Molecular classification and IDH mutations
Modern diagnostics of glial tumors rely on genetic markers. A key event is the mutation in the isocitrate dehydrogenase (IDH) gene, which differentiates prognostically favorable gliomas from aggressive ones.
- Tumors with the mutation (IDH-mutant) produce the oncometabolite D-2-hydroxyglutarate (2-HG), but paradoxically have a better prognosis and higher survival rates.
- Wild-type tumors (IDH wild-type), which account for 95% of glioblastomas, follow an extremely aggressive course. A diagnosis of wild-type glioma is established in the presence of EGFR overexpression or a TERT mutation.
Among IDH-mutant gliomas, oligodendrogliomas are distinguished, which obligatorily feature 1p19q codeletion (deletion of the short arm of chromosome 1 and the long arm of chromosome 19). These are tumors with the most favorable prognosis (survival of about 8 years). If the codeletion is absent (intact), diffuse astrocytoma is diagnosed (frequently accompanied by loss of ATRX expression).
Glioblastoma: MRI features
Glioblastoma (WHO Grade IV) is the most aggressive tumor. Typical neuroimaging features on MRI include:
- A space-occupying lesion with central necrosis.
- Intense, frequently nodular, ring-like contrast enhancement (gadolinium) along the periphery.
- An extensive zone of hyperintense signal on T2/FLAIR surrounding the tumor — vasogenic edema. This edema is infiltrated by tumor cells not visible on standard scans.
Standard treatment includes surgical resection and chemoradiotherapy. The presence of MGMT gene promoter methylation improves the response to temozolomide therapy. It is important to perform a postoperative MRI within the first 48 hours, before reactive enhancement develops along the resection margins.
Differential diagnosis and special MRI techniques
Standard structural MRI is supplemented by advanced physiological techniques to assess malignancy grade and differentiate post-treatment conditions:
- Perfusion-weighted MRI (PWI): evaluates neovascularization. The primary marker is rCBV (relative cerebral blood volume). It allows differentiation between low-grade and high-grade gliomas (threshold 1.75).
- Diffusion-weighted MRI (DWI): evaluates cellularity. Wild-type gliomas exhibit the lowest diffusivity (signal drop on ADC maps).
- Magnetic resonance spectroscopy (MRS): detects an elevated choline/NAA ratio (tumor sign), as well as lipid peaks (necrosis) and lactate peaks (hypoxia) in malignant forms.
A specific sign for IDH-mutant astrocytomas without 1p19q codeletion is the FLAIR-mismatch sign (FLAIR-mismatch sign) — signal suppression on the FLAIR sequence within a bright T2-weighted solid tumor.
Specific types of astrocytomas
In addition to typical diffuse gliomas, special forms exist:
- Pilocytic astrocytoma (Grade I): a non-infiltrative, frequently cystic tumor with an enhancing nodule. Typical for the posterior cranial fossa in children (IDH mutations are absent).
- Pleomorphic xanthoastrocytoma (Grade II): a rare lipid-containing tumor, frequently in the temporal lobe. Enhances with contrast, may mimic a "dural tail".
- H3 K27M-mutant midline glioma: an extremely aggressive tumor (prognosis < 1 year) located strictly in the midline in children and adolescents (previously some of these were termed DIPG).
Post-treatment complications
When monitoring gliomas on MRI, it is difficult to differentiate true tumor progression from treatment effects (RANO criteria are used):
| Condition | Onset timing | Features |
|---|---|---|
| Pseudoprogression | Up to 12 weeks | Transient inflammation with contrast enhancement. Frequently seen with MGMT methylation. |
| Radiation necrosis | From 6 months | Irreversible damage, resembles recurrence. |
| Pseudoresponse | Hours to days | False improvement during anti-angiogenic therapy due to rapid restoration of the BBB. |
Briefly: answers to topical questions
What are the WHO grades of astrocytoma malignancy (Grade I-IV)?
According to the WHO classification, the following malignancy grades of astrocytic tumors are distinguished:
- Pilocytic astrocytoma — WHO Grade I, characterized as a non-infiltrative neoplasm.
- Isocitrate dehydrogenase-mutant astrocytoma (IDH_{mut}1p19q_{int}) — WHO Grade II and III.
- Pleomorphic xanthoastrocytoma — typically WHO Grade II; anaplastic pleomorphic xanthoastrocytoma corresponds to WHO Grade III.
- Isocitrate dehydrogenase wild-type astrocytoma (IDH wild-type) — WHO II–III.
- Glioblastoma — WHO IV; it can be primary or secondary.
What hemodynamic parameters are evaluated in perfusion-weighted MRI (PWI) of brain tumors?
Perfusion-weighted MRI of brain tumors evaluates relative cerebral blood volume.
- Relative cerebral blood volume (rCBV) is used in characterizing neoplasms.
For example, low-grade oligodendrogliomas frequently demonstrate moderately elevated rCBV, which may overlap with features of wild-type tumors (IDH_{wt}). In DSC perfusion, rCBV helps differentiate infratentorial pilocytic astrocytomas from hemangioblastomas: it is higher in hemangioblastomas.
Class and exam questions
What does IDH wild-type mean and why is it a poor prognostic factor?
IDH wild-type means the absence of a mutation in the isocitrate dehydrogenase gene. Gliomas with this profile, even if they appear less malignant histologically, behave extremely aggressively and are considered an early stage of glioblastoma.
Which marker determines the best prognosis in gliomas?
The presence of the 1p19q codeletion combined with an IDH mutation. This is the profile of oligodendroglioma, which is most sensitive to treatment and has the highest survival rate among gliomas.
What is the essence of the FLAIR-mismatch sign?
This is an MRI sign where a solid tumor, which appears very bright (hyperintense) on T2-weighted images, exhibits an area of signal drop on the FLAIR sequence. It is highly specific for IDH-mutant astrocytomas without 1p19q codeletion.
What else to study on this topic
- Detailed analysis of RANO criteria in evaluating treatment efficacy.
- Features of perfusion MRI (DSC and rCBV) and mathematical post-processing.
- Diffusion tensor imaging (DTI) and evaluation of tumor invasiveness.
- Analysis of MR spectroscopy and screening for the 2-hydroxyglutarate (2-HG) peak.
- Specifics of the differential diagnosis between pilocytic astrocytoma and hemangioblastoma.
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