Medulloblastoma: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
April 24, 2026

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Medulloblastoma is a type of cancer that starts in the cerebellum, the part of the brain at the back of the head that controls balance, coordination, and fine movement. It is an embryonal tumor, which means it develops from immature cells that normally give rise to the nervous system during early development. Medulloblastoma is one of the most common cancerous brain tumors in children, although it can also occur in adults.

Medulloblastomas grow quickly and can spread through the cerebrospinal fluid (the clear fluid that surrounds the brain and spinal cord) to other parts of the central nervous system. Because of this, a full assessment of the brain and spine is always part of the workup after the tumor is first discovered.

Medulloblastoma is not a single disease. It is a group of related tumors that differ in the patient’s age, where the tumor arises in the cerebellum, how the cells look under the microscope, and — most importantly — the genetic changes driving the tumor’s growth. These differences matter because they help predict how the tumor will behave and what treatment is most likely to work.

This article will help you understand the findings in your pathology report — what each term means and why it matters for your care or the care of your child.

What are the symptoms of medulloblastoma?

The symptoms of medulloblastoma develop because the tumor grows inside the back of the skull, in a space called the posterior fossa. As the tumor enlarges, it blocks the normal flow of cerebrospinal fluid and puts pressure on nearby parts of the brain.

The most common early symptoms include headache, nausea, and vomiting, often worse in the morning. These symptoms are caused by a build-up of fluid and pressure inside the skull, a condition called hydrocephalus.

Because the tumor arises in the cerebellum, many patients also develop balance and coordination problems. Children may walk unsteadily, fall more often, or have difficulty with tasks that require fine movements. Older children and adults may notice clumsiness, double vision, or difficulty with eye movements.

Other symptoms can include irritability, fatigue, changes in personality, or unusual head tilting. In infants, the soft spot at the top of the head may bulge, and the head may grow faster than expected.

Because the cerebrospinal fluid carries tumor cells, some patients have back pain, leg weakness, or problems with bladder or bowel control if the tumor has spread down the spinal cord.

What causes medulloblastoma?

Medulloblastoma is caused by genetic changes in cells in the developing cerebellum. These changes affect genes that normally control how cells grow, divide, and mature. As a result, the cells begin to grow uncontrollably and fail to develop into normal brain tissue.

For most patients, the genetic changes occur by chance during development. There is no specific environmental cause, and nothing the parents did or did not do caused the tumor to form.

However, in a small but important group of patients, medulloblastoma develops in the setting of an inherited condition. These inherited conditions are caused by a genetic change that is present in every cell of the body from birth and can be passed from parent to child. Inherited conditions that increase the risk of medulloblastoma include:

• Gorlin syndrome (also called nevoid basal cell carcinoma syndrome) — caused by a change in the PTCH1 or SUFU gene. People with Gorlin syndrome develop multiple basal cell skin cancers, jaw cysts, and other features, and have an increased risk of medulloblastoma, usually in infancy or early childhood.
• Li-Fraumeni syndrome — caused by a change in the TP53 gene. This syndrome predisposes to many types of cancer, including medulloblastoma, usually in older children and young adults.
• Familial adenomatous polyposis (FAP, sometimes called Turcot syndrome) — caused by a change in the APC gene. FAP is best known for causing hundreds of polyps in the colon, which can lead to colon cancer, but it also increases the risk of medulloblastoma.
• Fanconi anemia and related conditions — caused by changes in DNA repair genes such as BRCA2 or PALB2. These conditions affect the body’s ability to fix damaged DNA and can increase the risk of several cancers, including medulloblastoma.
• Other rare syndromes — including Rubinstein-Taybi syndrome (CREBBP), constitutional mismatch repair deficiency, and conditions caused by changes in ELP1 or GPR161.

Because inherited conditions are common in medulloblastoma, especially in certain molecular groups, genetic counseling and testing are often recommended for patients and their families.

How common is medulloblastoma?

