Sen. John McCain (R-Ariz.), who is undergoing treatment for brain cancer, sustained a leg injury that will require him to wear a walking boot until he recovers, his office said Monday.

“Senator McCain was treated at Walter Reed Medical Center over the weekend for a minor tear in his right Achilles’ tendon, as well as for other normal and non-life-threatening side effects of cancer therapy,” McCain’s office said in a written statement.

His office did not specify what the other side effects were. McCain, 81, has “returned to work in the Senate and will be wearing a walking boot until his injured tendon is fully healed,” his office said.

McCain was diagnosed this past summer with glioblastoma, a serious form of brain cancer. He announced in September that he was receiving treatment in the Washington area while he continued working in the Senate.

The longtime lawmaker and current chairman of the Senate Armed Services Committee was a pivotal swing vote during the failed Republican effort to repeal and replace the Affordable Care Act, coming out against two efforts to accomplish that.

He could once again be a key swing vote in the party’s biggest current legislative priority: tax reform.