 Primitive Neuroectodermal Tumors (PNET)
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Primitive neuroectodermal tumors (PNET) form a group of tumors defined by their appearance. They are thought to develop from primitive (undifferentiated) nerve cells in the brain.
Under a microscope, these tumors are similar to a tumor called medulloblastoma and some will classify medulloblastoma as "PNET (cerebellum)," although it has been demonstrated that medulloblastoma behaves in a distinct manner compared to PNET in other locations in the brain.
According to the World Health Organization (WHO) classification of pediatric brain tumors, all undifferentiated tumors, regardless of location, are derived from common neuroepithelial cells (cells that give rise to the brain and spinal cord) and are therefore classified as PNET. Their location is frequently indicated in parentheses so as to separate medulloblastoma and pineoblastoma (PNET in the pineal region) from other PNET, such as tumors in the supratentorial compartment (meaning they occur in the cerebrum, the part of the brain that controls thought, movement and sensation) or in the spine. PNET are rare tumors and their incidence is not well defined.
As you read further below, you will find general information about PNET. If you would like to view summary information about brain tumors first, see the overview on brain tumors.
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Researchers have not been able to find an identifiable cause or risk factors for PNET. There doesn't appear to be a genetic predisposition, meaning the disease does not seem to run in families.
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The symptoms of a PNET depend on the location of the tumor. The following are the most common symptoms of a PNET, however each child may experience symptoms differently.
Symptoms of a PNET in the supratentorial compartment usually arise quickly and last up to several weeks. These tumors are often seen in the older child or teenager. Common symptoms may include:
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- increased pressure within the brain, which results in:
- headache (generally upon awakening in the morning)
- seizures
Pineoblastomas, or PNETs (pineal region), are usually seen in young children and occur in the area of the pineal gland, a tiny organ near the center of the brain. Common symptoms may include:
- increased pressure within the brain, which results in:
- headache (generally upon awakening in the morning)
- disturbances in ocular movements and/or complaints of double vision, because the tumor is close to the nuclei (centers in the brain) that regulate eye movements and coordination
Common symptoms of PNET of the spine may include
- back and/or leg pain
- slow development of weakness in the lower extremities
- impairment of bowel and/or bladder function, including incontinence
The symptoms of a brain tumor may resemble other conditions or medical problems. Always consult your child's physician for a diagnosis.
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Diagnostic procedure for PNET may include:
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- physical examination and history
- computerized tomography scan (also called a CT or CAT scan) - a diagnostic imaging procedure that uses a combination of x-rays and computer technology to produce cross-sectional images (often called slices), both horizontally and vertically, of the body. CT scans are more detailed than general x-rays.
- magnetic resonance imaging (MRI) - a diagnostic procedure that uses a combination of large magnets, radiofrequencies, and a computer to produce detailed images of organs and structures within the body. For PNET, MRIs of the brain and spine are usually done. Since these tumors are known to spread via the cerebrospinal fluid (CSF) pathways there is a high incidence of dissemination of the tumor and it is essential that the initial investigations include both a spine and brain MRI, both with and without the administration of contrast.
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Specific treatment for a PNET will be determined by your child's physician based on:
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- your child's age, overall health, and medical history
- type, location, and size of the tumor
- extent of the disease
- your child's tolerance for specific medications, procedures, or therapies
- how your child's doctors expect the disease to progress
- your opinion or preference
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Treatment may include:
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- surgery -utilized to obtain a biopsy for diagnosis and to remove the tumor. There are data to suggest that aggressive surgical removal is associated with better long-term outcome and, therefore, surgery is appropriately modified to attempt a complete removal. This is usually possible in the supratentorial tumors, but less frequently accomplished in the tumors of the pineal region and rarely in the spine.
- radiation therapy - for tumors that are incompletely removed or for tumors that recur, the recommended treatment is radiation therapy to the site of the tumor. Radiation therapy uses high-energy rays (radiation) from a specialized machine to kill cancer cells and shrink tumors.
Usually, the radiation therapy is delivered first, followed by the chemotherapy. In infants, or children under the age of 3, radiation therapy is withheld until they reach the age of 3 years.
- chemotherapy - a drug treatment that works by interfering with the cancer cell's ability to grow or reproduce
Different groups of chemotherapy drugs work in different ways to fight cancer cells and shrink tumors. Chemotherapy may be used alone for some types of cancer or in conjunction with other therapy such as radiation or surgery. Often, a combination of chemotherapy drugs is used to fight a specific cancer. Certain chemotherapy drugs may be given in a specific order depending on the type of cancer it is being used to treat. While chemotherapy can be quite effective in treating certain cancers, the agents do not differentiate normal healthy cells from cancer cells. Because of this, there can be many adverse side effects during treatment. Being able to anticipate these side effects can help the care team, parents, and child prepare, and, in some cases, prevent these symptoms from occurring, if possible.
Chemotherapy is systemic treatment, meaning it is introduced to the bloodstream and travels throughout the body to kill cancer cells. Chemotherapy can be given:
- as an injection into the muscle or fat tissue
- intravenously (directly to the bloodstream; also called IV)
- intrathecally - chemotherapy given directly into the spinal column with a needle
- ventriculoperitoneal shunt (also called a VP shunt) - a bypass, usually a type of tubing, that is placed in the head to drain excess cerebrospinal fluid (CSF) from inside the brain to space in the abdomen. A VP shunt helps control the pressure inside the brain. If there is presence of obstructive hydrocephalus (swelling of the brain) in a pineoblastoma, a CSF diversionary procedure, ETV (endoscopic third ventriculostomy) may be necessary. There have been reports of dissemination of pineoblastoma via shunts into the peritoneal cavity and therefore, all efforts are made to avoid placement of a shunt in these children.
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The side effects of therapy are those expected with surgery to the specific areas of the brain and those associated with radiation therapy to the brain and spine. These are managed depending on the specific side effects and may include physical and occupational therapy (if there is weakness or sensory loss), and speech therapy (if there is impairment of speech), all on either an inpatient or outpatient basis.
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The rarity of these tumors and the difficulty in their classification make it difficult to analyze outcome and survival rates. The overall 5-year survival of patients with PNET is between 50 and 60 percent, but is clearly worse in the sub-group of infants and pineoblastoma patients with incomplete tumor removal or incomplete response to radiation therapy.
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Relapsed PNET is almost always fatal and there are no good chemotherapy or other curative therapies available. There are trials looking at bone marrow transplantation with aggressive chemotherapy and use of novel biologic agents, but most of these are currently experimental.
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Children's Hospital and Dana-Farber Cancer Institute are conducting numerous research studies that will help clinicians better understand and treat PNET. For more information on current research, see the Brain Tumor Program.
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