Protoplasmic astrocytoma (historical)

Protoplasmic astrocytoma is a rare variant of diffuse low grade astrocytomas with histological and imaging features which are fairly characteristicoverlap with other entities. It has been suggested that protoplasmic astrocytomas represent variants of dysembryoplastic neuroepithelial tumours (DNET) as

Until recently they share histological as well as imaging features. Currently however they arewere classified as a subtype of diffuse low grade astrocytoma.

On imaging, in additionhowever, in the latest (2016) update to diffuse low grade astrocytoma features, a prominent involvementWHO classification of cortex and the presence of large portions of the tumour demonstrating high T2 signal which suppresses on FLAIR should favourCNS tumours, protoplasmic astrocytomas no longer exists as a protoplasmic subtypedistinct entity ⁶.

Epidemiology

Typically patients diagnosed with low grade infiltrative astrocytomas are young adults (mean 32 years of age) ⁴. A male predilection is described (M:F ~5:3) ⁴.

Clinical presentation

The most common presenting feature (~40% of cases) is seizure. This is particularly the case in adults. Headaches are often also present. Depending on the size of the lesion and its location other features may be present, such as hydrocephalus and focal neurological dysfunction including personality change.

Pathology

Protoplasmic astrocytomas, along with other variants of diffuse low grade astrocytomas, are considered WHO grade II tumours (see grading of diffuse low grade astrocytomas).

These tumours are composed of neoplastic astrocytes with rounded prominent nuclear contour and little cytoplasm. They have scant processes. The tumour matrix contains numerous and prominent microcystic spaces filled with mucinous fluid ³.

Mitoses, microvascular proliferation and necrosis are absent (if present they suggest a high grade tumour). Like all tumours derived from astrocytes, fibrillary astrocytomas stain with glial fibrillary acidic protein (gFAP) ².

Radiographic features

MRI is the modality of choice for characterising these lesions. These tumours appear to have a predilection for the frontal and temporal lobes ⁴, however, it is important to consider that frontal lobe is the largest and temporal lobe the second largest in volume. 

CT

Typically protoplasmic low grade infiltrating astrocytomas appear as hypodense regions of positive mass effect, usually without any enhancement (in fact presence of enhancement would suggest high grade tumours). Areas of the tumour appear of fluid attenuation, due to the aforementioned prominent mucinous microcystic component.

MRI

These tumours have fairly characteristic appearances ⁴:

• T1: hypointense compared to white matter
• T2: strikingly hyperintense
• FLAIR: large areas of T2 hyperintensity suppressing on FLAIR (these are not macrocystic but rather represent the areas with abundant microcystic change)
• T1 C+ (Gd): usually little or no enhancement
• MR spectroscopy: elevated choline:creatine ratio
• MR perfusion: there is reduced rCBV

The key features which should prompt a protoplasmic astrocytoma being raised as the favoured diagnosis are: 

1. Prominent involvement of cortex.
2. Large portions of the tumour demonstrating high T2 signal which suppresses on FLAIR.

Treatment and prognosis

These tumours, along with with the more common fibrillary astrocytoma, tend to be relatively indolent. Treatment depends on clinical presentation, size of the tumour and location. In general the options are:

• observe
• biopsy to confirm diagnosis and observe
• resection
• radiotherapy
• chemotherapy may have a role in recurrent/de-differentiated tumours

Differential diagnosis

On MR imaging consider

• fibrillary astrocytoma
  • absence of FLAIR suppressing T2 high signal components
• dysembryoplastic neuroepithelial tumours (DNET)
  • many similarities on imaging and histology ⁴
  • smaller
  • more purely cortical involvement

Practical points

• protoplasmic astrocytomas show a prominent involvement of cortex
• large portions of the tumour usually demonstrate high T2 signal which suppresses on FLAIR