Pilocytic astrocytoma

by Jason Wasserman MD PhD FRCPC
August 6, 2024

This article was written by pathologists for patients. It aims to help patients understand their pathology report for pilocytic astrocytoma. It includes sections describing the symptoms, causes, genetic changes, associated syndromes, WHO grade, diagnosis, and microscopic features of this tumour.

What is pilocytic astrocytoma?

Pilocytic astrocytoma is a type of brain tumor originating from star-shaped brain cells called astrocytes. These tumors are typically slow-growing and are considered low-grade, meaning they are less aggressive and less likely to spread compared to high-grade brain tumors. Pilocytic astrocytomas are most commonly found in children and young adults and often occur in areas of the brain, like the cerebellum, which is responsible for coordination and balance.

What are the symptoms of pilocytic astrocytoma?

The symptoms of pilocytic astrocytoma can vary depending on the tumor’s location in the brain, but they may include:

• Headaches.
• Nausea and vomiting.
• Balance and coordination problems.
• Vision problems.
• Seizures.
• Behavioral changes.

What causes pilocytic astrocytoma?

The exact cause of pilocytic astrocytoma is not fully understood. It is believed to arise from random genetic mutations. These mutations cause the astrocytes to grow and divide uncontrollably, forming a tumour. In most cases, pilocytic astrocytomas are not inherited and do not run in families.

What genetic changes are found in pilocytic astrocytoma?

Pilocytic astrocytomas often have genetic changes in the MAPK/ERK pathway, which controls cell growth and division. The most common genetic change in these tumours involves a gene called BRAF. In many pilocytic astrocytomas, the BRAF gene is altered so that it is always active, leading to uncontrolled cell growth. This alteration can occur through a specific fusion between the BRAF gene and another gene called KIAA1549.

What syndromes are associated with pilocytic astrocytoma?

Pilocytic astrocytoma can sometimes be associated with genetic conditions, including:

• Neurofibromatosis type 1 (NF1): A hereditary disorder that causes tumours to form in the brain, spinal cord, or from nerve tissue. People with NF1 have an increased risk of developing pilocytic astrocytomas, especially in the optic nerve, which is responsible for vision.
• Noonan syndrome: A genetic disorder that can cause various physical and developmental issues. People with Noonan syndrome have an increased risk of developing various types of tumours, including pilocytic astrocytomas.

How is the diagnosis of pilocytic astrocytoma made?

The diagnosis of pilocytic astrocytoma is made through a combination of imaging studies and biopsy. Imaging studies such as MRI (Magnetic Resonance Imaging) can reveal the presence and location of the tumour in the brain. A biopsy, which involves taking a small sample of the tumor tissue, is then performed to confirm the diagnosis. The biopsy sample is examined under a microscope by a pathologist, who looks for specific characteristics of pilocytic astrocytoma.

WHO grade

The World Health Organization (WHO) grade is a system used to classify the aggressiveness of brain tumours. Tumors are graded on a scale from I to IV, with grade I being the least aggressive and grade IV being the most aggressive. All pilocytic astrocytomas are classified as WHO grade I. This means they are considered low-grade tumours, which are slow-growing and less likely to spread to other parts of the brain or body. Being WHO grade I also indicates that these tumours generally have a better prognosis and are more manageable with treatment compared to higher-grade tumours.

What are the typical microscopic features of pilocytic astrocytoma?

When a pathologist examines a biopsy from a pilocytic astrocytoma, they look for specific features under the microscope, including:

• Biphasic pattern: The tumour often has two different types of tissue patterns – one that is dense and another that is loose and cyst-like.
• Rosenthal fibers: These are thick, elongated protein structures found within the tumour cells.
• Eosinophilic granular bodies: These are small, round, pink-staining structures seen within the tumour.

It is important to note that high-grade features, which indicate a more aggressive tumour, are typically not seen in pilocytic astrocytoma. These high-grade features include:

• Necrosis: Areas of dead tissue within the tumor.
• Increased mitotic activity: A high number of dividing cells, indicating rapid growth.
• Microvascular proliferation: An increased number of small blood vessels within the tumor.

Can pilocytic astrocytoma recur after treatment?

Yes, pilocytic astrocytoma can recur after treatment, although this is relatively uncommon. Recurrence may happen if the tumour is not completely removed during surgery. Regular follow-up with imaging studies, such as MRI, is essential to monitor for any signs of recurrence. If the tumour does recur, additional treatments, such as surgery, radiation therapy, or chemotherapy, may be needed to manage the condition.

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