Sen. Edward M. Kennedy was released from Massachusetts General Hospital Wednesday morning after being diagnosed with a malignant tumor in his brain.

A biopsy revealed that the Massachusetts Democrat has a malignant glioma on his left parietal lobe, according to a statement releasd Tuesday by Massachusetts General Hospital in Boston.

His prognosis is bleak, .

"It's treatable but not curable. You can put it into remission for a while but it's not a curable tumor," said Dr. Suriya Jeyapalan, a neuroncologist at Beth Israel Deaconess Medical Center in Boston.

Kennedy, 76, was taken to the Boston hospital Saturday after suffering two seizures at the Kennedy compound in Hyannis Port.

“Glioma is a term that is used for any tumor that arises in the brain, so it has not spread from anywhere else,” said Dr. Lynne Taylor, a neuro-oncologist at Virginia Mason Medical Center in Seattle, Wash., who has not treated Kennedy. “It’s not good news. This will ultimately be what ends his life.”

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More than 18,000 primary malignant brain tumors are diagnosed each year in the United States; about 9,000 of those are malignant gliomas, according to the National Cancer Institute.

In general, half of all patients die within a year.

These tumors are the second-most common cause of cancer deaths in the 15 to 44 age group. Median survival for patients with moderately severe malignant gliomas is three to five years.

Taylor, who is also a fellow with the American Academy of Neurology, told FOXNEWS.com that treatment for a malignant glioma usually includes radiation, chemotherapy and surgery.

However, because the tumor is usually fatal, doctors most often use a concept known as neurologic progression-free survival, a surgery that would attempt to make the patient’s quality of life better during his or her end-stages of life.

“Usually, it’s an attempted total gross resection,” said Taylor of the surgery. “However, these days, if you can do that, you would do that first, and the fact that they went in and did a biopsy and not the resection makes me think the tumor is in a spot where they can’t remove it. However, that is just speculation.”

The resection attempts to remove the tumor, however, malignant cells are usually left behind in nearby parts of the brain, making the tumor likely to grow back.

The tumor may eventually affect Kennedy’s overall health, Taylor said.

“The left parietal lobe means it could partially affect his speech and (cause) swelling (which) could affect other parts of the left side of the brain,” Taylor said.

If doctors are able to perform a successful resection that might reduce the number of seizures Kennedy will suffer, Taylor said.

It is unknown at this time what type of tumor Kennedy has, but there are three types of gliomas, all of which are malignant.

Glioblastoma multiforme, or GBM, which is the most dangerous kind of glioma, is also the most common type of glioma for someone in Kennedy’s age group.

For patients with the most severe type of this tumor, survival is nine to 18 months, Taylor said.

“Obviously, he could beat the odds,” she said.

Anaplastic astrocytoma is basically a mid-range tumor that has about a three-year survival rate, Taylor said.

Patients with low-grade astrocytomas may be able to stave off the growth of the tumor even longer, she said.

Taylor said medical experts are unsure as to what causes brain tumors. Studies have looked at cell phone usage, links to airline fuel and power lines, but all reports came back negative, she said.

“No one knows,” she said. “At some point, a DNA strand breaks and then you get unregulated cell growth.”

Kennedy has suffered other health problems over the years.

In October 2007, doctors performed surgery to clean out a partially blocked neck artery, which left untreated, could have trigged a stroke.

In 1964, Kennedy suffered several fractured bones in his back, broken ribs, and internal bleeding after he was involved in a plane crash.

Two people died in that crash.

— The Associated Press contributed to this story