(CNN) -- Sen. Edward Kennedy has the most common type of brain tumor, but the size and nature of the tumor will determine how life threatening it is, CNN's chief medical correspondent Dr. Sanjay Gupta said Tuesday.
Kennedy, 76, was hospitalized over the weekend after having a seizure.
Kennedy's doctors at the Massachusetts General Hospital said Tuesday that the preliminary results from the brain biopsy showed a tumor in the left parietal lobe was responsible for the seizure.
Kennedy hasn't had any seizures since his hospitalization, doctors said.
Gupta, who is a neurosurgeon, said a tumor in that area of the brain could affect the senator's ability to speak and understand speech as well as the strength on the right side of his body. Learn more about the parietal lobes and gliomas »
The parietal lobes are also responsible for simultaneously interpreting signals from other parts of the brain that focus on vision, hearing, motor, sensory and memory, according to the American Association of Neurological Surgeons Web site. That information, combined with a person's memory, gives meaning to objects.
Gupta said while he hasn't seen Kennedy's brain scans, doctors would normally perform surgery to try to remove as much of the tumor as possible. Doctors then would likely begin chemotherapy or radiation to try and kill the remaining cancerous cells.
Several Massachusetts Democratic sources close to the Kennedy family told CNN that the senator and his wife, Victoria, are awaiting more test results and information from doctors before deciding whether surgery is a viable treatment option.
"He is in remarkably good spirits and everyone else around them is just stunned," said a source in regular contact with the family. "It is just such a crushing blow." Dr. Gupta: Watch more on Kennedy's condition »
As evidence of the senator's good spirits, this source said Kennedy asked his doctors if he could leave the hospital to participate in a sailing regatta off Cape Cod this weekend
But surgery can be tricky, said Dr. Wendy Wright, assistant professor of neurology at Emory University School of Medicine in Atlanta.
"The problem with glioma is, when surgeons go in and try to work on this, it's nearly impossible to tell what is tumor and what is brain cells, because this is a tumor of the brain cells," she said. The result, sometimes, is that the surgery causes brain damage, Wright said.
And the area of the brain where Kennedy's tumor is located is "high-risk real estate, near a lot of centers that can affect the ability to move the right side of the body, the ability to speak and understand," she said.
In cases where surgery is deemed too dangerous, doctors opt to avoid surgery altogether, and stick with chemotherapy and radiation, she said.
Kennedy will remain in the hospital, his doctors said, and a decision about treatment will be made after more tests are done.
Doctors say Kennedy's tumor is a malignant glioma, but they have not said how large it is or how quickly it may be spreading.
Average survival can range from three to five years for moderately severe malignant gliomas to less than a year for very advanced and aggressive types such as glioblastomas.
"We have limited information about Senator Kennedy's situation, and it would not be appropriate for us to speculate on his condition or outlook," said Dr. Otis W. Brawley, chief medical officer for the American Cancer Society, in a written statement. Watch more on surviving brain tumors »
He estimated that 21,810 malignant tumors of the brain or spinal cord will be diagnosed this year in the United States and that approximately 13,070 people (7,420 men and 5,650 women) will die from the tumors.
The cancers account for about 1.3 percent of all cancers and 2.2 percent of all cancer-related deaths in the country, he said.
About 42 percent of brain tumors, including benign ones, are gliomas, and about 77 percent of malignant tumors are gliomas, Brawley said.
Uncommon among children, their incidence goes up with age and peaks among people ages 75 to 84. Survival tends to drop with increasing age, he said.
-- CNN's John King contributed to this report.