What are the key considerations for treatment planning evaluation in radiotherapy (Radiation Therapy)?

Treatment Planning Evaluation in Radiotherapy

Core Planning Requirements

CT-based treatment planning with conformal blocking and dosimetry is considered standard care for radiotherapy, with CT slice thickness of 2-3 mm recommended to permit generation of high-resolution digitally reconstructed radiographs and facilitate accurate tumor delineation 1, 2. Advanced dose calculation algorithms (type B) are mandatory for thoracic radiotherapy because they allow for more accurate computation of dose distributions, and absolute doses calculated with type A versus type B algorithms cannot be compared 1.

Imaging and Simulation Standards

• Intravenous contrast should be used during CT simulation to improve delineation of centrally located primary tumors and lymph nodes 1, 2
• 4D-CT scanning is strongly preferred for treatment planning in thoracic malignancies to account for respiratory motion and avoid systematic errors from capturing random tumor positions 1, 2
• FDG-PET reduces the risk of missing pathologic lymph nodes and should be incorporated in target volume definition, acquired in treatment position and coregistered with planning CT using rigid registration methods 1, 2
• Planning CT scans should include the entire lung volume, typically extending from the cricoid cartilage to the second lumbar vertebra 1

Target Volume Delineation

Recommended CT window settings for tumor delineation are: lung parenchyma (width 1,600, level 600) and mediastinum (width 400, level 20) 1, 2. These standardized settings prevent significant variations in target delineation that occur with inconsistent window/level settings 2.

Margin Recommendations

• A fixed 5-mm CTV margin from GTV is recommended, though adjustment according to tumor histology and lymph node size may be appropriate 1, 2
• CTV to PTV margins typically range from 5-10 mm for non-stereotactic treatments depending on immobilization and image guidance techniques 2
• For SBRT treatments, smaller CTV-PTV margins (3-5 mm) may be used with advanced image guidance 2
• Manual PTV adjustments are not permitted, as the PTV accounts for setup errors and breathing motion 1
• Planning organ at risk volume (PRV) margins should be applied around critical serial organs 1, 2

Patient Positioning and Immobilization

Stable and reproducible patient positioning is essential, with patients positioned with both arms above the head when possible to permit greater beam access and improve target coverage while sparing normal tissues 1. However, individualized immobilization techniques are required for patients unable to use standard positioning due to arthritis, debility, or inability to raise arms, as long as they remain reproducible and stable 1.

Motion Management

• Tumor delineation in the middle or average ventilation position with calculated adequate margin is the recommended strategy for generating PTV 1
• Gating and tracking are technically challenging and may only be valuable in the small subgroup of patients whose tumors show significant motion with online tumor-based setup corrections 1
• Respiratory gating and breath-hold techniques can reduce tumor motion but must be tolerated by the patient 1
• External surrogates (chest/abdominal wall movement) may not adequately reflect internal tumor position and displacement of implanted fiducials 1

Treatment Delivery Verification

Corrections based on tumor positions derived from cone beam CT (CBCT) are superior to setup with bony anatomy using electronic portal images 1. Online corrections are superior to offline protocols because of large interfraction baseline tumor variations 1.

Image Guidance Requirements

• Frequent CBCTs (more than once weekly) are relevant when large changes are anticipated, such as atelectasis or bulky small cell lung cancer treated with concurrent chemoradiation 1
• Online verification is needed for stereotactic treatment 1
• Repeated imaging is particularly important when treatment delivery times are prolonged as in stereotactic radiotherapy 1
• CBCT increases radiation doses to patients and should be factored into the final plan 1

Dose Prescription and Constraints

Dose prescriptions and reporting should follow international ICRU standards 1. For lung cancer specifically, the V20 and mean lung dose should be kept at 35-37% and 20-23 Gy respectively when possible 1.

Critical Organ Tolerances

• Spinal cord maximum dose should be limited to the equivalent of 13 Gy in a single fraction or 20 Gy in three fractions for SBRT to prevent myelopathy (risk <1%) 1
• Doses to central bronchi in excess of 80 Gy may increase the risk of late toxicity including severe bronchial stenosis and fistula occurring after 2 years or more 1
• Coplanar techniques are the first choice to limit treatment time and intrafraction motion while maintaining adequate dose distributions 1

Treatment Planning Optimization

Advanced treatment planning systems now provide superior dose estimations through improved modeling of radiotherapy beams as they pass through normal lung tissue 1. The radiation oncologist must have sufficient training to define target volumes and critical structures, interpret treatment planning information, and guide the physicist or dosimetrist in achieving the best dose distribution 3.

Quality Assurance

• Quality assurance of radiotherapy plans with peer review is important in improving outcomes, with studies showing 22% of treatment plans changed with peer review 1
• Protocol deviations in radiotherapy delivery are associated with increased risks of treatment failure and overall mortality 1
• The radiation oncologist must be competent to judge the quality of dose distribution and technical feasibility of the proposed plan 3

Response and Toxicity Evaluation

Morphologic imaging bears large interobserver variability and may be hampered by treatment-related changes, especially pneumonitis and lung fibrosis 1. FDG-PET scans may be helpful despite false-positive accumulations in intratumor macrophages and early postirradiation inflammation, as the decrease in FDG accumulation is a strong predictor of prognosis 1.

Toxicity Assessment

• Toxicity should be scored using validated instruments such as the Common Terminology Criteria of Adverse Events version 4.0 1
• For response evaluation and detection of recurrence, the negative predictive value of FDG-PET is high 1
• Response after radiotherapy or chemoradiation should be defined according to the criteria of Green et al., as RECIST criteria used for chemotherapy response are often inadequate 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 result in marginal recurrences 2
• Prolonged treatment duration has adverse effects on outcome, with approximately 0.5-1% decrease in pelvic control and cause-specific survival for each extra day beyond 6-8 weeks 1
• Planning on scans that capture random tumor positions at extremes of the respiratory cycle may result in systematic errors 1
• No computer software can correct the radiation oncologist's errors of clinical judgment or misunderstanding of physical concepts 3