What CT acquisition parameters and anatomical structures should be used for contouring intracranial targets in radiation therapy planning?

CT Anatomy of Brain for Radiation Contouring

CT Acquisition Parameters

For intracranial radiation therapy planning, CT slice thickness should be ≤5 mm, with 2-3 mm preferred for optimal target delineation and digitally reconstructed radiograph generation. 1

Technical Specifications

• Slice thickness: 2-3 mm is strongly recommended as it permits generation of high-resolution digitally reconstructed radiographs and facilitates accurate tumor delineation 1
• Maximum acceptable thickness: ≤5 mm (0.5 cm) for intracranial targets, particularly skull base meningiomas 1
• Scan coverage: Entire brain volume, typically extending from vertex to skull base, ensuring complete coverage of all intracranial structures 1

Contrast Administration

• Intravenous contrast can improve delineation of centrally located primary tumors and lymph nodes, though its use should be determined based on tumor location and type 1
• Contrast is optional for CT simulation but may enhance visualization of vascular structures and tumor boundaries 1

Mandatory MRI Fusion Protocol

MRI fusion with planning CT is mandatory for accurate intracranial target delineation, particularly for skull base extension, perineural involvement, and marrow infiltration. 1, 2

Required MRI Sequences

• Volumetric T1 post-gadolinium: Essential for tumor detection and characterization 1, 3
• High-resolution T2-weighted sequences (FIESTA/CISS): Critical for nerve visualization and anatomical detail 1, 3
• Pre-contrast T1-weighted: Baseline tissue characterization 1
• T2 FLAIR: Demonstrates CSF spaces and edema 3

Image Fusion Requirements

• Rigid coregistration methods should be used to align MRI with planning CT 1
• Review MRI-CT fusion with a neuroradiologist when uncertainty exists regarding disease extent 1, 4
• Total maximum image distortion for 3T MRI scanners is approximately 4 mm, which is acceptable for treatment planning 5

Anatomical Structures for Contouring

Gross Tumor Volume (GTV) Delineation

The GTV is defined as macroscopic visible disease based on pre- and post-operative post-contrast T1-weighted MR sequences, with CT window settings of width 400 and level 20 for soft tissue structures. 1, 2

• Window settings for CT: Width 400, Level 20 for mediastinal/soft tissue structures; Width 1600, Level -600 for lung windows 1, 2
• Include all visible tumor on clinical examination and imaging 2
• For meningiomas, include the dural tail and any abnormal bone on radiology 1
• Tumor bed should be included in post-operative cases 1

Critical Anatomical Boundaries

• Skull base structures: Parasellar/cavernous location, infratentorial location including cerebellopontine angle and clivus require careful delineation 1
• Bony anatomy: CT provides superior detection of cortical bone invasion compared to MRI 1
• Natural barriers: Trim CTV at bone without invasion and air spaces 2
• Perineural spread: MRI is superior for detecting perineural disease without bone involvement 1, 4

Clinical Target Volume (CTV) Margins

Apply a 5 mm margin from GTV to CTV for intracranial targets to account for microscopic disease extension, with adjustment at anatomical barriers. 2

• Standard CTV margin: 5 mm from GTV 2
• Adjust CTV according to normal tissues (e.g., bones) as appropriate 1
• For meningiomas: 0.5-1.0 cm margins for benign/atypical tumors, 1.0-1.5 cm for anaplastic meningiomas 1

Planning Target Volume (PTV) Expansion

Use 3-5 mm CTV-to-PTV expansion when daily cone-beam CT or kV imaging is performed; increase to 5-10 mm with less frequent image guidance. 2

• With daily image guidance: 3-5 mm expansion 2
• Without daily image guidance: 5-10 mm expansion 2
• PTV accounts for setup errors and must not be adjusted or trimmed 1

Planning Risk Volume (PRV) for Organs at Risk

A PRV margin around critical serial organs (optic nerves, chiasm, brainstem, spinal cord) should be applied to account for setup uncertainty. 1

• PRV margins are encouraged for all critical serial structures 1
• Dose Volume Histograms should be constructed for both PTVs and organs at risk 1

Common Pitfalls to Avoid

• Do not reduce GTV to post-chemotherapy volumes if induction chemotherapy was given; preinduction tumor extent within bony anatomy should receive full dose 1, 2
• Avoid relying on CT alone for small lesions, particularly in the internal auditory canal or cerebellopontine angle, as they are frequently missed 3
• Do not use symmetric geometric expansions without considering anatomical boundaries 2
• Ensure quality of image coregistration is checked before contouring, as patient movements may lead to incorrect fusion even with PET-CT 1
• Account for MRI distortion: Total maximum distortion of approximately 4 mm should be considered in margin design 5

Quality Assurance Considerations

• Collaborative review with neuroradiology significantly impacts target delineation accuracy, with 55% of cases requiring alterations and 61% of those having clinically significant changes 4
• Cases with challenging patterns of infiltrative, intracranial, or perineural spread warrant intensive imaging-based review with a diagnostic neuroradiologist 4
• Dose specifications should follow ICRU 50/62 standards 1