Your pathology report for pilocytic astrocytoma

by Jason Wasserman MD PhD FRCPC
December 3, 2025

Pilocytic astrocytoma is a type of brain tumor that develops from astrocytes, which are star-shaped support cells in the brain and spinal cord. This tumor is considered low grade, meaning it grows slowly and does not usually spread into surrounding brain tissue the way high-grade tumors do. Pilocytic astrocytoma most often affects children, teenagers, and young adults, and is one of the most common brain tumors diagnosed in these age groups.

These tumors frequently arise in the cerebellum, the part of the brain responsible for balance and coordination, but they can also occur in the optic nerves, brainstem, hypothalamus, spinal cord, or other locations.

What are the symptoms?

Symptoms  of pilocytic astrocytoma depend on the tumor’s location and how much it presses on nearby structures. Because pilocytic astrocytomas tend to grow slowly, symptoms may develop gradually.

Common symptoms include:

  • Headaches.

  • Nausea and vomiting.

  • Problems with balance or coordination.

  • Vision changes or vision loss (especially with optic pathway tumors).

  • Seizures.

  • Behavioral or personality changes.

Some tumors are discovered incidentally when brain imaging is performed for another reason.

What causes pilocytic astrocytoma?

For most people, pilocytic astrocytoma occurs sporadically, meaning there is no known cause. The tumor develops when certain genetic changes occur within astrocytes, causing them to grow and divide more than they should. These genetic changes are acquired rather than inherited and do not usually run in families.

Genetic changes in the MAPK pathway

Most pilocytic astrocytomas have changes in a cellular signaling system called the MAPK/ERK pathway, which helps control normal cell growth. When this pathway becomes overactive, tumor cells grow more easily. The most common genetic change involves the BRAF gene.

In many tumors, a fragment of the BRAF gene fuses with another gene called KIAA1549, creating the KIAA1549–BRAF fusion. This fusion keeps the growth pathway “turned on,” allowing the tumor to develop. Less commonly, other BRAF alterations or mutations are found.

What syndromes are associated with pilocytic astrocytoma?

Most pilocytic astrocytomas occur independently, but a small number are associated with inherited genetic conditions:

  • Neurofibromatosis type 1 (NF1). People with NF1 have a higher risk of developing pilocytic astrocytomas, especially in the optic nerves.

  • Noonan syndrome. This condition affects growth and development and is also associated with a slightly increased risk of pilocytic astrocytoma.

In most families, however, these tumors are not inherited.

How is this diagnosis made?

Imaging

Diagnosis often begins with MRI, which provides detailed images of the brain. Pilocytic astrocytomas typically appear as well-defined masses and may contain a cyst (a fluid-filled cavity) with a small, solid nodule. When they arise in the cerebellum, this cyst-nodule pattern is classic. Tumors in the optic pathway or hypothalamus often appear more solid.

MRI can strongly suggest pilocytic astrocytoma, but imaging alone cannot confirm the diagnosis.

Biopsy and surgery

Pilocytic astrocytoma is usually diagnosed after surgical removal of the tumor or, less commonly, after a biopsy when the tumor cannot be safely removed. The tissue is examined by a pathologist, who studies the microscopic appearance and performs additional tests to determine the diagnosis.

Surgery is often curative when the tumor can be removed completely.

Microscopic features

Under the microscope, pilocytic astrocytoma has a characteristic appearance:

  • Biphasic pattern. The tumor contains alternating areas of dense, fiber-rich tissue and looser, more cyst-like spaces.

  • Rosenthal fibers. These are thick, elongated, pink structures within tumor cells, representing accumulations of abnormal proteins.

  • Eosinophilic granular bodies. These appear as small, round, pink granules and are common in pilocytic astrocytoma.

The tumor cells themselves usually look less aggressive than those seen in higher-grade astrocytomas. High-grade features such as necrosis (areas of dead tumor), microvascular proliferation (abnormal blood vessel growth), and many mitotic figures (dividing cells) are typically not present. If these features are seen, either the diagnosis needs re-evaluation, or the tumor may represent a rarer, more aggressive variant.

Immunohistochemistry

Immunohistochemistry (IHC) uses antibodies linked to dyes to highlight specific proteins in tumor cells. In pilocytic astrocytoma, IHC helps confirm glial origin and distinguish it from other low-grade tumors.

Common findings include:

  • GFAP positivity, confirming that the tumor arises from glial cells.

  • OLIG2 positivity, supporting astrocytic differentiation.

  • BRAF V600E staining, which may be positive in some tumors but is less common than the KIAA1549–BRAF fusion.

Molecular tests

Molecular testing looks for genetic changes that are characteristic of pilocytic astrocytoma. These tests may include:

  • Fusion testing for KIAA1549–BRAF.

  • BRAF mutation testing.

  • NF1 gene analysis in patients with suspected hereditary syndromes.

Detecting these changes supports the diagnosis and may help guide treatment in select cases.

Integrated diagnosis

The final diagnosis combines:

  • Clinical information and imaging.

  • Microscopic appearance.

  • Immunohistochemistry results.

  • Molecular findings.

Because pilocytic astrocytoma has distinct microscopic and genetic features, integrating this information allows the pathologist to distinguish it from other pediatric low-grade gliomas.

WHO grade

The World Health Organization (WHO) assigns brain tumors a grade from 1 to 4 based on how they behave. Pilocytic astrocytoma is always WHO grade 1.

WHO grade 1 means:

  • The tumor grows slowly.

  • It rarely spreads beyond its original location.

  • It generally responds well to treatment, especially surgery.

  • Long-term survival is common.

Even though pilocytic astrocytoma is considered benign in terms of behavior, it can still cause significant symptoms depending on where it is located, which is why treatment and follow-up are important.

Can pilocytic astrocytoma recur?

Most pilocytic astrocytomas do not recur after complete surgical removal. When recurrence happens, it is usually because the tumor could not be entirely removed due to its location near critical brain structures.

For this reason, regular follow-up MRI scans are important. If the tumor returns, treatment options may include additional surgery, targeted therapy, chemotherapy, or radiation, depending on the age of the patient and tumor location.

What happens after the diagnosis?

Your healthcare team will discuss treatment options with you based on the tumor’s location, size, symptoms, and molecular features. Surgery is the primary treatment and is often curative. In tumors that cannot be completely removed, additional treatments may be recommended.

Children with pilocytic astrocytoma typically do very well, and many lead full, healthy lives after treatment. Ongoing monitoring helps ensure that any recurrence is detected early.

Questions to ask your doctor

  • Was my tumor completely removed during surgery?

  • What genetic changes (such as BRAF alterations) were identified?

  • Are additional treatments needed, or is observation appropriate?

  • How often will I need MRI scans?

  • Should my child or family be evaluated for hereditary syndromes such as NF1 or Noonan syndrome?

  • What symptoms should I watch for that might indicate tumor recurrence?

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