NeuroradiologyAstrocytoma
primary brain
tumour of astrocytic origin. Based on specific histological subtypes of proliferating astrocytes, the tumours are classified as pilocytic, fibrillary (including gemistocytic and protoplasmatic), subependymal giant cells, astrocytomas and pleomorphic xanthoastrocytomas. The varying degree of histopathological differentiation or anaplasia of the neoplastically transformed astrocytes is utilized by different grading systems to classify astrocytomas into a benignancymalignancy scale (Table I).
Astrocytoma, Table 1. Grading of malignancy for astrocytomas.
| WHO CLASSIFICATION | KERNOHAN CLASSIFICATION |
|---|
| Low grade or "benign" (grade II) | Benign (grade 1) |
| Low grade (grade 2) |
| Anaplastic (grade III) | Anaplastic (grade 3) |
| Glioblastoma multiforme (grade IV) | Glioblastoma multiforme (grade 4) |
The WHO system does not include juvenile pilocytic astrocytoma in this classification and considers this tumour as grade I.
Depending on biological behaviour, astrocytomas are divided into diffuse (or infiltrative) and localized (or noninfiltrative). Each category comprises a group of tumours graded by histological criteria, which differ in their natural history, clinical behaviour and response to therapy (Table 2).
Astrocytoma, Table 2. Classification of astrocytic brain tumours.
| Diffuse fibrillary astrocytoma | |
| astrocytoma low-grade or benign |
| anaplastic astrocytoma (protoplasmatic, gemistocytic) |
| glioblastoma multiforme |
| Localized astrocytoma | |
| pilocytic astrocytoma |
| pleomorphic xanthoastrocytoma |
| subependymal giant cell astrocytoma |
Regardless of the degree of histological malignancy, the prognosis of these neoplasms greatly depends on patient age, location and infiltrating tendency of the tumour. Anaplastic astrocytoma and glioblastoma are more frequent in adults, whereas pilocytic astrocytoma is found primarily in children. Astrocytomas are typically found in certain brain sites. In adults infiltrative astrocytomas are most often located in the cerebral hemispheres (75%). In children, infiltrative astrocytomas are typically within the brain stem, and pilocytic astrocytoma in the cerebellum (Fig.1). A cerebellar astrocytoma in an adult is more commonly infiltrative than pilocytic.
Infiltrative astrocytomas have a worse prognosis, owing to their tendency to dedifferentiate into more malignant types and to spread locally and to the leptomeninges.
The clinical presentation widely depends on the location and extension of the tumour. Focal neurological deficits or seizures, and signs of increased intracranial pressure such as increasing headache, nausea, vomiting, visual disturbances and lethargy can be the presenting symptoms.
CT scan is the first imaging procedure usually performed to screen for suspected brain tumours. MR better defines the infiltrative or localized behaviour of the mass, its extension and structure.
Pilocytic astrocytomas are round/oval sharply demarcated masses. They can be solid or cystic therefore appearing hypo-isointense on T1-weighted and hyperintense on T2-weighted images. Mural nodules and solid tumours enhance strongly. Infiltrating astrocytomas are usually ill-defined masses hypointense on T1-weighted and homogeneously hyperintense on T2-weighted, without or with mild variable contrast enhancement. More malignant astrocytomas have a more heterogeneous signal due to the presence of haemorrhage, necrosis and abnormal vessels within the tumour. Marked and irregular contrast enhancement is usually evident.
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a. MR, coronal T1-weighted image. Left cerebellar hemispheric pilocytic astrocytoma, partially cystic and with isointense nodular components.
b. MR, axial T2-weighted image. The nodular component is isointense. The tumour grows toward the cerebellopontine angle cistern. Compression of the cistern and a thin rim of cerebellar tissue anteriorly allow a diagnosis of intra-axial tumour versus extra-axial.
c. MR, coronal T1-weighted image following contrast injection. Enhancement of the nodular components.
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Astrocytoma, Fig.1 (a) | | Astrocytoma, Fig.1 (b) | | Astrocytoma, Fig.1 (c) |