The family is still in shock according to McCain's daughter, Meghan.

She however said her father was the "most confident and calm" of them all as he prepared for a new battle against cancer.

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.

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.