Early Tuesday afternoon, news broke that a seizure Sen. Edward Kennedy (D-Mass.) suffered over the weekend was caused by a malignant brain tumor. With this diagnosis, Kennedy, 76, joins the more than 44,865 new cases of primary brain tumors the Des Plaines-based American Brain Tumor Association estimates for 2008.

Kennedy’s preliminary biopsy results show that the tumor is a malignant glioma located in the left parietal lobe, according to media reports.

A brain tumor, as defined by the Brain Tumor Association, is a “mass of unnecessary cells growing in the brain.” Not all tumors are cancerous; a tumor can be characterized as benign if it is slow-growing with distinct borders. If the tumor is invasive, spreading or located in a critical part of the brain, it is classified as malignant.

Kennedy’s tumor has so far only been classified as a glioma, a general term for any tumor which arises from the supportive tissue called glia. The Brain Tumor Association reports that just over half of all gliomas are glioblastomas, although information on Kennedy’s complete diagnosis was not yet available Tuesday evening.

Kennedy’s seizure points to glioblastoma, said Dr. Nicholas Vick, a neurologist at Evanston Northwestern Healthcare, who has no inside information about the case. “[A glioblastoma] is what anyone would have expected once they said it wasn’t a stroke,” he said. If so, it’s “a very serious tumor.”

Glioblastoma is most common in older adults, and is seen in men at a slightly higher frequency than in women, Vick said. The median survival with standard treatment is 15 months, he said, but stressed that number is an average. “If you take 1,000 patients with that tumor, 15 months is average,” he said. Some live much longer, while some die sooner.

Three treatments are commonly used for brain tumors: surgery, radiation and chemotherapy, Vick said. Oftentimes, doctors will employ a combination of two or three for the most effective care.

Surgery is not always an option, but depends on the location of the tumor. “You don’t operate when the operation would impair the patient,” Vick said.

A tumor, he said, “is not like a smooth golf ball. It has fingers, roots. It’s invasive.” Thus, it is not always easy to remove.

Radiation therapy uses radiation energy to shrink the tumor. However, while this treatment works, it does not usually totally eradicate the tumor, according to Vick, who is also a professor of neurology at Northwestern University’s Feinberg School of Medicine.

Chemotherapy uses anti-cancer drugs to treat the tumors. The drug most commonly used now for glioblastoma is Temodor, but a new drug, Bevacizumab (trade name: Avastin) is in clinical trials. Although Avastin is already used to treat other cancers, such as lung and colon cancer, it isn’t yet approved to treat brain tumors. Rather than attacking the tumor itself, this drug blocks the blood vessels that feed the tumor in an attempt to kill it.