He continued speaking in front of Congress and making public appearances almost up until the time of his death on Wednesday morning at his home on Cape Cod.
"For a man in his 70s, he did very, very well," Fox News quoted Dr. Michael Gruber, professor of neurology and neuro-surgery at NYU School of Medicine and Director of the Brain Tumor Center in Summit, New Jersey.
"He was walking unassisted (up until the end), he was lucid," Dr. Gruber added.
Dr. Suriya Jeyapalan, a neuroncologist at Beth Israel Deaconess Medical Center in Boston, said that Kennedy's condition was treatable, but not curable.
More than 18,000 primary malignant brain tumors are diagnosed each year in the United States; about 9,000 of those are malignant gliomas, according to the National Cancer Institute.
In general, half of all patients die within a year.
However, patients with malignant gliomas often maintain a very good quality of life after their diagnosis, Gruber said.
Gruber said the fate of a brain tumor patient depends on the location of the tumor. For example, if the tumor is located on the frontal or temporal lobe, then the patient's speech might be affected.
Since Kennedy's tumor was on the left parietal lobe, he suffered seizures. Other brain tumor patients may lose the ability to walk, lose vision or lose comprehension skills, depending on where the tumor lies or if the tumor invades other parts of the brain.
Kennedy underwent targeted brain surgery on June 2, 2008 at Duke University Medical Center. The surgery lasted for about 3 1/2 hours and Kennedy spent some of that time awake.
Targeted brain surgery is a delicate balance - removing as much tumor as possible improves cancer control, but there's also the risk of harming the healthy brain tissue that lets patients walk and talk.
This is why doctors keep patients awake and talking during the surgery to make sure they're steering clear of delicate areas of the brain. The surgery, considered a success, was followed by months of chemo and radiation therapy.
Kennedy has suffered other health problems over the years.
In October 2007, doctors performed surgery to clean out a partially blocked neck artery, which left untreated, could have triggered a stroke.
In 1964, Kennedy suffered several fractured bones in his back, broken ribs, and internal bleeding after he was involved in a plane crash.
Two people died in that crash.