US Senator John McCain, the 2008 Republican presidential nominee known for political independence during more than three decades in the Senate, has been diagnosed with an aggressive form of brain cancer, his office said on Wednesday.

The 80-year-old lawmaker and former Navy pilot, who was re-elected to a sixth Senate term in November, has been recovering at home in Arizona since undergoing surgery at the Mayo Clinic in Phoenix last Friday to remove a blood clot from above his left eye.

Tissue analysis since that procedure revealed that a brain tumor known as a glioblastoma was associated with the clot, his office said.

McCain’s doctors said he was recovering from surgery “amazingly well” and that his underlying health was excellent. Treatment options include a combination of chemotherapy and radiation.

However, glioblastoma is considered a grade IV tumor, the most malignant of gliomas. Medical experts said it can be very aggressive and spread into other parts of the brain quickly.

“It takes people’s lives almost uniformly … The tumor cells are very resistant to conventional therapy, such as radiation and chemotherapy. It’s a poor prognosis,” said Dr. Richard Ellenbogen, who chairs the Department of Neurological Surgery at the University of Washington.

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McCain’s daughter, Meghan McCain, said the family was shocked by the diagnosis but that her father was the “most confident and calm” of them all as he prepared for a new battle against cancer.

McCain has had non-invasive melanomas removed at least three times. He also overcame injuries suffered as a prisoner of war in Vietnam, where he endured beatings and torture by his North Vietnamese captors.

Questions about McCain’s health arose during a recent Senate hearing when the lawmaker, normally a keen interrogator of witnesses, rambled during questioning of former FBI Director James Comey. His doctors told CNN on Wednesday, however, that he had no sign of neurological impairment before or during his surgery.

His fellow members of Congress rushed to offer tributes to McCain and wishes for his quick recovery. Known for an independent political streak, ready wit and strong opinions, McCain is one of the best-known and most popular US lawmakers among his peers and the media.

Senator Lindsey Graham, a long-time friend, said McCain was “resolved and determined” when they spoke by telephone. “This disease has never had a more worthy opponent.”

While known as a fierce advocate for strong US military action overseas, McCain also has a reputation for working with Democrats on issues from clamping down on campaign finance abuses to immigration reform. This week, McCain called for a bipartisan approach to overhauling the US healthcare system. (Reuters)