What are the typical magnetic resonance imaging characteristics of a World Health Organization Grade 3 glioma (anaplastic astrocytoma, anaplastic oligodendroglioma, or anaplastic oligo‑astrocytoma)?

WHO Grade 3 Glioma MRI Characteristics

Key Imaging Features by Molecular Subtype

WHO Grade 3 gliomas demonstrate distinct MRI characteristics that vary significantly between IDH-mutant astrocytomas and oligodendrogliomas, with contrast enhancement being the most consistent predictor of grade 3 histology across both subtypes. 1

Anaplastic Astrocytoma (IDH-mutant, Grade 3)

Contrast Enhancement Pattern:

• Contrast enhancement shows a stepwise positive association with WHO grade in IDH-mutant astrocytomas, with significantly higher likelihood of enhancement in grade 3 compared to grade 2 tumors (OR 7.79, p = 0.003) 1
• Ring-like enhancement patterns are common when enhancement is present 2
• Enhancement typically emerges from previously existing T2-hyperintense areas that initially showed no enhancement 3

T2-Weighted Signal Characteristics:

• Hyperintense signal on T2-weighted images is the predominant finding 1, 3
• T2-weighted high-signal intensity volumes are critical for surgical planning, with median preoperative volumes around 56.1 cm³ in grade 3 tumors 4
• The T2-hyperintense region represents infiltrative tumor extending beyond any contrast-enhancing component 4

Advanced MRI Metrics:

• ADC (Apparent Diffusion Coefficient): Negatively associated with WHO grade, meaning lower ADC values predict grade 3 histology (OR 0.74, p = 0.002) 1
• rCBV (Relative Cerebral Blood Volume): Positively associated with WHO grade, with higher rCBV values predicting grade 3 tumors (OR 2.33, p = 0.002) 1

T1-Weighted Characteristics:

• Hypointense signal on T1-weighted images is typical 2

Associated Features:

• Cystic regions within the mass are common (approximately 50% of cases) 2
• Hemorrhage may be present but is not a defining feature 2
• Contact with lateral ventricle is very common (>90% of cases) 2
• Mass effect with surrounding edema is typically present 3

Anaplastic Oligodendroglioma (IDH-mutant, 1p/19q-codeleted, Grade 3)

Contrast Enhancement Pattern:

• Strong positive association between contrast enhancement and grade 3 histology (OR 15.33-20.00, p = 0.003-0.008) 1
• Ring-like enhancement is the predominant pattern when enhancement occurs (approximately 80% of enhancing cases) 2

T2-Weighted Signal Characteristics:

• Hyperintense signal on T2-weighted images 1
• Oligodendrogliomas characteristically contact the brain surface (meninges) significantly more often than astrocytomas (87.5% vs 50%, p = 0.046) 2

Advanced MRI Metrics - Critical Distinction:

• ADC values do NOT predict WHO grade in oligodendrogliomas (unlike astrocytomas) 1
• rCBV does NOT predict WHO grade in oligodendrogliomas (unlike astrocytomas) 1
• However, low-grade oligodendrogliomas generally show higher baseline rCBV (median 3.68) compared to low-grade astrocytomas (median 0.92), suggesting oligodendrogliomas are inherently more vascular 5

T1-Weighted Characteristics:

• Hypointense signal on T1-weighted images 2

Associated Features:

• Cystic regions are very common (75% of cases) 2
• Hemorrhage is present in approximately 37% of cases 2
• Contact with lateral ventricle is very common (87.5% of cases) 2
• Cortical/subcortical location with meningeal contact is characteristic 2

Histologic Correlates

Grade 3 Oligodendroglioma (formerly anaplastic):

• High mitotic activity, microvascular proliferation, and frequently necrosis are the defining histologic features 6
• Characteristic "fried-egg" pattern with monomorphic rounded nuclei and delicate "chicken-wire" capillary networks 6

Grade 3 Astrocytoma (formerly anaplastic):

• Increased cellularity and mitotic activity distinguish grade 3 from grade 2 6
• Necrosis and microvascular proliferation are absent (their presence would indicate grade 4) 6

Clinical Pitfalls

Common Misinterpretation:

• Early grade 3 astrocytomas may present as T2-hyperintense lesions without enhancement and can be misinterpreted as ischemic lesions, infarction, or demyelinating processes 3
• In some cases, neoplastic lesions may not be detectable on initial MRI, even retrospectively 3

Molecular Testing Imperative:

• MRI features alone cannot reliably distinguish between astrocytoma and oligodendroglioma 2
• Oligodendroglioma diagnosis requires both IDH mutation and complete 1p/19q codeletion 7
• Isolated 1p deletion without 19q codeletion indicates astrocytoma, not oligodendroglioma 7

PET Imaging Adjunct

Amino Acid PET for Grade 3 Detection:

• Dynamic 18F-FET PET analysis provides 95% sensitivity and specificity for detecting grade 3 foci in non-contrast-enhancing gliomas 6
• Grade 3 gliomas typically show early activity peak around 10-20 minutes followed by decreasing uptake, versus steadily increasing curves in grade 2 tumors 6
• This is particularly valuable when MRI shows non-enhancing lesions suspected of harboring anaplastic foci (present in ~40% of such cases) 6