Republican Sen. John McCain of Arizona has brain cancer, his office says.

The discovery was made during surgery on a blood clot near his eye, his office says.

He was the Republican presidential candidate in 2008, losing to Barack Obama.

His office issued this statement from the Mayo Clinic:

"On Friday, July 14, Sen. John McCain underwent a procedure to remove a blood clot from above his left eye at Mayo Clinic Hospital in Phoenix. Subsequent tissue pathology revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot.

“Scanning done since the procedure (a minimally invasive craniotomy with an eyebrow incision) shows that the tissue of concern was completely resected by imaging criteria. “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 Senator’s doctors say he is recovering from his surgery ‘amazingly well’ and his underlying health is excellent.”

The senator's office 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, Meagan, released a statement on social media.

According to the American Brain Tumor Association, glioblastoma is caused by increased pressure in the brain. It represents about 15% of all primary brain tumors and can be difficult to treat because it contains many different types of cells.

Adults with more aggressive glioblastoma, treated with chemo and radation, can expect a median survival of 14.6 months, and two year survival is 30 percent. A 2009 study, however, reported almost 10 percent of patients may live five years or longer.

Colleagues began to send their condolences about Sen. McCain, including President Trump who issued a statement saying that McCain "has always been a fighter" and that he and Mrs. Trump "send our thoughts and prayers to Senator John McCain, Cindy, and their entire family," adding, "Get well soon."

Former President Barack Obama who wrote "cancer doesn't know what it's up against. "Give it hell, John."

McCain has been in office since 1987. He was a POW in Vietnam, and wrote about his experiences in "Faith of My Fathers."

Information from CBS was used in this report.