John McCain's junior Senate colleague tells how he learned of veteran senator's cancer diagnosis

Flake said he can't imagine a Senate without his colleague.

Flake added, "He's optimistic, obviously. He's John McCain; that's what we expect."

"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," reads a statement from the clinic, released at the request of McCain. "Subsequent tissue pathology revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot."

McCain's doctors say he is recovering from surgery "amazingly well" and "his underlying health is excellent," according to the statement.

Dr. David Reardon of the Dana-Farber Cancer Institute in Boston, one of the doctors who treated the late Sen. Ted Kennedy, confirmed to ABC News that Kennedy's tumor was also glioblastoma.

Reardon said glioblastoma is the most common kind of brain cancer for adults, with approximately 13,000 cases diagnosed in the United States each year. It is also one of the most aggressive types of brain cancer.

"This is a type of cancer that we do have effective treatments that can help patients," Reardon told "GMA" co-anchor Robin Roberts in an interview today. "But unfortunately, like many aggressive cancers, the durability of that benefit is something we have to continue to work on and try to improve."

McCain's diagnosis has several similar features to Kennedy's case. But Kennedy's case was a "very challenging one," according to Reardon.

"These tumors depending on where they are in the brain and their size, their location relevant to important functional areas of the brain can be even more challenging to treat -- and unfortunately, those were the circumstances for Sen. Kennedy," Reardon said in the interview on "GMA."

"In Sen. McCain, what we know thus far is that the tumor appears to be fairly small, fairly superficially located, not in a critical functional area of the brain and able to undergo an effective surgical resection," he added.

Reardon said "the gold standard" in treatment would entail a few weeks recovering from surgery, then starting radiation therapy, which typically lasts for about six weeks, and during that same period taking a daily dose of an oral chemotherapy medicine.

Flake, who began his career in Congress as an intern decades ago, said it's unclear when and if McCain will return to Capitol Hill. Flake said he can't imagine a Senate without him.