The 80-year-old Arizona Republican took the floor on Tuesday afternoon, with a scar above his left brow and some bruising underneath his eye. As McCain entered the Senate to vote, senators on both sides of the aisle rose in a standing ovation.
"Many of you have reached out in the last few days with your concern and your prayers, and it means a lot to me," McCain said.
"He had great energy, fluent speech and no signs of weakness," said CNN Chief Medical Correspondent Dr. Sanjay Gupta, a practicing neurosurgeon. With McCain's permission, Gupta previously spoke with McCain's doctors in an exclusive conversation, but was not part of the senator's care. "These are all things that could be affected by a tumor in that location, but he seems to have recovered."
His timely return gave the Republicans the votes they needed, albeit by the narrowest of margins, to advance the Obamacare repeal and replace efforts. The vote passed 51-50, with Vice President Mike Pence breaking a tie.
Shortly after voting, McCain expressed frustration with the Republicans' closed-door efforts. He called for more trust among his colleagues and lamented Congress' wide partisan divide.
"We keep trying to find a way to win without help from across the aisle," said McCain, without whom the vote would have failed. "Why don't we try the old way of legislating in the Senate?"
After Tuesday's procedural vote, the Senate will open a debate to amend the bill that passed the House
in May -- the American Health Care Act. McCain clarified that his yes vote was not an endorsement of the health care bill in its current form.
"I will not vote for this bill as it is today," he said. "It's a shell of a bill right now. We all know that."
There had been speculation that McCain would return to Washington until his office confirmed it Monday night.
His office said in a statement
that he would return to the Senate "to continue working on important legislation, including health care reform, the National Defense Authorization Act, and new sanctions on Russia, Iran and North Korea."
What doctors found
Before brain scans showed a five-centimeter blood clot above McCain's left eye, the senator was feeling fatigued and had bouts of double vision, according to Mayo Clinic doctors directly involved in McCain's care.
The surgeons opened a small piece of bone, removed the clot, and found an associated brain tumor -- called a glioblastoma
-- which had originated in the brain.
"Any time you're basically opening the bone to gain access to the brain, it's a significant operation," Gupta said.
The doctors completely removed what they saw of the tumor, but that does not mean that the cancer is gone.
"It's a difficult tumor to treat," Gupta said. "It's a malignant cancer, so the concern is that it comes back even after you operate on it."
Treatments like chemotherapy, radiation and surgery have pushed the median survival for glioblastoma over 14 months
-- meaning half of patients live longer, and half live shorter. However, anyone's prognosis can change based on their age, the size and location of the tumor at diagnosis, what symptoms they have and several other factors.
McCain's doctors said that they would wait several weeks for his surgical wounds to heal before potentially pursuing additional treatment such as radiation.
McCain, who has also survived multiple bouts of skin cancers over the years, was in surgery for several hours. When he finally came to, he was making jokes with the recovery room staff. McCain was sent home the following day to recover with his family.
"He's 80 years old, he had general anesthesia, he just had a brain operation (and) he goes home the next day," Gupta said. "That's quick recovery by any standard, even for a person much, much younger."
Gupta said that McCain's doctors encouraged him to stay home and rest for a couple of weeks, but ultimately allowed him to travel. One concern after surgery is that air can get trapped in tissues, which can expand on a plane, Gupta said. However, 11 days after the procedure, it is unlikely that any significant air bubbles would still be there, he added.
This past Saturday, his daughter, Meghan McCain
, tweeted another reassuring photo to well-wishers, showing father and daughter on a hike in Arizona.
McCain said he would stay in Washington "for a few days" to manage the floor debate on the defense authorization bill.
"After that, I'm going home for a while to treat my illness," he said, though he remained lighthearted.
He quipped, "I have every intention of returning here and giving many of you cause to regret all the nice things you said about me."