Medulloblastoma is one of the most common cancerous brain tumors in children. It accounts for about 20% of all brain tumors in childhood. The overall annual incidence is approximately 6 cases per million children and 0.6 per million adults, and these numbers have not changed substantially over time.

The median age at diagnosis is about 9 years, with peaks at ages 3 and 7. About one quarter of medulloblastomas occur in adults, but medulloblastoma accounts for less than 1% of all adult brain tumors. The tumor is slightly more common in males than in females.

How is the diagnosis made?

The diagnosis of medulloblastoma begins when brain imaging, usually magnetic resonance imaging (MRI), reveals a cerebellar mass. The tumor typically enhances brightly after intravenous contrast is given and is most often located in or near the midline of the cerebellum, blocking the normal flow of cerebrospinal fluid.

The diagnosis is confirmed after a tissue sample is examined under the microscope by a pathologist. In nearly all cases, the tissue is obtained during surgery to remove the tumor. A neurosurgeon opens the skull through an operation called a craniotomy and removes as much of the tumor as can be safely taken out. Surgery, therefore, serves two purposes: it relieves pressure on the brain and provides tissue for diagnosis. Only rarely, when a tumor cannot be safely removed, is a small biopsy performed first.

Under the microscope, the pathologist sees sheets of small, densely packed cells with dark, round or oval nuclei and very little cytoplasm. The cells divide rapidly and many show signs of cell death. This pattern is sometimes called a “small round blue cell tumor.” To confirm the diagnosis and rule out other tumors that can look similar, the pathologist uses immunohistochemistry, a laboratory test that uses antibodies to detect specific proteins in the tumor cells. Medulloblastomas typically express proteins such as synaptophysin and NeuN, which indicate that the cells are developing along a nerve cell pathway.

Once the diagnosis of medulloblastoma is made, additional testing is used to determine the molecular group and histological pattern. Molecular testing — including DNA methylation profiling, sequencing of specific genes, and testing for gene amplifications — is now considered an essential part of the workup because the results guide treatment and help predict how the tumor will behave.

After surgery, imaging and fluid testing are used to complete the picture. An MRI of the entire spine is performed to look for tumor cells that may have spread down the spinal cord. A lumbar puncture is also performed to collect cerebrospinal fluid, which is then examined under the microscope for tumor cells. Together, these tests determine whether the tumor has spread beyond the original site, which is called metastasis.

Molecular groups of medulloblastoma

Medulloblastomas are now divided into four molecular groups based on the genetic changes driving the tumor’s growth. The four groups differ in the typical age of patients, how the tumor behaves, and which treatments are most likely to work. Knowing the molecular group is one of the most important pieces of information in the pathology report.

WNT-activated medulloblastoma

WNT-activated medulloblastoma accounts for about 10% of all medulloblastomas. It usually occurs in older children aged 7-14 years, but it also accounts for 15–20% of medulloblastomas in adults. It is rare in infants.

These tumors are driven by abnormal activation of a cell signaling pathway called WNT. The WNT pathway is involved in normal brain development, but in these tumors, it is activated inappropriately, leading to uncontrolled cell growth. The most common cause is a mutation in the CTNNB1 gene, found in about 85% of WNT-activated tumors. A smaller number of cases occur in patients with familial adenomatous polyposis (FAP), caused by an inherited change in the APC gene.

WNT-activated tumors have the best outcome of all medulloblastoma groups, with survival rates approaching 100% in children. Because of this excellent prognosis, researchers are now studying whether these patients can be safely treated with less intense therapy to reduce long-term side effects. Adults with WNT-activated tumors tend to have a less favorable outcome than children, although it is still better than outcomes for other medulloblastoma groups.

SHH-activated medulloblastoma, TP53-wildtype

SHH-activated medulloblastoma is driven by an abnormal activation of the sonic hedgehog (SHH) signaling pathway. This pathway normally controls the growth of specific cells in the developing cerebellum. When it is switched on inappropriately, these cells continue to grow and form a tumor. The pathway can be activated by changes in one of several genes, most commonly PTCH1, SMO, and SUFU.

