Rosette-forming glioneuronal tumours

Rosette-forming glioneuronal tumours (RGNTs) are rare, usually midline, tumours that involve the fourth ventricle and/or aqueduct of Sylvius. 

Although relatively well-circumscribed on MRI and clinically indolent, they often invade surrounding tissues, involving the cerebellum, pons and even the pineal region. There are often cystic components and they tend to have heterogeneous contrast enhancement. They are considered WHO grade 1 tumours in the current (2021) WHO classification of CNS tumours. 

Epidemiology

Rosette-forming glioneuronal tumours are typically diagnosed in younger adult patients with a mean age of 30 years ^1,6. 

Clinical presentation

Symptoms are non-specific and mainly depend on the location of the tumour. Commonly they present with ataxia and headache. 

Pathology

Location

Rosette-forming glioneuronal tumours are originally described in the region of the 4th ventricle (~60%) often with variable local parenchymal extension ³. A recent literature review ³ has demonstrated these tumours outside the characteristic location including the cerebellum hemispheres, cerebellopontine angle, pineal gland, midbrain tectum, thalamus, third ventricle, optic chiasm and spinal cord.

Microscopic features

Rosette-forming glioneuronal tumours contain both neurocytic and astrocytic components. The neurocytic component, which is a minor component but allows for diagnosis of this tumour, forms perivascular pseudorosettes or neurocytic rosettes surrounding eosinophilic neuropil cores ⁶. The larger glial component resembles that of pilocytic astrocytomas, including the presence of Rosenthal fibres ⁶. 

Immunophenotype

Immunohistochemistry is consistent with the biphasic nature of this tumour with both glial and neurocytic patterns identified.

The glial component demonstrates: 

• GFAP: positive
• S100: positive

The neurocytic component demonstrates: 

• MAP2: positive
• neurone-specific enolase: positive
• synaptophysin: positive in the centre of the neurocytic rosettes

Proliferation index (Ki-67) is usually low, <3% ⁶. 

Radiographic features

Rosette-forming glioneuronal tumours have variable solid-cystic components ³:

• entirely solid: 40%
• mixed solid and cystic: 35%
• entirely cystic: 25%  

CT

On CT these tumours are of variable appearance reflecting the solid and cystic components. Calcification is seen in approximately a quarter of cases ³. 

MRI

As expected, the cystic components demonstrate high T2 signal, whereas the solid components demonstrate variable gadolinium enhancement ³. 

Treatment and prognosis

If complete resection is achievable, which depends mostly on the location of the tumour, then a cure is possible to surgery alone although due to their location neurological deficits following resection are common ⁶. 

Differential diagnosis

The main differential diagnosis is pilocytic astrocytoma. The other differential diagnoses will, to a degree, depend on the location and include: 

• pilocytic astrocytoma
• gangliogliomas and gangliocytomas
• pleomorphic xanthoastrocytoma (almost always hemispheric)
• papillary glioneuronal tumour (usually hemispheric)