25 Questions
What is the typical appearance of a pilocytic astrocytoma on T1 imaging?
Isointense to hypointense
What is the average annual age-adjusted incidence rate of pilocytic astrocytoma per 100,000 population?
0.91 cases
What percentage of all childhood primary brain tumours are pilocytic astrocytomas?
17.6%
What is the characteristic of optic nerve tumours associated with neurofibromatosis type 1 (NF1)?
They rarely extend beyond the optic pathway
What is the percentage of all primary brain tumours that are pilocytic astrocytomas?
5%
What is a characteristic of the KIAA1549::BRAF fusion?
It is common in cerebellar tumours
What is a common feature of pilocytic astrocytomas?
Low to moderate cellularity
In which type of tumours are BRAF p.V600E mutations more common?
Supratentorial tumours
What is a characteristic of optic nerve tumours?
They often circumferentially infiltrate the optic sheath
What is a characteristic of the nuclei of neoplastic cells in pilocytic astrocytomas?
They are round to elongate
What is a characteristic feature of pilocytic astrocytomas in some cases?
Rare mitotic figures
What is a common feature of pilocytic astrocytomas?
Myxoid background with microcystic change
What is a histological pattern seen in pilocytic astrocytomas?
Biphasic pattern
What is a rare feature of pilocytic astrocytomas?
Palisading necrosis
What is associated with the oligodendrocyte-like pattern in pilocytic astrocytomas?
FGFR1 alterations
What is the primary mechanism behind the activation of NTRK fusion proteins?
Constitutive dimerization of the NTRK fusion proteins
Which of the following genes is typically fused with BRAF in pilocytic astrocytomas?
KIAA1549
What type of mutations are commonly seen in the FGFR1 gene in pilocytic astrocytomas?
Hotspot point mutations
Which of the following chromosomes are reportedly involved in polysomies in pilocytic astrocytomas?
Chromosomes 5, 6, 7, 11, and 15
What is the effect of the internal tandem duplication of the kinase domain of FGFR1?
Constitutive activation of the FGFR1 protein
What is the most common age range for pilocytic astrocytomas with anaplasia?
Adults with a mean age of 32 years
What is the percentage of pilocytic astrocytomas with anaplasia that show loss of nuclear ATRX expression?
57%
What is the term used to describe the distinct methylome signature found in 81% of pilocytic astrocytomas with anaplasia?
DNA methylation class anaplastic astrocytoma with piloid features
What is the percentage of cerebellar glioblastomas that show a similar DNA methylation profile to pilocytic astrocytomas with anaplasia?
36%
What is the current understanding of the relationship between pilocytic astrocytomas with histological anaplasia and high-grade astrocytoma with piloid features?
They overlap partially, but their relationship remains to be elucidated
Study Notes
Imaging Characteristics
- Pilocytic astrocytomas can appear as a cyst-like mass with a central non-enhancing zone or as a predominantly solid mass.
- The cyst wall enhancement is variable, and enhancement does not necessarily indicate tumour involvement.
- Calcification may be present.
- Pilocytic astrocytomas are often contrast enhancing, with the solid tumour component typically being isointense to hypointense on T1 imaging and hyperintense on T2.
Epidemiology
- Pilocytic astrocytoma accounts for 5% of all primary brain tumours.
- It is most common during the first two decades of life, with an average annual age-adjusted incidence rate of 0.91 cases per 100,000 population.
- Pilocytic astrocytoma accounts for 17.6% of all childhood primary brain tumours and is the most common glioma in children.
- The incidence rate is highest in young children and decreases with advancing age.
- Pilocytic astrocytoma is rare in older adults.
Genetics
- Alterations include NF1 mutations, BRAF p.V600E mutations, BRAF fusions, BRAF insertions, KRAS mutations, FGFR1 mutations or fusions, and rare NTRK family receptor tyrosine kinase fusions.
- NTRK genes fuse with several different 5′ partners that contain a dimerization domain, leading to constitutive dimerization of the NTRK fusion proteins and activation of the kinase.
- FGFR1 alterations seen in pilocytic astrocytomas overlap with those seen in other paediatric low-grade glial and glioneuronal tumours.
Macroscopic Appearance
- Most pilocytic astrocytomas are soft, grey, and relatively discrete.
- Intratumoural or paratumoural cyst formation, including mural tumour nodules, is common.
- Chronic lesions may be calcified.
- Spinal tumours may be associated with syrinx formation.
- Optic nerve tumours often circumferentially infiltrate the optic sheath.
Histopathology
- Pilocytic astrocytomas have low to moderate cellularity.
- Neoplastic cells range widely in their morphology and include varying proportions of piloid and oligodendrocyte-like cells.
- Nuclei are round to elongate.
- Multinucleated cells with horseshoe-shaped nuclear clusters (pennies-on-a-plate pattern) are often seen.
- In some cases, hyperchromasia and pleomorphism are obvious, but mitotic figures are rare.
Learn about the characteristics of pilocytic astrocytomas, including appearance, enhancement, and calcification. Test your knowledge of these brain tumors.
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