SHH-activated medulloblastomas account for about 20–30% of all medulloblastomas. They occur in two main age groups: infants under 4 years of age and adults. A subset of these tumors occurs in patients with Gorlin syndrome, who are born with an inherited change in the PTCH1 or SUFU gene.

The TP53-wildtype subgroup means that the TP53 gene is normal (unmutated). The prognosis in this subgroup is intermediate overall but varies significantly depending on additional features. Infants with SHH-activated, TP53-wildtype tumors that also have a specific histological pattern (desmoplastic/nodular or medulloblastoma with extensive nodularity, described below) have excellent outcomes, often over 90%. Adults with SHH-activated tumors also tend to do relatively well.

A group of drugs called SHH pathway inhibitors (such as vismodegib and sonidegib) is being used in clinical trials to treat SHH-activated medulloblastomas in adults. These drugs work best in tumors with specific mutations (for example, PTCH1 and SMO) and are usually not used in young children because of concerns about long-term effects on bone growth.

SHH-activated medulloblastoma, TP53-mutant

The TP53-mutant subgroup is also driven by activation of the sonic hedgehog pathway, but in addition has a mutation in the TP53 gene. TP53 is one of the most important tumor suppressor genes in the body; it normally helps damaged cells either repair themselves or die, preventing cancer from forming. When TP53 is lost or damaged, cells can keep dividing even when their DNA is abnormal.

These tumors are most often diagnosed in children between the ages of 5 and 17. More than half of SHH-activated, TP53-mutant medulloblastomas are associated with Li-Fraumeni syndrome, meaning that the TP53 mutation is present from birth rather than arising within the tumor. Because of this, genetic testing of blood (to look for an inherited TP53 mutation) is strongly recommended whenever a TP53-mutant tumor is identified.

SHH-activated, TP53-mutant medulloblastomas have a worse prognosis than other SHH tumors. They are often accompanied by MYCN amplification (extra copies of a growth-promoting gene) and have a large cell or anaplastic histological pattern. Patients usually require more intensive treatment and close follow-up.

Non-WNT/non-SHH medulloblastoma (group 3 and group 4)

Non-WNT/non-SHH medulloblastomas include two related but distinct categories called group 3 and group 4. Together, they account for about 60–65% of all medulloblastomas and are the most common groups among children.

Group 3 medulloblastomas account for about 25% of all medulloblastomas and are most common in infants and young children. Many of these tumors have amplification (extra copies) of the MYC gene, a strong driver of cell growth. MYC amplification is associated with an aggressive tumor and a worse prognosis. Group 3 tumors are also the most likely medulloblastomas to have already spread through the cerebrospinal fluid at the time of diagnosis.

Group 4 medulloblastomas are the largest single group, accounting for about 40% of all medulloblastomas. They are most common in children aged 5–15 years. Group 4 tumors often show changes involving chromosome 17, including an isochromosome 17q. Their prognosis is intermediate — better than group 3 but not as good as WNT-activated tumors.

Unlike WNT-activated and SHH-activated tumors, non-WNT/non-SHH medulloblastomas are rarely associated with known inherited syndromes, although a small number of cases occur in people with changes in BRCA2, PALB2, or CREBBP. At present, there are no targeted therapies specifically approved for this group, and treatment relies on surgery, radiation, and standard chemotherapy.

Research using DNA methylation profiling has shown that non-WNT/non-SHH medulloblastomas can be further divided into eight subgroups, some of which have distinct risk profiles. These subgroups may not appear on every pathology report, but are increasingly being used in clinical trials and specialized centers.

Histological patterns of medulloblastoma

In addition to the molecular group, medulloblastomas are described by their appearance under the microscope. There are four recognized histological patterns, and each has its own clinical associations. The histological pattern often provides a clue to the molecular group and can influence treatment decisions.

