PROVIDENCE — The malignant glioma diagnosed in Massachusetts Sen. Edward M. Kennedy is a fast-growing form of cancer that originates in the brain and is difficult to treat, a Rhode Island Hospital neurosurgeon told reporters yesterday.

“The prognosis is usually poor, with a life expectancy of one year or less,” said Dr. Curtis E. Doberstein, interim chief of neurosurgery at the hospital. Doberstein said he knows of cases of younger patients who have survived several years with similar brain tumors.

Doberstein has no more specific information about Kennedy’s condition than was released to the public yesterday. But in response to media requests, he agreed to speak in general about what to expect with the type of cancer diagnosed in Kennedy.

Doctors rank tumors from one to four, with four being the most serious, Doberstein said. The diagnosis of malignant glioma given by Kennedy’s doctors means he has a grade three or grade four tumor, Doberstein said.

A grade three tumor is called an anaplastic astrocytoma; the median survival of a patient with that grade of tumor is two to three years.

But Doberstein said, statistically it is far more likely that Kennedy has a grade four tumor, called a glioblastoma. The prognosis is poor because such tumors grow so fast, doubling on average every seven days.

A glioblastoma spreads rapidly through the brain, making surgery difficult, Doberstein said. He said it can strike at any age. The cause is generally a genetic mutation.

“It’s anybody’s guess how long it’s been there, but probably not long,” Doberstein said. Doberstein said the hospital sees about 50 to 70 such tumors a year, statistically more than one would suspect in Rhode Island.

People aren’t screened for such cancers because they are relatively rare, Doberstein said. But once a patient has such symptoms as seizures, headaches or nausea, it’s relatively easy to diagnose a glioblastoma.

Doberstein said some 18,000 to 20,000 brain tumors are diagnosed each year in the United States. In Rhode Island there are about 80 to 100 cases each year with about 50 to 70 of them diagnosed as the worst case, glioblastomas.

“The numbers are higher in elderly patients, but we see it in all age groups,” Doberstein said.

He said he has never seen a case of glioblastoma cured, but it can be treated to improve the length and quality of a patient’s life.

He said Kennedy could be expected to leave the hospital soon and go home, if his treatment is confined to radiation and chemotherapy.

The goal in treating patients is to strike a balance between quality of life and longevity.

plord@projo.com