Tissue analysis since that procedure revealed that a primary brain tumour known as a glioblastoma was associated with the clot, his office said.
Doctors for the veteran Republican said he is recovering from his surgery “amazingly well” and his “underlying health is excellent”.
The Mayo Clinic of Phoenix said in a written statement: “The Senator and his family are reviewing further treatment options with his Mayo Clinic care team. Treatment options may include a combination of chemotherapy and radiation.”
The veteran Republican, who ran for President against Barack Obama in 2008, was rushed into surgery when the clot was discovered above his eye following a routine check up.
The office of Senator John McCain also released the following statement: “Senator McCain appreciates the outpouring of support he has received over the last few days. 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.”
The Senator’s daughter Meghan released a statement on social media saying “the news of my father’s illness has affected every one of the us in the McCain family”.
She goes on to write that her father is “the toughest person I know” and that her love for her father is boundless.
Meghan added: “My fears for him are overwhelmed by one thing above all: gratitude for our years together, and the years still to come.”
In a statement US President Donald Trump said: “He and First Lady Melania send thoughts and prayers to Senator McCain, Cindy, and their entire family.”
Senate Majority Leader Mitch McConnell called Mr McCain, the chairman of the Armed Services Committee, and a prisoner of war during the Vietnam War, a hero and said he looked forward to having him back in Washington.
He said in a statement: “We all look forward to seeing this American hero again soon.”
Barack Obama tweeted: “John McCain is an American hero & one of the bravest fighters I’ve ever known. Cancer doesn’t know what it’s up against. Give it hell, John.”
Hillary Clinton tweeted: “John McCain is as tough as they come. Thinking of John, Cindy, their wonderful children, & their whole family tonight.”