Classic medulloblastoma

Classic medulloblastoma is the most common pattern, accounting for 70–80% of cases. Under the microscope, the tumor is made up of densely packed small cells with dark, round nuclei and very little cytoplasm. The cells divide rapidly, and small rosette-like arrangements called Homer-Wright rosettes are sometimes seen. Classic medulloblastomas can occur in any of the four molecular groups but are especially common in WNT-activated and non-WNT/non-SHH tumors.

Desmoplastic/nodular medulloblastoma

Desmoplastic/nodular medulloblastoma makes up about 20% of medulloblastomas overall, but up to half of cases in children under 3 years of age. Under the microscope, the tumor has a two-part appearance: pale, round islands of mature-looking cells are separated by bands of densely packed immature cells and a supportive tissue called stroma. A special stain called reticulin highlights this pattern clearly. Nearly all desmoplastic/nodular medulloblastomas are SHH-activated and TP53-wildtype. Infants with this pattern often have excellent outcomes, even when treatment avoids radiation therapy.

Medulloblastoma with extensive nodularity

Medulloblastoma with extensive nodularity is a rare pattern (about 3–4% of cases overall, but up to 20% in infants). It is considered a more mature form of the desmoplastic/nodular pattern. The pale nodules are larger and contain cells that have begun to take on the appearance of mature nerve cells, set within a soft, neuron-like background tissue. Like desmoplastic/nodular tumors, these tumors are almost always SHH-activated and carry a very good prognosis, with survival rates approaching 100% in representative studies.

Large cell / anaplastic medulloblastoma

Large cell / anaplastic medulloblastoma accounts for about 10% of cases and is considered the most aggressive histological pattern. Under the microscope, the cells are larger than in classic medulloblastoma, with prominent nucleoli (small dense structures inside the nucleus) and marked nuclear pleomorphism, meaning the nuclei vary greatly in size and shape. There are many mitotic figures (cells caught in the act of dividing) and many apoptotic cells (cells dying by programmed cell death). This pattern is most common in group 3 tumors with MYC amplification and in SHH-activated tumors with TP53 mutation. Large cell/anaplastic histology is considered a high-risk feature that often warrants more intensive treatment.

WHO grade

All medulloblastomas are assigned World Health Organization (WHO) grade 4, the highest grade used for tumors of the central nervous system. This means the tumor is considered cancerous and grows quickly. Unlike some other cancers, the WHO grade does not vary between medulloblastomas — every medulloblastoma is WHO grade 4, regardless of its molecular group or histological pattern. Prognosis therefore depends on the molecular group, histological pattern, extent of spread, and other features rather than on the WHO grade itself. The very good prognosis seen in some subgroups (especially WNT-activated medulloblastoma) reflects the effectiveness of modern treatment, not a lower WHO grade.

Staging

Unlike most cancers that start outside the central nervous system, medulloblastoma is not staged using the TNM system. Instead, it is staged using the Chang M system, which describes the extent of spread within the central nervous system. Staging is determined by a combination of brain and spine MRI performed after surgery and examination of cerebrospinal fluid collected by lumbar puncture.

• M0 — No evidence of tumor spread. Imaging of the brain and spine is clear, and no tumor cells are seen in the cerebrospinal fluid.
• M1 — Microscopic tumor cells are found in the cerebrospinal fluid, but no visible spread is seen on imaging.
• M2 — Visible tumor deposits are seen in the fluid-filled spaces surrounding the brain (the cerebral or cerebellar subarachnoid space) or within the brain’s fluid-filled chambers (the third or lateral ventricles).
• M3 — Visible tumor deposits are seen in the fluid-filled space surrounding the spinal cord (the spinal subarachnoid space).
• M4 — The tumor has spread outside the central nervous system, which is rare but most often involves bone or bone marrow.

In addition to the M stage, your report or discharge summary may describe how much of the tumor was removed during surgery. Complete or near-complete removal (called gross total resection) is associated with a better outcome than partial removal. Tumors with residual disease greater than 1.5 cm² on post-operative imaging are generally considered higher risk.

