Posterior Fossa Ependymoma: Understanding Your Pathology Report

By Jason Wasserman MD PhD FRCPC and Anne F Buckley MD
September 10, 2026

A posterior fossa ependymoma is a brain tumor that grows from ependymal cells, which normally line the fluid-filled spaces inside the brain and spinal cord. The posterior fossa is the lower back part of the skull. It contains the cerebellum, which controls balance, and the brainstem, which connects the brain to the spinal cord.

These tumors usually arise in and around a space called the fourth ventricle. They are typically well defined, but they can still press on nearby brain structures or block the normal flow of cerebrospinal fluid. That blockage causes pressure to build up inside the skull. This article explains what appears on a posterior fossa ependymoma pathology report, and what separates group A from group B. It also explains why the chromosome results now carry more weight than the grade. For an overview of the whole family, see our article on ependymoma.

What are the symptoms of a posterior fossa ependymoma?

The symptoms of a posterior fossa ependymoma depend on the tumor’s size and location, and on whether it blocks the flow of cerebrospinal fluid. Most come from the resulting build-up of pressure inside the skull rather than from the tumor itself.

Headaches. Often worse in the morning or when lying down.
Nausea and vomiting. Caused by the raised pressure.
Loss of balance, dizziness, or clumsiness. From pressure on the cerebellum.
Double vision or unusual eye movements.
Neck stiffness or a head tilt.
In babies and young children. Irritability, tiredness, poor feeding, and a head growing faster than expected.

Adults tend to notice gradual headaches or balance problems, while a baby whose skull can still expand may present with a rapidly enlarging head from fluid build-up, called hydrocephalus.

What causes a posterior fossa ependymoma?

For most people, the cause of a posterior fossa ependymoma is unknown, and the tumor is not inherited. It is not linked to diet, activity, injury, or anything in the home environment.

Research has shown that these tumors are driven mainly by epigenetic changes, meaning changes in how genes are switched on and off rather than changes to the genes themselves. That is unusual among cancers, and it is why posterior fossa ependymomas are now sorted into molecular groups that predict behavior better than microscopic appearance ever did.

How is the diagnosis made?

Diagnosing a posterior fossa ependymoma requires a combination of tests, and the modern report brings them together into an integrated diagnosis.

MRI shows the tumor’s location, size, and effect on the surrounding structures. Surgery follows, and a pathologist examines the tissue under a microscope. Immunohistochemistry and molecular tests then determine the group and identify additional changes that affect the outlook.

Because ependymomas can spread through the cerebrospinal fluid, doctors may order an MRI of the whole spine and sometimes take a fluid sample as part of the initial assessment. This is standard for building the full picture, not a sign that spread is expected.

What does a posterior fossa ependymoma look like under the microscope?

Under the microscope, a posterior fossa ependymoma is made of small, round cells that often arrange themselves around blood vessels. When the tumor cells form a ring around a vessel, with a clear zone of fine fibers between them, it is called a perivascular pseudorosette. A true ependymal rosette is a ring of cells around a small space, recreating the channel that ependymal cells normally line.

The pathologist also comments on mitotic activity, which describes how often the cells are dividing. Some tumors show microvascular proliferation, meaning tiny new blood vessels forming to feed the growing mass, and some show necrosis, meaning dead tumor tissue. These features, along with how crowded the cells look, are used to assign the CNS WHO grade.

Immunohistochemistry

Immunohistochemistry uses antibodies to highlight specific proteins inside the tumor cells. In a posterior fossa ependymoma, these patterns confirm the tumor type and help determine its molecular group.

Ependymomas typically produce GFAP, a marker of glial origin, and EMA, which often shows a distinctive dot-like or ring-shaped pattern inside the cells.

One stain matters more than the others here. H3K27me3, also written H3p. K28me3, looks for a specific chemical tag on a histone protein inside the cell nucleus.

Loss of staining. The tumor cells no longer carry the tag. This supports posterior fossa group A.
Retained staining. The tag is still present. This makes posterior fossa group B more likely.

Normal brain and blood vessel cells in the same sample retain the staining and act as an internal control, confirming that the test worked properly. Your report may mention this.

Molecular tests

Molecular tests provide deeper information about the DNA of a posterior fossa ependymoma and are essential for accurate classification. Your report will usually list which tests were performed, the results, and what those results mean.

DNA methylation profiling. Reads thousands of chemical marks on DNA to identify a characteristic pattern. Each tumor type has its own pattern, allowing classification as group A, group B, or occasionally another ependymoma type. This is considered the most reliable method for confirming the group.
Copy-number analysis. Looks for missing or extra chromosome segments. Gain of chromosome 1q and loss of chromosome 6q are the two findings that matter most, and they are covered in their own section below. Loss of chromosome 22q may also be reported.
Histone and EZHIP testing. Many group A tumors overproduce a protein called EZHIP, which mimics a mutation known as H3 K27M. Both lead to loss of the normal H3 K27me3 mark. Rarely, an actual H3 K27M mutation is found instead, in roughly 1 in 30 group A tumors.

Group A and group B

Posterior fossa ependymomas are separated into groups because their biology, typical patient age, and expected behavior differ substantially. The group is the most useful single piece of information on the report.

Posterior fossa group A, written PFA. Occurs mostly in infants and young children. These tumors usually show loss of the H3K27me3 marker and are classified as group A by methylation profiling. They return after treatment more often than group B tumors, particularly when complete removal was not possible or when a chromosome 1q or 6q change is present.
Posterior fossa group B, written PFB. More common in adolescents and adults. These tumors retain H3K27me3 staining and are classified as group B by methylation. They behave more favorably than group A tumors, and long-term control is often achieved.
NOS or NEC. Sometimes a tumor has the microscopic appearance of a posterior fossa ependymoma, but the group cannot be confirmed. NOS means molecular testing could not be performed, for example because there was too little tissue. NEC means testing was done, but the pattern did not fit a recognized group. The diagnosis of posterior fossa ependymoma still stands, and care is guided by imaging, the extent of removal, and any available chromosome results.

