Slideshow ( 2 images )

WASHINGTON (Reuters) - U.S. Senator John McCain was undergoing treatment at a military hospital outside Washington for side effects of cancer therapy, his office said on Tuesday.

McCain, 81, who was diagnosed with brain cancer during the summer, will return to work as soon as possible, a statement from his office said.

“Senator McCain is currently receiving treatment at Walter Reed Medical Center for normal side effects of his ongoing cancer therapy,” the statement said. “Senator McCain looks forward to returning to work as soon as possible.”

McCain, an Arizona Republican who ran unsuccessfully for president in 2008, was found to have an aggressive form of brain tumor, glioblastoma, after surgery in July for a blood clot above his left eye.

He has been receiving chemotherapy and radiation treatment in the Washington area while continuing to work in the Senate. He has missed Senate votes this week.

A critical vote on the Republican tax overhaul is expected in the Senate early next week. McCain’s absence would make it more difficult, but not impossible, for Republicans to pass the bill.

McCain was re-elected to a sixth Senate term last year.