Biomarker and molecular testing

Molecular testing is an essential part of the workup for every medulloblastoma. The results are used to assign the molecular group, identify high-risk features, predict treatment response, and detect inherited conditions that may have implications for the patient and their family.

DNA methylation profiling

DNA methylation profiling is now considered the gold-standard test for classifying medulloblastoma. DNA methylation refers to small chemical tags attached to DNA that help control which genes are turned on or off. Different tumor types and molecular groups have distinct methylation patterns, almost like fingerprints. By comparing a tumor’s methylation profile with a large reference database, pathologists can determine the molecular group (WNT, SHH, or non-WNT/non-SHH) and the subgroup with high accuracy. Your report will typically describe the molecular group and, in many cases, the specific subgroup.

Immunohistochemistry for molecular group

Before DNA methylation profiling became widely available, pathologists used immunohistochemistry (IHC) to predict the molecular group of medulloblastoma, and many labs still use IHC as an initial step. Several antibodies are used together:

• Beta-catenin — In WNT-activated tumors, beta-catenin moves into the nucleus of the tumor cells and produces a distinctive nuclear staining pattern. In tumors from other molecular groups, beta-catenin stays in the cytoplasm and does not enter the nucleus.
• GAB1 — GAB1 is expressed in SHH-activated tumors and is usually negative in other groups. It is a useful marker for confirming SHH activation.
• YAP1 — YAP1 is expressed in both WNT-activated and SHH-activated tumors but is negative in non-WNT/non-SHH tumors.
• p53 — Strong and widespread nuclear staining for the p53 protein suggests a TP53 mutation, which is then confirmed by DNA sequencing. This finding is especially important in SHH-activated tumors because it identifies the TP53-mutant subgroup and raises the possibility of Li-Fraumeni syndrome.

TP53 sequencing

Sequencing of the TP53 gene is used to confirm the presence of a mutation in SHH-activated tumors, and sometimes in WNT-activated tumors where it has a different meaning. Because more than half of TP53 mutations in SHH-activated medulloblastoma are inherited (Li-Fraumeni syndrome), DNA from a blood sample is also tested to determine whether the mutation is present throughout the body (germline) or only in the tumor (somatic). This distinction has important implications for the patient’s future cancer risk and for family members.

MYC and MYCN amplification

MYC and MYCN are genes that normally help control cell growth. When a tumor has many extra copies of one of these genes — a change called amplification — the affected cells grow more aggressively. MYC amplification is found in about 17% of group 3 medulloblastomas and is associated with a much worse prognosis. MYCN amplification is found in about 5–10% of SHH-activated medulloblastomas, most often in the TP53-mutant subgroup, and is also associated with worse outcomes. Amplification is usually detected using fluorescence in situ hybridization (FISH) or next-generation sequencing.

Germline genetic testing

Because inherited conditions are common in medulloblastoma, especially in the SHH-activated group, germline genetic testing is recommended for many patients. Testing typically uses a panel of genes that includes PTCH1, SUFU, TP53, ELP1, GPR161, APC, BRCA2, and PALB2. Genetic counseling is an important part of this process and helps families understand what the results mean for the patient and for relatives who may also be at risk.

For more information about biomarkers and molecular testing across all cancer types, visit the Biomarkers and Genetic Testing section.

What is the prognosis?

The prognosis for medulloblastoma has improved substantially over the past several decades, largely due to advances in surgery, radiation therapy, and chemotherapy, as well as a better understanding of the disease’s molecular subgroups. Overall, about 70–75% of children diagnosed with medulloblastoma are alive five years after diagnosis, and many are cured. Outcomes differ significantly among molecular groups and are influenced by several other features identified in the pathology report.

By molecular group, typical five-year survival rates are:

• WNT-activated — More than 95%, approaching 100% in children.
• SHH-activated, TP53-wildtype — 70–80% overall; higher (often above 90%) in infants with desmoplastic/nodular or extensively nodular tumors.
• SHH-activated, TP53-mutant — 40–50%.
• Non-WNT/non-SHH, group 3 — 40–60%, with worse outcomes when MYC is amplified.
• Non-WNT/non-SHH, group 4 — 70–80%.

