What are the guidelines for post-operative contouring for a glioma CNS (Central Nervous System) grade 2?

Post-Operative Contouring Guidelines for CNS Grade 2 Glioma

For CNS grade 2 gliomas, the gross tumor volume (GTV) should be defined using T2/FLAIR signal abnormalities on MRI, with a clinical target volume (CTV) expansion of 10 mm, and a total radiation dose of 50.4 Gy in 28 fractions (or 50-54 Gy in 1.8-2.0 Gy fractions). 1, 2

Imaging Requirements for Target Delineation

Pre-Treatment Imaging

• MRI is the standard imaging modality for both diagnosis and target volume delineation in grade 2 gliomas 2
• Obtain postoperative MRI within 24-72 hours to establish baseline and assess extent of resection 2
• Use both pre-operative and post-operative MRI fused together for optimal target definition 2, 1

Imaging Sequences for Contouring

• T2-weighted and FLAIR sequences are the primary imaging modalities for defining the GTV in grade 2 gliomas 2, 1
• The GTV should include the entire T2/FLAIR signal abnormality, as this represents the infiltrative tumor extent 1
• T1 post-contrast sequences should be reviewed, though grade 2 gliomas typically do not enhance 3

Target Volume Delineation Algorithm

Step 1: Define the Gross Tumor Volume (GTV)

• Include all residual tumor visible on postoperative imaging 1
• Include the entire resection cavity 1
• Include all T2/FLAIR signal abnormalities that represent tumor infiltration 1
• For optimal resection cases, the GTV encompasses the tumor volume defined in T2 and/or FLAIR sequence 2

Step 2: Create the Clinical Target Volume (CTV)

• Expand the GTV by 10 mm to create the CTV for grade 2 gliomas 1
• This margin accounts for microscopic tumor infiltration beyond visible imaging abnormalities 1
• The more recent ESTRO-EANO guideline specifically recommends 10 mm for grade 2 tumors, which is more conservative than older recommendations of 1-2 cm 1, 2

Step 3: Planning Target Volume (PTV)

• Add institutional setup margin (typically 3-5 mm) to account for daily setup variability and patient motion 1

Radiation Dose and Fractionation

The recommended dose is 50.4 Gy in 28 fractions (1.8 Gy per fraction), based on the most recent ESTRO-EANO guidelines 1. Alternative acceptable regimens include:

• 50-54 Gy in 1.8-2.0 Gy fractions 2
• The dose range of 45-54 Gy is considered standard, with preference for 50-54 Gy 2

Technical Considerations

Radiation Technique

• IMRT (Intensity-Modulated Radiation Therapy) is the preferred approach for grade 2 gliomas 1
• Proton therapy may be considered when available 1
• Stereotactic radiosurgery (SRS) has NOT been established as appropriate for low-grade gliomas and should not be used as initial treatment 2

Critical Pitfalls to Avoid

• Do not use whole-brain radiation therapy - localized treatment fields are standard to minimize toxicity 4
• Do not include vasogenic edema in the GTV if it can be distinguished from tumor infiltration (though this is less of an issue in grade 2 than grade 3-4 gliomas) 1
• Do not delay postoperative imaging beyond 72 hours, as this baseline is critical for future response assessment 2

Special Considerations by Histology

Oligodendroglioma Grade 2

• Follow the same contouring principles as above 1
• These tumors are typically well-demarcated and may contain calcifications 3
• Molecular testing for IDH mutation and 1p/19q codeletion should be confirmed, as this affects overall treatment strategy 3

Astrocytoma Grade 2

• More infiltrative pattern may be present 1
• T2/FLAIR abnormalities are particularly important to capture the full extent 1

Quality Assurance

• Ensure experienced neuropathologist review of tissue 2
• Multidisciplinary consultation should guide treatment decisions 2
• Use neuronavigation software to accurately fuse pre- and post-operative imaging 1