"I'd say that we're getting pretty close to a remission," said Harper's neuro-oncologist Dr. Jeremy Rudnick. "It defies the odds."
Harper was diagnosed with leptomeningeal carinomatosis, a form of brain cancer that attacks the membranes surrounding the brain, that was expected to be terminal.
But "The Mary Tyler Moore Show" actress, who is featured in an hour-long documentary on NBC September 19, has bought herself more time.
Rudnick cautions Harper and her husband, Tony Cacciotti, that the good news still comes with a caveat: the cancer will probably develop resistance to the therapy, and that she is still terminal.
"It's not a matter of if," said Rudnick, "it's a matter of when."
Still, Harper can't help but be pleased by the good news.
"Going from having three months to live, or less; we're into our sixth month," Cacciotti said, "and now there's even hope beyond right now we're looking at...."
"We're looking at Christmas!" said Harper.
Celebrity Couples of 2014 [PHOTOS]