CT Simulation Protocol for Radiation Therapy
CT simulation with intravenous and/or oral contrast should be used when clinically appropriate to aid in target localization, with all patients positioned supine using immobilization devices for reproducible daily setup. 1
Patient Positioning and Immobilization
- All patients must be simulated and treated in the supine position to ensure reproducibility across treatment sessions 1
- Use of an immobilization device is strongly recommended (not optional) for reproducibility of daily setup 1
- For thoracic malignancies, patients should be immobilized with a customized mold with arms positioned overhead when possible to maximize available beam angles 1
- For testicular cancer, patients should have arms at sides with legs separated by a rolled towel and scrotal shield placement 1
CT Simulation Technical Specifications
CT simulation with 3-dimensional treatment planning is strongly encouraged across all disease sites 1. The American Society of Clinical Oncology recommends CT scans with slice thickness of 2-3 mm for accurate tumor delineation 2.
Contrast Administration
- Intravenous and/or oral contrast should be used when clinically appropriate to aid in target localization 1
- Intravenous contrast is specifically recommended to improve delineation of centrally located tumors and lymph nodes 2
- For gastric cancer, patients should avoid heavy meals for 3 hours before simulation and treatment 1
Respiratory Motion Management
For thoracic malignancies, either free-breathing or 4D CT scans are recommended with slice spacing of 2.5-3 mm 1. 4D-CT scanning is strongly preferred for treatment planning in thoracic malignancies to account for respiratory motion 2. When using 4D-CT scans, an internal target volume (ITV) approach should be used to account for respiratory motion 2.
Advanced Imaging Integration
PET/CT scans should be obtained before contouring when appropriate for restaging and optimal target delineation 1. FDG-PET reduces the risk of missing pathologic lymph nodes and should be incorporated in target volume definition 2. PET scans for radiotherapy planning should be acquired in the treatment position and coregistered with planning CT using rigid registration methods 2.
MRI scans are superior for detecting T3-T4 disease in mesothelioma and should be incorporated when appropriate and available, particularly for lung-sparing approaches with gross disease 1. T1, T2, fat-suppressing sequences, and diffusion-weighted MRI can greatly aid target delineation 1.
Disease-Specific Considerations
Thoracic Malignancies
- Free-breathing or 4D CT should include entire thorax from lung apex to at least L3 vertebra to cover the lowest insertion point of the diaphragm with inferior margin 1
- Nuclear perfusion renal scan is recommended before simulation for hemithoracic radiation to assess relative urinary flow, as the ipsilateral kidney typically receives moderate radiation approaching tolerance 1
- Wire markers and optional 0.5 cm thick bolus with 3.0-3.5 cm diameter may be placed over scars and drain sites 1
Gastroesophageal Malignancies
- CT scans, barium swallow, endoscopic ultrasound, endoscopy reports, and PET/CT should be reviewed by multidisciplinary team before simulation 1
- Planning target volume should include tumor plus nominal 5-cm cephalad/caudal margin and 1.5-2 cm radial margin, accounting for respiratory motion 1
Testicular Cancer
- Non-contrast CT simulation should be performed with patient supine, arms at sides, in treatment position 1
- All patients except those with bilateral orchiectomy should be treated with scrotal shield 1
- Immobilization with cast may be used to improve reproducibility 1
Common Pitfalls to Avoid
Failure to account for respiratory motion leads to systematic errors in thoracic radiotherapy 2. 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 2.
The uncertainties from variations in stomach filling and respiratory motion must be taken into consideration when planning gastric cancer treatments 1.