Research has divided group A further into subgroups labeled PFA-1 and PFA-2, with several subtypes beneath them. These are not yet used to guide treatment, and you may see them mentioned without any change to your plan.

Chromosome 1q and 6q

Two chromosome findings now carry more weight in posterior fossa ependymoma than almost anything else on the report, and both apply to group A tumors. Your report may list them under copy-number analysis.

Gain of chromosome 1q means the tumor cells carry extra copies of part of chromosome 1. Loss of chromosome 6q means part of chromosome 6 is missing. Each is associated with a higher chance of tumor recurrence, and together they identify the highest-risk group.

One study followed 663 group A tumors. The proportion still free of progression at five years was roughly 50 percent when both chromosomes were balanced, and roughly 32 percent with 1q gain alone. It was lower again with 6q loss. The lowest figures were in the small group with both changes. Loss of 6q is found in roughly 1 in 11 group A tumors and predicts outcome independently of 1q status.

These findings also change where a tumor tends to return. Tumors with 1q gain are more likely to come back at a distance from the original site rather than in the same place. This is one reason imaging of the whole brain and spine continues during follow-up.

What does the WHO grade mean?

All posterior fossa ependymomas are given a CNS WHO grade, usually grade 2 or grade 3. The system used is the World Health Organization classification of central nervous system tumors, 5th edition, published in 2021. The grade is based on microscopic features such as mitotic activity and microvascular proliferation.

Your report lists the grade because it remains part of the official classification. Research has repeatedly shown that grade alone does not reliably predict how a posterior fossa ependymoma will behave. The molecular group, extent of removal, and chromosome 1q and 6q results are stronger predictors. Modern reports therefore combine these into an integrated diagnosis rather than relying on grade alone.

How much of the tumor was removed

How completely a posterior fossa ependymoma was removed is the strongest factor that treatment can influence, and it is weighed alongside the molecular findings.

Brain tumors are not reported with inked surgical margins in the way tumors elsewhere in the body are. Instead, the neurosurgeon describes what was achieved, and an MRI within a day or two afterward confirms it. Gross total resection means no visible tumor remains. Subtotal or partial resection means some remains.

Complete removal is harder in this location than in many others. The tumor can wrap around the brainstem and the nerves emerging from it, so taking every last piece risks lasting harm. Where tumor remains, a second operation is often considered, sometimes after other treatment. This is a planned part of care, not a sign that the first operation failed.

What is the prognosis?

Outcomes in posterior fossa ependymoma depend on the molecular group, how completely the tumor was removed, the chromosome findings, and age. The range is wide, so a single figure for the whole group would be misleading.

Group B. The more favorable group. Most patients achieve long-term control, particularly after complete removal.
Group A. Less favorable. In a large international study, roughly 56 percent of patients with group A tumors were alive 10 years after diagnosis.
Group A with balanced chromosomes and complete removal. Considerably better. In one clinical trial, survival in this situation was reported at roughly 81 percent.
Group A with 1q gain or 6q loss. The highest-risk situation, with a higher chance of the tumor returning and of it returning at a distance from the original site.

Across all intracranial ependymomas, reported 10-year survival ranges from roughly 50 to 75 percent, reflecting how much these factors matter. Complete removal followed by radiation gives the best reported outcomes in both groups.

These figures come from groups of patients treated over many years and cannot predict what will happen to one person. Your neuro-oncology team can give numbers that match your group, chromosome results, and the extent of removal.

What happens after the diagnosis?

A team plans care for a posterior fossa ependymoma, including a neurosurgeon, a neuro-oncologist, a radiation oncologist, and a neuropathologist. Because the molecular results are central to the diagnosis, the complete report can take longer than families expect, and some decisions wait for it.

Doctors address hydrocephalus first, or at the same time as the tumor. This may mean a temporary drain or a shunt, a thin tube that carries excess fluid away from the brain. In many patients, the fluid build-up settles once the tumor is removed.

Surgery is the mainstay of treatment, and removing as much tumor as can safely be taken is the goal. Radiation aimed at the tumor bed follows for most patients, including young children, because the risk of the tumor returning without it is high. Chemotherapy has a more limited role and is used mainly in infants, where it can delay radiation until the brain is more developed.

Follow-up MRI continues for many years, because these tumors can return late. It covers the whole brain and spine, not just the original site. Long-term follow-up also monitors treatment effects on learning, hormone function, hearing, and balance.

Questions to ask your doctor

Does my report say group A, group B, or NOS or NEC, and how was that determined?
What did the H3 K27me3 stain show?
Was DNA methylation profiling performed, and did it confirm the group?
Was chromosome 1q gain or 6q loss found?
If those results aren’t in my report, can that testing still be done?
What CNS WHO grade is listed, and how does it relate to the molecular findings?
Was the whole tumor removed, and was an MRI done afterward to confirm?
If tumor remains, is a second operation being considered?
Was the rest of my spine imaged, and was the cerebrospinal fluid examined?
Is radiation recommended, and how was age weighed in that decision?
Would chemotherapy be used, and if so, for what? what
How often will I need follow-up MRI, and for how many years?
Has a pathologist who specializes in nervous system tumors reviewed the slides?
Is treatment being given on a clinical trial or a standard protocol?