CAPITAL REGION — The type of brain tumor Sen. Edward Kennedy has is often lethal and difficult to treat, with the odds of survival going down with age, Capital Region doctors said Thursday.

Kennedy, 76, was hospitalized Saturday after a seizure. On Tuesday, doctors at Massachusetts General Hospital said he had a malignant glioma in his left parietal lobe. Doctors were awaiting further tests before choosing a treatment but said radiation and chemotherapy were the standards of treatment.

About 9,500 people per year in the United States are diagnosed with malignant gliomas. Besides age, who lives the longest depends on the grade of glioma, whether surgeons can cut most of it out and the condition of the patient.

“It’s hard to predict Kennedy’s survival odds,” said Dr. Yu-Hung Kuo, a neurosurgeon specializing in brain surgery at Albany Medical Center. “When we talk about the most malignant, which is a grade 4, the most average time we give people is around 14 to 16 months. It’s a little longer for a grade 3 — I would say two to three years, but there’s a number of things it’s dependent upon, one of which is how well the patient is doing before the operation. Kennedy was obviously functional.”

If the tumor cannot be removed, the outcome is not as good, said doctors.

Kuo said most malignant gliomas are caused by haywire genes in cells that serve as support tissue to neurons, the nerve cells that do the real work of the brain. Among these support cells are astrocytes and oligodendrocytes.

“One of these support cells, probably an astrocyte, became cancerous in Kennedy,” said Kuo. “From the sounds of it, he probably has either a grade 3 or more likely a grade 4.”

Dr. Robert J. Smith, division chief of radiation/oncology at Ellis Hospital, said whether doctors remove the tumor depends on its location.

“Sometimes, a tumor will be adjacent to a structure that you don’t want to damage with surgery,” said Smith. “The part of the brain where Kennedy’s tumor is located is near to areas that can affect motor function and speech.”

Smith said Kennedy will most likely have radiation treatment five days a week for six to seven weeks for a total of 30 to 35 treatments.

“Each treatment lasts a few minutes and is painless,” he explained.

Side effects include some fatigue, hair loss in the area around the treatment and some minor skin discomfort like a sunburn effect.

“Many patients getting radiation treatment are able to work throughout it,” said Smith.

The chemotherapy is often given in pill form, and side effects vary.

“It’s very individual in terms of who has which side effects,” said Smith.

Because Kennedy had a seizure, he will most likely be given medication to prevent future seizures for several months, as well as steroids to prevent swelling of the brain.

Dr. Tia Olds, radiation oncologist for New York Oncology Hematology in Rexford, said the type of tumor Kennedy has is most likely a high-grade, invasive brain tumor called a glioblastoma. It is the most common primary brain tumor in adults.

“Unfortunately, it’s starting to affect younger populations as well,” she said.

Typically, the peak age is 45 to 55.

“The problem with these tumors is that even after optimal treatment, which would be surgical resection or removal, meaning you remove all the visible disease, you typically find that it recurs,” said Olds. “In terms of a cure, it’s not something that can be cured. If it’s not able to be operated on, the chance of a cure is essentially none.”

Olds predicted that Kennedy’s median life expectancy after undergoing chemotherapy and radiation is probably from nine to 12 months.

Over the next few months, depending on where the tumor is situated, Kennedy could have problems moving his limbs or verbalizing his thoughts, or experience personality changes.

“If I were Kennedy, I would ask for the most aggressive treatment available and see if there are any protocols [or clinical trials] that are open that I would be eligible to participate in,” said Olds. “If we don’t have an acceptable treatment or cure, then protocols are really useful.

“Unfortunately, Kennedy’s prognosis does not look good.”