region with a margin at the mediastinal/pleural interface of 0.5 cm 3
Total dose of 45-54 Gy in 1.8-2.0 Gy fractions is recommended 3

The European Society for Medical Oncology recommends IMRT or volumetric modulated arc therapy for complex treatment sites to spare organs at risk and reduce toxicity 7
When using 4D-CT scans, an internal target volume (ITV) approach can be used to account for respiratory motion 4
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

Failure to account for respiratory motion can lead to systematic errors in thoracic radiotherapy 2
Inadequate margins for microscopic disease extension may result in marginal recurrences 4
Inconsistent window/level settings during contouring can lead to significant variations in target delineation 1