What is the recommended diagnostic approach for glioblastoma?

How to Diagnose Glioblastoma

The diagnosis of glioblastoma requires MRI of the brain with and without gadolinium-based contrast agent, followed by tissue acquisition (surgical resection or stereotactic biopsy) for histopathological and molecular analysis according to the WHO Classification of CNS Tumors. 1

Initial Imaging

MRI brain without and with gadolinium contrast is the diagnostic gold standard for detecting suspected glioblastoma 1, 2. This imaging modality provides superior spatial resolution and tissue contrast compared to CT, allowing accurate delineation of tumor extent, vascular involvement, and mass effect.

Key MRI features suggestive of glioblastoma include:

• Irregular ring-enhancing lesion with central necrosis
• Surrounding vasogenic edema on T2/FLAIR sequences
• Mass effect with midline shift
• Infiltrative margins crossing white matter tracts

Advanced MRI sequences such as perfusion imaging and MR spectroscopy may aid in pretreatment differential diagnosis and surgical planning 2. Amino acid PET (fluciclovine) is emerging as a useful adjunct for diagnosis and treatment planning 2.

Tissue Acquisition: The Definitive Diagnostic Step

Clinical decision-making without obtaining tissue diagnosis should only occur in exceptional circumstances - specifically when biopsy risk is prohibitively high or prognosis is extremely poor (e.g., patients with severe comorbidities, large typical-appearing lesions, and rapid neurological deterioration) 1. Even in palliative situations, histological diagnosis aids patient and caregiver counseling 1.

Surgical Options

Microsurgical resection is preferred when safely feasible, serving both diagnostic and therapeutic purposes 1. The surgery should be performed at high-volume specialist centers with experienced neurosurgeons 1.

When resection is not safely feasible (due to tumor location or patient condition), stereotactic biopsy must be performed 1. Both frame-based and frameless techniques provide high diagnostic accuracy with low morbidity. Critical technical points include:

• Obtain serial samples along the biopsy trajectory to avoid sampling bias 1
• Sample different tumor areas for markers without proven homogeneity 1
• Consider 5-aminolevulinic acid fluorescence guidance to ensure adequate tumor sampling 1

Molecular and Histopathological Analysis

The tissue diagnosis must follow an integrated histomolecular classification combining morphology with molecular markers 1. This is not optional - it defines the disease entity and guides treatment decisions.

Essential Molecular Testing Algorithm

Step 1: IDH Status Assessment

• Perform immunohistochemistry for IDH1 R132H mutant protein on all specimens 1
• Exception: In patients >55 years with histologically typical glioblastoma, no pre-existing lower-grade glioma, non-midline location, and retained nuclear ATRX, IDH1 R132H immunonegativity suffices for IDH-wild-type classification 1
• If IDH1 R132H immunohistochemistry is negative in all other cases: sequence IDH1 codon 132 and IDH2 codon 172 to detect non-canonical mutations 1

Step 2: Confirm Glioblastoma Diagnosis in IDH-Wild-Type Tumors

For IDH-wild-type diffuse astrocytic gliomas:

• If microvascular proliferation or necrosis present: Diagnosis is IDH-wild-type glioblastoma, WHO grade 4 1
• If microvascular proliferation and necrosis absent: Test for molecular features of glioblastoma 1:
  • TERT promoter mutation
  • EGFR amplification
  • Combined chromosome 7 gain/chromosome 10 loss (+7/-10 signature)
  • If any of these present: Classify as IDH-wild-type glioblastoma despite lacking necrosis/microvascular proliferation 1

Step 3: Additional Molecular Markers

• ATRX immunohistochemistry: Assess nuclear expression routinely 1
• H3 K27M status: Must be assessed in all diffuse gliomas involving midline structures (thalamus, brainstem, spinal cord) 1
• H3.3 G34 mutations: Should be evaluated in young patients (<50 years) with IDH-wild-type gliomas, particularly with nuclear ATRX loss 1

Step 4: MGMT Promoter Methylation

MGMT promoter methylation status should be determined in all glioblastomas 1, particularly in elderly or frail patients, as it predicts benefit from temozolomide chemotherapy 1. Use methylation-specific PCR, pyrosequencing, or methylation arrays (MGMT-STP27 model) - never immunocytochemistry 1.

Critical Pitfalls to Avoid

Sampling error: IDH mutations, 1p/19q codeletion, and MGMT promoter methylation are homogeneously distributed, so sampling error risk is low 1. However, for other markers without proven homogeneity, multiple tumor areas must be sampled 1.

Infratentorial tumors: Brainstem and cerebellar diffuse astrocytomas have ~80% frequency of non-canonical IDH mutations versus <10% in supratentorial tumors 1. Do not rely on IDH1 R132H immunohistochemistry alone - DNA sequencing is mandatory 1.

Molecular testing delays: Molecular diagnostic algorithms must be standardized and should not delay radiotherapy or tumor-specific pharmacotherapy 1. Arrange for rapid turnaround molecular testing protocols.

Pseudoprogression: Consider this diagnosis when imaging abnormalities increase within 3-6 months after radiotherapy completion, especially if the patient is clinically stable 1. This is a common pitfall in post-treatment surveillance that can mimic tumor progression.

Preoperative Management

Before surgery, multidisciplinary tumor board discussion is essential, including neuroradiologists, neuropathologists, neurosurgeons, radiation oncologists, and neuro-oncologists 1.

Corticosteroids can be administered to decrease symptomatic tumor-associated edema, unless primary CNS lymphoma or inflammatory lesions are suspected 1.

Anticonvulsant drugs should be given to patients with prior seizures. Primary prophylaxis does not reduce first seizure risk and is not recommended 1.