The majority (more than 60%) of childhood brain tumours are located in the posterior fossa (the back compartment of the brain). The posterior fossa includes the cerebellum, the brainstem, and the fourth ventricle.
Tumours in this area include Primitive Neuroectodermal Tumours (PNETs), Medulloblastomas , Brainstem Gliomas, Cerebellar Astrocytomas, and Ependymomas.
Please refer to the Brain Tumour Glossary for explanations of any terms that you do not understand.
If there are terms in this article that do not appear in the glossary, you can also make use of the NCI Dictionary of Cancer Terms (link on the right hand side of every blog page if you ever need them again – look for Bennie Bookworm)
These are the types of Malignant/Cancerous brain tumours that occur in the posterior fossa:
Primitive Neuroectodermal Tumours (PNETs)
Although PNETs can occur anywhere a child’s brain, the most common area they occur is in the back of the brain near the cerebellum. PNETs that occur in this area are called medulloblastomas.
Symptoms of PNETs depend on where they are located in the brain, but typically a child with PNET experiences increased intracranial pressure. PNETs grow very fast and are often malignant, with occasional spreading throughout the brain or spinal cord.
Medulloblastomas are the most common malignant brain tumour occurring in children, making up around 20% of all paediatric tumours, and generally occur in children from 4 to 10 years of age.
Medulloblastomas occur more often in boys than in girls and originate in the middle of the cerebellum. They interfere with the flow of cerebrospinal fluid (CSF), which bathes the brain and spinal cord. This can cause hydrocephalus, an abnormal condition in which cerebrospinal fluid collects in the ventricles of the brain, resulting in abnormally rapid growth of the head,bulging fontanelles and a small face.
Medulloblastoma cells can metastasize (spread) to other parts of the of the central nervous system, especially around the spinal cord, through the CSF.
A child with Medulloblastomas may have headaches, vomit, or walk unsteadily. Sometimes there is pain at the back of the head. Treatment is generally surgical removal, followed by radiation therapy of the entire head and spinal cord and/or chemotherapy.
Brainstem Gliomas (DIPGs)
Brainstem gliomas are also often referred to as Diffuse Intrinsic Pontine Gliomas (DIPGs).
DIPG are highly aggressive and difficult to treat brain tumours that originate in the pons, the part of the brainstem on the lower back of the brain, near the top of the spinal cord. The pons controls many of the body’s most vital functions such as breathing, blood pressure, heart rate and the nerves and muscles that help us see, hear, walk, talk and eat.
DIPGs are glial tumours, because they arise from the brain’s glial tissue that is made up of the cells that help support and protect the brain’s neurons.
DIPGs account for approximately 10% – 15% of childhood brain tumours, and most commonly affect children between the ages of 5 and 10 years, but can occur at any age, even in adulthood.
DIPGs fall under the Glioma Staging System, which means that they can be classified according to the four stages below based on how the cells look under the microscope. The grades are from the least severe to the most severe.
- Low Grade: Grade I or II means that the tumour cells are the closest to normal.
- High Grade: Grade III or IV means that these are the most aggressive tumours.
One of the big issues with DIPG is that most of them are not classified by grade because surgery to obtain tissue by biopsy or to remove the tumour is unsafe due to the location of the tumour. When DIPGs are biopsied, they are usually grade III or grade IV tumours, which tend to behave extremely aggressively. Most DIPGs are diagnosed by how they appear on an MRI.
Brainstem gliomas may cause sudden dramatic symptoms, such as difficulty swallowing, clumsiness, double vision, and weakness, due to their location,
Unlike many other paediatric cancers, there has been little progress in improving treatments and cure rates for DIPG over the last few decades. Unfortunately, fewer than 10% of children with DIPG survive two years from diagnosis.
Cerebellar astrocytomas are tumors that arise from a type of glial (supporting) cell in the nervous system. One of the more common paediatric brain tumours is the cerebellar astrocytoma, an astrocytoma that occurs in the cerebellum, a region in the lower-back portion of the brain that controls movement and balance.
Cerebellar astrocytomas comprise approximately 15% to 25% of all brain tumours in children. They can occur at any time in childhood or adolescence and have the same symptoms as medulloblastomas.
Most cerebellar astrocytomas are low-grade (slow-growing) tumours, and the prognosis for children diagnosed with this type of tumour is excellent. High-grade (or malignant) astrocytomas also may occur in the cerebellum, but they are very rare.
The most common symptoms associated with cerebellar astrocytomas are: problems with walking, clumsiness, nausea, headache, and vomiting. Some initial symptoms of brain tumors, such as headache and nausea, are the result of increased intracranial pressure caused by the bulk of the tumor or a backup of the cerebrospinal fluid that surrounds the brain and spinal cord.
Treatment is surgical removal, which is the cure in most cases if the tumour is totally removed. If the tumour has grown into the brainstem, radiation therapy or chemotherapy (depending on the child’s age) is sometimes needed.
Although ependymomas are the third most common type of brain tumour in children (following astrocytoma and medulloblastoma), they are relatively rare. Ependymomas account for 6-12% of brain tumours in children less than 18 years of age, but 30% of brain tumours in children less than 3 years of age.
Ependymomas are tumours that arise from the cells lining the ventricles (the fluid containing spaces within the brain) and central canal within the spinal cord.
According to the World Health Organization (WHO) classification of tumours, there are four types of ependymal tumors. The microscopic appearance and clinical behaviour of these tumours vary considerably:
- Myxopapillary ependymomas are well-circumscribed tumors that occur primarily at the base of the spine in an area called the filum terminale. They are relatively uncommon in children.
- Subependymomas are well-circumscribed tumors that are usually located in or around the ventricles. Most are incidental tumors, but some may be large enough to cause symptoms. Following surgical resection, the long-term prognosis is generally excellent because these tumors rarely recur. Subependymomas are also uncommon in children.
- Classic ependymomas are fairly well delineated tumors. They can arise anywhere in the central nervous system, but commonly develop in the posterior fossa (the portion of the skull containing the cerebellum and brain stem), usually around the fourth ventricle, the spinal cord or the lateral ventricles (within the cerebral cortex).
- Anaplastic ependymomas, in general, exhibit a high growth rate and have been associated with a less favorable prognosis. However, there is uncertainty and controversy regarding the microscopic characteristics that distinguish the anaplastic form of ependymoma from the classic (lower grade) ependymoma.
Ependymomas tend to grow relatively slowly and displace, rather than invade adjacent brain or spinal cord tissue. They are described as plastic because they grow outside the ventricle through paths of least resistance within the brain. Some ependymomas, particularly those located in the posterior fossa, spread to other parts of the brain or spinal cord through the cerebrospinal fluid. Cerebrospinal dissemination occurred in approximately 13% of patients in one study. Ependymomas rarely metastasize to sites outside of the central nervous system. When ependymomas recur after treatment, they tend to grow back locally (i.e. at or near the site of the original tumor), rather than spreading to other sites.