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Posterior Fossa Tumors

Practice Essentials

A brain tumor is one of the most devastating forms of human illness, especially when occurring in the posterior fossa. Brainstem compression, herniation, and death are all risks in tumors which occur in this critical location. Tumors in the posterior fossa are considered critical brain lesions, primarily because of the limited space within the posterior fossa and the potential involvement of vital brain stem nuclei. The clinical presentation depends on the site of the tumor, biological behavior and aggressiveness of the tumor, and the rate of growth. At the time of presentation, the patient may be very ill from severe headache or frequent vomiting due to associated hydrocephalus. Symptoms may be caused by focal compression of the cerebellum or brain stem centers and increased intracranial pressure.

No specific causes for posterior fossa tumors exist. However, genetic factors, such as dysfunction of some tumor suppressor genes (p53 gene) and activation of some oncogenes, may play a role in their development.
Environmental factors such as irradiation and toxins may also play a role.

Cushing probably was the first to report a large series of posterior fossa tumors. He published information about 61 patients with cerebellar medulloblastoma with mostly fatal outcome.

Advances in brain surgery for tumors primarily were due to discovery of anesthesia, asepsis, neurologic localization, and the ability to achieve hemostasis. Hippocrates, who likely performed it for headache, epilepsy, fractures, and blindness, first described trephination.

An image depicting posterior fossa anatomy can be seen below.

Posterior fossa anatomy.

Posterior fossa anatomy.

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Posterior fossa tumors are more common in children than in adults. Central nervous system tumors are the most common solid tumors in children; between 54% and 70% of all childhood brain tumors originate in the posterior fossa.
About 15-20% of brain tumors in adults occur in the posterior fossa.

Certain types of posterior fossa tumors, such as medulloblastoma, pineoblastoma, ependymomas, primitive neuroectodermal tumors (PNETs), and astrocytomas of the cerebellum and brain stem, occur more frequently in children.Some glial tumors, such as mixed gliomas, are unique to children; they are located more frequently in the cerebellum (67%) and are usually benign.

Hydrocephalus is common in children with posterior fossa tumors, occurring in 71-90% of pediatric patients; approximately 10-40% demonstrate persistent hydrocephalus after posterior fossa tumor resection.

Patients older than 3 years are considered at standard risk or high risk and have long-term survival rates of approximately 85% and 70%, respectively. In patients 3 years or younger, survival rates are generally lower.

Patients who present with posterior fossa tumors undergo surgery for the following goals
:

To decompress the posterior fossa for the purpose of relieving pressure on the brain stem and/or to release intracranial pressure and avert the risk of herniation

To diagnose the tumor based on histopathology

To determine further plan of management depending on the nature of the tumor

When indicated, to treat hydrocephalus by shunting cerebrospinal fluid (CSF) to the peritoneal cavity (external CSF drainage or even no drainage is considered in some cases; still, many disagree regarding the ideal shunting procedure)

CT scan of the posterior fossa is inferior to MRI in diagnostic value because of the artifact produced from the surrounding thick bone. However, CT scan is helpful for postoperative follow-up. Cerebral angiography is useful to assess the vascular supply of the tumor, but with the wide availability of MRI, cerebral angiography is no longer used as the first option in brain tumor assessment.

Some patients should undergo an emergency operation, especially if they present with acute symptoms of brain stem involvement or herniation.
 (See the images below.)

A 28-year-old female patient presented with headac

A 28-year-old female patient presented with headache, ataxia, and blurring of vision. A T1-weighted MRI image, sagittal view, shows dilatation of the lateral ventricles.

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A 28-year-old female patient presented with headac

A 28-year-old female patient presented with headache, ataxia, and blurring of vision. A T1-weighted MRI image, sagittal view, shows an infratentorial mass with a large cystic component and small nodule. The mass is compressing the brainstem.

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The tumor (the infratentorial mass) after excision

The tumor (the infratentorial mass) after excision shows its vascular nature (see previous images).

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Performing a lumbar puncture in patients with suspected posterior fossa tumors is contraindicated. A risk of central herniation exists even with a very small amount of CSF drainage. To avoid this catastrophe, a thorough clinical examination, including funduscopy and a CT scan of the brain should be performed before lumbar puncture.

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