Specific pathologic and clinical features associated with a worse prognosis include:

• Metastatic disease at diagnosis — M1 or higher is one of the strongest predictors of a worse outcome.
• Incomplete surgical removal — More than 1.5 cm² of residual tumor on post-operative imaging.
• Large cell / anaplastic histological pattern — Associated with more aggressive behavior.
• MYC amplification in group 3 tumors — A strong predictor of poor outcome.
• MYCN amplification in SHH-activated tumors — Associated with worse outcomes, especially when a TP53 mutation is also present.
• TP53 mutation in SHH-activated tumors — Associated with worse outcomes and often inherited (Li-Fraumeni syndrome).
• Age under 3 years — Infants historically had worse outcomes, although this has improved substantially with modern infant-specific treatment regimens, especially for desmoplastic/nodular tumors.

Adults with medulloblastoma generally have a slightly lower survival rate than children, although this varies by molecular group. Because treatment decisions increasingly depend on molecular findings, the overall prognosis for any individual patient is best discussed with the treating medical team after the full pathology and molecular results are available.

What happens after the diagnosis?

Medulloblastoma is treated by a team of specialists that typically includes a neurosurgeon, a pediatric or adult oncologist, a radiation oncologist, a neuropathologist, and a neuroradiologist, often with support from a geneticist, neurologist, endocrinologist, and rehabilitation specialists.

The first step in treatment is almost always surgery to remove as much of the tumor as possible. Complete or near-complete removal is an important goal because it improves the chance of cure and reduces the amount of additional treatment needed.

After surgery, most patients older than 3 years receive a combination of radiation therapy and chemotherapy. Radiation is usually given to the whole brain and spine (craniospinal radiation) to treat tumor cells that may have spread through the cerebrospinal fluid, followed by a higher dose focused on the area where the tumor was removed. Chemotherapy usually combines several drugs, such as cisplatin, lomustine, vincristine, and cyclophosphamide.

For children under 3 years of age, the approach is different. Because radiation to the developing brain can cause significant long-term effects on learning and development, treatment in this age group relies on chemotherapy, sometimes given directly into the cerebrospinal fluid, with radiation avoided or delayed when possible.

Treatment decisions are also increasingly guided by the molecular group. Patients with WNT-activated tumors may be offered less intensive therapy in clinical trials, whereas patients with high-risk features (such as MYC amplification, TP53 mutation, large cell/anaplastic histology, or metastatic disease) may be offered more intensive treatment. Clinical trials are available for many molecular subgroups and are an important option to discuss with your team.

Long-term follow-up is essential. Patients are monitored with regular brain and spine MRIs to detect recurrence and with physical, developmental, and cognitive assessments to identify and manage the long-term effects of treatment. Hormonal, hearing, and growth problems can occur and are managed by specialists as needed.

Questions to ask your doctor

• What molecular group does my (or my child’s) medulloblastoma belong to?
• What histological pattern was seen under the microscope, and what does it mean?
• Was the tumor completely removed during surgery, or is there residual tumor?
• What is the M stage, and did the tumor spread to the cerebrospinal fluid or spine?
• Were any high-risk features identified, such as MYC or MYCN amplification, TP53 mutation, or large cell / anaplastic histology?
• Do we need germline (inherited) genetic testing, and should our family members also be tested?
• Is DNA methylation profiling being performed, and will it change the treatment plan?
• What treatment do you recommend, and will it include radiation to the brain and spine, chemotherapy, or both?
• Are there clinical trials available for my (or my child’s) molecular group?
• What are the expected short-term and long-term side effects of treatment?
• How will we know if the treatment is working?
• How often will follow-up MRIs be done, and for how long?
• What are the realistic chances of cure and of the tumor coming back?
• What support is available for the long-term effects of treatment, such as hormone, hearing, and learning problems?

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