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Primitive Neuroectodermal Tumors (PNETs)

After leukemia and lymphoma, brain tumors are the most common type of childhood cancer. Primitive neuroectodermal tumors (PNETs) are malignant tumors of the central nervous system that usually are found in infants, children, and young adults. These tumors are thought to arise from primitive (undifferentiated) nerve cells left over from the gestational development of the nervous system. Because the tumors are malignant, they tend to spread easily through the cerebrospinal fluid (CSF) that bathes the brain and spinal cord. In some cases, the tumors may spread beyond the central nervous system.

PNETs include relatively common tumors known as medulloblastomas — the most common primary malignant brain tumors in children, accounting for as many as 30 percent of all solid brain tumors — as well as rarer tumors, such as neuroblastomas, pineoblastomas, and retinoblastomas. While PNETs can be found throughout the central nervous system, medulloblastomas usually occur near the base of the skull and can spread to the spine or to other parts of the body.

Peripheral PNETs (pPNETs) also can occur at sites outside of the central nervous system — usually within bone — in the extremities, pelvis, and the chest wall.

Symptoms

Medulloblastomas typically arise near the fourth ventricle, a chamber in the brain filled with cerebrospinal fluid. As a result, these tumors often result in a condition called hydrocephalus, in which the flow of CSF is disrupted and pressure inside the skull increases, resulting in symptoms, including:

  • Irritability
  • Nausea and vomiting
  • Morning headache or headache that goes away after vomiting
  • Weakness or change in sensation on one side of the body
  • Seizures
  • Unusual sleepiness or change in energy level
  • Change in personality or behavior
  • Unexplained weight loss or weight gain

More advanced symptoms associated with PNETs include

  • Loss of coordination
  • Problems with walking and balance
  • Dizziness, and
  • Double vision

Diagnosis

Diagnosis begins with a physical examination and history. As with other brain tumors, imaging studies are the key component in the diagnosis of PNETs.

  • Magnetic resonance imaging (MRI) scans primarily are used; this imaging test can show the size and characteristics of the tumor, as well as the presence of hydrocephalus.
  • Computed tomography (CT) scans also are used.

For either study, an agent that provides contrast in the image is administered intravenously so neurological surgeons can visualize the tumor against the normal brain in the background. Sometimes a PNET can be diagnosed by ultrasound before a child is born.

If a brain tumor is suspected, a biopsy is done by removing part of the skull and using a needle to remove a sample of brain tissue, which is examined immediately under a microscope by a pathologist. If cancer cells are found, the doctor will remove as much tumor as safely possible during the same surgery.

Treatment

To determine and implement optimum therapy, treatment planning by a multidisciplinary team of cancer specialists who have experience treating childhood brain tumors is required. Treatment options depend on:

  • The age of the child, overall health and medical history when the tumor is found
  • The type, location and size of the tumor
  • The amount of tumor remaining after surgery
  • Whether the cancer has spread to other parts of the central nervous system, such as the meninges (membranes covering the brain) or spinal cord
  • Whether the cancer has spread to other parts of the body, such as the bone or lung
  • The child's tolerance for specific medications, procedures, or therapies
  • How doctors expect the child's disease to progress

PNETs may be treated with surgery, radiation therapy, chemotherapy and sometimes, by implanting of a ventriculoperitoneal shunt. The primary goal of surgery for a PNET is to remove as much tumor as possible and to restore the flow of CSF without producing neurological deficits. Surgery also provides tissue samples that can be used to obtain an accurate diagnosis. However, these tumors tend to spread quickly and in an erratic manner, making total surgical removal of the tumor difficult and follow-up treatment a necessity. Several studies have recommended aggressive surgical resection followed closely by both radiation therapy and chemotherapy.

  • Radiation therapy, using high-energy x-rays or other types of radiation to kill cancer cells, is used when tumors are incompletely removed by surgery, or when they recur. Radiation therapy is not recommended for young developing brains and is avoided in children under the age of two. Radiation therapy, if needed, is technically very demanding and should be carried out in centers that have experience in that area to ensure optimal results.
  • Chemotherapy uses drugs to stop the growth of cancer cells, either by killing the cells or by stopping the cells from dividing. Chemotherapy is introduced to the bloodstream and travels throughout the body to kill cancer cells. Chemotherapy can be given:
  • by mouth, as a pill
  • by injection into the muscle or fat tissue
  • intravenously (directly to the bloodstream)
  • intrathecally - injected by needle directly into the spinal column
  • Clinical trials are studying ways of using chemotherapy to delay or reduce the need for radiation therapy in children younger than 2-3 years.
  • Ventriculoperitoneal shunt (also called a VP shunt): In approximately 30 percent of cases, the flow of CSF will not be returned to normal, and the child will require the implantation of a permanent shunt after surgery. The shunt is a type of tube that maintains a normal level of pressure inside the skull by draining excess cerebrospinal fluid from inside the brain to space in the abdomen.

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