On Friday, McCain underwent surgery to remove a blood clot from above his left eye at the Mayo Clinic Hospital-Phoenix in Arizona. Tissue pathology revealed a cancerous tumor was associated with the blood clot.
The senator and his family are looking at treatment options for the glioblastoma, an aggressive cancer. The treatments may include a combination of chemotherapy and radiation, Mayo Clinic officials said.
The tumor digs tentacle-like roots into normal brain tissue. Patients fare best when surgeons can cut out all the visible tumor, which happened with McCain's tumor, according to his office. That isn't a cure; cancerous cells that aren't visible still tend to lurk, the reason McCain's doctors are considering further treatment.
It's the same type of tumor that struck McCain's close Democratic colleague in legislative battles, the late Ted Kennedy of Massachusetts.
Doctors say McCain is recovering from his surgery amazingly well and his underlying health is excellent, according to the statement.
McCain's office also released the following statement, saying the 80-year-old appreciates the outpouring of support he has received."
"He is in good spirits as he continues to recover at home with his family in Arizona. He is grateful to the doctors and staff at Mayo Clinic for their outstanding care, and is confident that any future treatment will be effective. Further consultations with Senator McCain's Mayo Clinic care team will indicate when he will return to the United States Senate."
Daughter Meghan McCain spoke out about the news on Twitter, saying her father is the toughest person she knows, and the disease "will not make him surrender. Nothing ever has."
President Donald Trump also spoke about the lawmaker's strength, saying, "Senator McCain has always been a fighter. Melania and I send our thoughts and prayers to him, Cindy, and their entire family. Get well soon."
Dr. Santosh Kesari is a neuro oncologist at the John Wayne Cancer Institute at Providence St. John's Hospital in Santa Monica. He said a good prognosis is very possible.
"Because of his health, functional status and because of the ability to get most of the tumor that we could see out, he may end up having a good prognosis relative to the average person, if everything else lines up," he said.
McCain was the GOP's presidential nominee in 2008, when he and running mate Sarah Palin lost to Barack Obama. A Navy pilot, he was shot down over Vietnam and held as a prisoner for 5 years.
The Associated Press contributed to this report.