Kennedy, who was 76 when he was diagnosed, received aggressive treatments, such as surgery, chemotherapy and possibly radiation, Ashton said.
The prognosis for brain tumors like Kennedy's, Ashton said, is "very poor," with only a 50 percent survival rate within the first year of diagnosis, and only three months of survival if left untreated.
However, Kennedy survived 14 months after he was diagnosed, Ashton pointed out.
Contributing factors to survival rates, Ashton said, include the age of the patient and the person's general state of health. Ashton said Kennedy was in a generally good state of health.
Ashton added the length of time Kennedy lived after he learned of the tumor was a testament to his steadfastness. She said, referring to his months of treatment, "It really, in a lot of ways, exemplified how strong he was, and what kind of battle he was able to wage against this particularly aggressive form of cancer."
The American Cancer Society estimates that 22,070 malignant tumors of the brain or spinal cord will be diagnosed in the U.S. this year. About 12,920 people will die from these tumors in 2009.