Protoplasmic astrocytoma (historical)
Updates to Article Attributes
Protoplasmic astrocytoma is a rarehistorical term previously applied to an uncommon variant of diffuse low-gradeadult-type 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, inIn the latestupdated 4th edition (2016) update toof the WHO classification of CNS tumours, protoplasmic astrocytomas no longer existwere removed as a distinct 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 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.
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 were composed of neoplastic astrocytes with rounded prominent nuclear contourcontours and little cytoplasm. They havehad scant processes. The tumour matrix containscontained numerous and prominent microcystic spaces filled with mucinous fluid, that presumably accounts for their T2/FLAIR appearance 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 infiltratingProtoplasmic astrocytomas appeartypically appeared 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 appearappeared of near fluid attenuation, due to the aforementioned prominent mucinous microcystic component.
MRI
These tumours havehad fairly characteristic appearances 4:
- T1: hypointense compared to white matter
- T2: strikingly hyperintense
-
FLAIR: large areas of T2 hyperintensity suppressing on FLAIR
(these- these are not macrocystic but rather represent the areas with abundant microcystic change)- and is known as T2/FLAIR mismatch sign, more recently shown to be predictive of absence of 1p19q co-deletion - 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:
Prominent involvement of cortex.Large portions of the tumour demonstrating high T2 signal which suppresses on FLAIR.
Treatment and prognosis
These tumours, along with the more common fibrillary astrocytoma, tend to be relatively indolent. Treatment depends on clinical presentation, the size of the tumour and location. In general, the options are:
observebiopsy to confirm the diagnosis and observeresectionradiotherapychemotherapy may have a role in recurrent/dedifferentiated tumours
Differential diagnosis
On MR imaging consider
-
fibrillary astrocytomaabsence of FLAIR suppressing T2 high signal components
-
dysembryoplastic neuroepithelial tumours (DNET)-
many similarities on imaging and histology4 smallermore purely cortical involvement
-
Practical points
protoplasmic astrocytomas show a prominent involvement of cortexlarge portions of the tumour usually demonstrate high T2 signal which suppresses on FLAIR
-<p><strong>Protoplasmic</strong><strong> astrocytoma</strong> is a rare variant of <a href="/articles/diffuse-astrocytoma-1">diffuse low-grade astrocytomas</a> with histological and imaging features which overlap with other entities. </p><p>Until recently, they were classified as a subtype of low-grade diffuse astrocytoma. However, in the latest (2016) update to <a href="/articles/who-classification-of-cns-tumours-1">WHO classification of CNS tumours</a>, protoplasmic astrocytomas no longer exist as a distinct entity <sup>6</sup>. </p><h4>Epidemiology</h4><p>Typically patients diagnosed with low-grade infiltrative astrocytomas are young adults (mean 32 years of age) <sup>4</sup>. A male predilection is described (M:F ~5:3) <sup>4</sup>.</p><h4>Clinical presentation</h4><p>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.</p><h4>Pathology</h4><p>Protoplasmic astrocytomas, along with other variants of diffuse low-grade astrocytomas, are considered WHO grade II tumours (see <a href="/articles/diffuse-astrocytoma-grading">grading of diffuse low-grade astrocytomas</a>).</p><p>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 <sup>3</sup>.</p><p>Mitoses, microvascular proliferation and necrosis are absent (if present they suggest a <a href="/articles/diffuse-astrocytoma-grading">high-grade tumour</a>). Like all tumours derived from astrocytes, fibrillary astrocytomas stain with glial fibrillary acidic protein (gFAP) <sup>2</sup>.</p><h4>Radiographic features</h4><p>MRI is the modality of choice for characterising these lesions. These tumours appear to have a predilection for the frontal and temporal lobes <sup>4</sup>, however, it is important to consider that frontal lobe is the largest and temporal lobe the second largest in volume. </p><h5>CT</h5><p>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.</p><h5>MRI</h5><p>These tumours have fairly characteristic appearances <sup>4</sup>:</p><ul>- +<p><strong>Protoplasmic</strong><strong> astrocytoma</strong> is a historical term previously applied to an uncommon variant of <a href="/articles/astrocytoma-idh-mutant">diffuse adult-type astrocytomas</a>. In the updated 4th edition (2016) of the <a href="/articles/who-classification-of-cns-tumours-1">WHO classification of CNS tumours</a>, protoplasmic astrocytomas were removed as a distinct entity <sup>6</sup>. </p><h4>Pathology</h4><p>Protoplasmic astrocytomas were composed of neoplastic astrocytes with rounded prominent nuclear contours and little cytoplasm. They had scant processes. The tumour matrix contained numerous and prominent microcystic spaces filled with mucinous fluid, that presumably accounts for their T2/FLAIR appearance <sup>3</sup>.</p><h4>Radiographic features</h4><h5>CT</h5><p>Protoplasmic astrocytomas typically appeared as hypodense regions of positive mass effect, usually without any enhancement. Areas of the tumour appeared of near fluid attenuation, due to the aforementioned prominent mucinous microcystic component.</p><h5>MRI</h5><p>These tumours had fairly characteristic appearances <sup>4</sup>:</p><ul>
-<strong>FLAIR:</strong> large areas of T2 hyperintensity suppressing on FLAIR (these are not macrocystic but rather represent the areas with abundant microcystic change)</li>- +<strong>FLAIR:</strong> large areas of T2 hyperintensity suppressing on FLAIR - these are not macrocystic but rather represent the areas with abundant microcystic change - and is known as <a href="/articles/t2-flair-mismatch-sign">T2/FLAIR mismatch sign</a>, more recently shown to be predictive of absence of 1p19q co-deletion</li>
-</ul><p>The key features which should prompt a protoplasmic astrocytoma being raised as the favoured diagnosis are: </p><ol>-<li>Prominent involvement of cortex.</li>-<li>Large portions of the tumour demonstrating high T2 signal which suppresses on FLAIR.</li>-</ol><h4>Treatment and prognosis</h4><p>These tumours, along with the more common <a href="/articles/fibrillary-astrocytoma">fibrillary astrocytoma</a>, tend to be relatively indolent. Treatment depends on clinical presentation, the size of the tumour and location. In general, the options are:</p><ul>-<li>observe</li>-<li>biopsy to confirm the diagnosis and observe</li>-<li>resection</li>-<li>radiotherapy</li>-<li>chemotherapy may have a role in recurrent/dedifferentiated tumours</li>-</ul><h4>Differential diagnosis</h4><p>On MR imaging consider</p><ul>-<li>-<a href="/articles/fibrillary-astrocytoma">fibrillary astrocytoma</a><ul><li>absence of FLAIR suppressing T2 high signal components</li></ul>-</li>-<li>-<a href="/articles/dysembryoplastic-neuroepithelial-tumour">dysembryoplastic neuroepithelial tumours (DNET)</a><ul>-<li>many similarities on imaging and histology <sup>4</sup>-</li>-<li>smaller</li>-<li>more purely cortical involvement</li>-</ul>-</li>-</ul><h4>Practical points</h4><ul>-<li>protoplasmic astrocytomas show a prominent involvement of cortex</li>-<li>large portions of the tumour usually demonstrate high T2 signal which suppresses on FLAIR</li>
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