Pilocytic astrocytomas, also known as juvenile pilocytic astrocytomas, are circumscribed astrocytic gliomas that typically occur in young patients. The majority of sporadic pilocytic astrocytomas arise from the cerebellum, whereas in the setting of neurofibromatosis type 1, they often involve the optic chiasm and pathway. In adults, they are typically supratentorial 11.
Pilocytic astrocytomas are considered WHO grade 1 tumors in the current WHO classification of CNS tumors and correspondingly have a relatively good prognosis.
These tumors have a range of imaging appearances, with the majority presenting as a large cystic lesion with a brightly enhancing mural nodule. Calcification can be present in around one-fifth of cases.
Optic pathway gliomas and spinal cord pilocytic astrocytomas are discussed separately. The remainder of this article focuses on a general discussion of pilocytic astrocytomas, particularly those in the cerebellum.
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Epidemiology
Pilocytic astrocytomas are tumors of young people, with 69% occurring in the first two decades of life, typically late in the first decade (9-10 years) 11. They do, however, also occasionally arise in adults 11.
Although they only account for 0.6-5.1% of all intracranial neoplasms (1.7-7% of all glial tumors), they are the most common primary brain tumor of childhood (15%) 7 and the second most common pediatric posterior fossa tumor (25-35%) after medulloblastomas 10.
Associations
There is a strong association with neurofibromatosis type 1 (NF1). NF1-associated tumors have a tendency to affect the optic nerves and chiasm (see: optic pathway glioma). The association between NF1 and pilocytic astrocytomas is so strong that up to 20% of all patients with NF1 will develop these tumors, typically in early childhood. Conversely, approximately one-third of pilocytic astrocytomas involving the optic nerves have associated NF1.
Clinical presentation
Presentation depends on location. In posterior fossa tumors, there is predominantly a mass effect with signs of raised intracranial pressure, especially when hydrocephalus is present. Bulbar symptoms or cerebellar symptoms may also be present.
Pathology
Location
By far the most common location is the cerebellum, with the optic pathway being the next most common, particularly in patients with neurofibromatosis type 1. In adults, most tumors are supratentorial 11.
The distribution within the cerebellum varies with many tumors involving both the vermis and the cerebellar hemisphere.
In general, pilocytic astrocytomas typically arise from midline structures.
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cerebellum
60%
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optic pathway (optic nerve, optic chiasm, hypothalamus, optic radiation)
25-30%, particularly common in NF1
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other less common locations
brainstem
cerebral hemispheres: more frequent in adults 11
cerebral ventricles
velum interpositum
spinal cord: see spinal pilocytic astrocytoma
Microscopic appearance
The term pilocytic refers to the elongated hair-like projections from the neoplastic cells 4. The presence of eosinophilic Rosenthal fibers is a characteristic feature and hyalinization of blood vessels is also common. The histological features are, however, fairly heterogeneous even within the one tumor, with some areas mimicking diffuse astrocytomas and even oligodendrogliomas 6.
Immunophenotype
Immunohistochemistry reflects astrocytic differentiation 6:
GFAP: positive
S100: positive
OLIG2: positive
IDH R132H mutation: negative
p53 protein: negative or weak
Genetics
Pilocytic astrocytoma, as well as pleomorphic xanthoastrocytomas, frequently have BRAF alterations (present in ~70% of cases). Importantly they, along with other pediatric low-grade gliomas, lack IDH mutations and TP53 mutations 6,7.
Radiographic features
General
Pilocytic astrocytomas range in appearance:
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large cystic component with a brightly enhancing mural nodule: 67%
non-enhancing cyst wall: 21%
enhancing cyst wall: 46%
heterogeneous, mixed solid and multiple cysts and central necrosis: 16%
completely solid: 17%
Enhancement is almost invariably present (~95%). Up to 20% may demonstrate some calcification. Hemorrhage is an uncommon complication.
MRI
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T1
solid component: iso- to hypointense compared to adjacent brain
cystic component: fluid signal unless hemorrhagic
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T1 C+ (Gd)
vivid contrast enhancement
the cyst wall enhances in ~50% of cases
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T2
solid component: hyperintense compared to adjacent brain
cystic component: high signal
T2*/GRE/SWI: signal loss if calcification or hemorrhage present
Treatment and prognosis
They are slow-growing well-circumscribed tumors with an overall good prognosis in children following treatment (5-year and 10-year survival >95%) 6. Cystic tumors have an even better prognosis while fibrillary variants tend to do worse.
Surgical resection, if complete, is usually curative. Some surgeons advocate that only the nodule needs to be resected to achieve a cure, as the cyst walls are non-neoplastic, even if enhancing 2.
Adults have a poorer prognosis, with only 53% 5-year survival 11,12.
It is also worth noting that historically some patients with neurofibromatosis type 1 and high-grade cerebellar tumors diagnosed as pilocytic astrocytomas, actually represented high-grade astrocytoma with piloid features.
History and etymology
In 1931, Harvey Cushing was the first who described this tumor, based on his studies of 76 cases of cerebellar astrocytomas 5. Pilocytic means "hair-like" and is derived from the Latin word pilus for hair.
Differential diagnosis
General imaging differential considerations include:
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high-grade astrocytoma with piloid features
particularly in adults and in patients with neurofibromatosis type 1
terrible prognosis
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usually seen in adults. In children, usually associated with von Hippel-Lindau disease
cyst wall usually does not enhance
doesn't show calcifications
smaller mural nodule with angiographic contrast blush
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typically arise from the midline (especially the vermis and roof of the fourth ventricle) rather than the cerebellar hemisphere
usually seen in younger patients (2-6 years of age)
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atypical teratoid/rhabdoid tumor (AT/RT)
larger heterogeneously enhancing mass
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tends to fill the fourth ventricle and protrude out of the foramina of Luschka and foramen of Magendie
large cystic component less common
