About 20,000 people in the U.S. each year are diagnosed with a glioblastoma. The American Cancer Society puts the five-year survival rate for patients over 55 at about 4 percent.

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 including chemotherapy and radiation.

The senator and chairman of the Armed Services Committee had been recovering at his Arizona home. His absence had forced Majority Leader Mitch McConnell, R-Ky., to delay action on health care legislation.

In a statement, McConnell said: "John McCain is a hero to our Conference and a hero to our country. He has never shied from a fight and I know that he will face this challenge with the same extraordinary courage that has characterized his life. The entire Senate family's prayers are with John, Cindy and his family, his staff, and the people of Arizona he represents so well."

U.S. Rep. Martha McSally, R-Arizona, said McCain is "one of the bravest men I know — a hero, a warrior and a survivor."