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

— -- The Republican junior senator who serves with Sen. John McCain representing Arizona described how he learned of his elder colleague's brain cancer diagnosis through a casual comment in a conversation Wednesday.

"I called him before we heard of the diagnosis and spoke to him for several minutes about what was going on on Capitol Hill and what he was missing," Sen. Jeff Flake (R-Ariz.) said in an interview today with ABC News Chief Anchor George Stephanopoulos on "Good Morning America."

"And only at the end of the conversation, I asked him how he was feeling today, and he said, 'I'm feeling fine but I might have some chemotherapy in my future,'" Flake said. "And that's how I learned of it. So it was almost in passing about his diagnosis."

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

Wednesday night, the Mayo Clinic and the Arizona Republican's office officially announced McCain's diagnosis, a primary brain tumor of a type called glioblastoma which was related to a blood clot above the senior senator's eye that he had removed last week.

"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."

The statement continues, "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."

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.

"He is a steady force; one who stands for the institution and bipartisanship, and I cannot overstate what an impact he has in the Senate," Flake said on "GMA" today. "We need him back here and he wants to be back here."