Protoplasmic astrocytoma is a rare variant of diffuse low-grade astrocytomas with histological and imaging features which overlap with other entities.
Until recently, they were classified as a subtype of low-grade diffuse astrocytoma. However, in the latest (2016) update to WHO classification of CNS tumors, protoplasmic astrocytomas no longer exist as a distinct entity .
Typically patients diagnosed with low-grade infiltrative astrocytomas are young adults (mean 32 years of age) . A male predilection is described (M:F ~5:3) .
The most common presenting feature (~40% of cases) is a 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.
Protoplasmic astrocytomas, along with other variants of diffuse low-grade astrocytomas, are considered WHO grade II tumors (see grading of diffuse low-grade astrocytomas).
These tumors are composed of neoplastic astrocytes with rounded prominent nuclear contour and little cytoplasm. They have scant processes. The tumor matrix contains numerous and prominent microcystic spaces filled with mucinous fluid .
Mitoses, microvascular proliferation and necrosis are absent (if present they suggest a high-grade tumor). Like all tumors derived from astrocytes, fibrillary astrocytomas stain with glial fibrillary acidic protein (gFAP) .
MRI is the modality of choice for characterizing these lesions. These tumors appear to have a predilection for the frontal and temporal lobes , however, it is important to consider that frontal lobe is the largest and temporal lobe the second largest in volume.
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 tumors). Areas of the tumor appear of fluid attenuation, due to the aforementioned prominent mucinous microcystic component.
These tumors have fairly characteristic appearances :
- 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 favored diagnosis are:
Treatment and prognosis
These tumors, along with the more common fibrillary astrocytoma, tend to be relatively indolent. Treatment depends on clinical presentation, the size of the tumor and location. In general, the options are:
- biopsy to confirm the diagnosis and observe
- chemotherapy may have a role in recurrent/dedifferentiated tumors
On MR imaging consider
- fibrillary astrocytoma
- absence of FLAIR suppressing T2 high signal components
- dysembryoplastic neuroepithelial tumors (DNET)
- many similarities on imaging and histology
- more purely cortical involvement
- protoplasmic astrocytomas show a prominent involvement of cortex
- large portions of the tumor usually demonstrate high T2 signal which suppresses on FLAIR