Harper, 74, announced in March she had been diagnosed with an aggressive form of brain cancer and given as few as three months to live.
"I'd say that we're getting pretty close to a remission. It defies the odds," neuro-oncologist Dr. Jeremy Rudnick, Harper's physician, is seen telling the actress in a clip from an NBC documentary about her cancer battle, set to air Sept. 19.
Harper said in the program, which was previewed on Thursday's edition of "Today," how she underwent traditional chemotherapy, as well as having acupuncture and drinking medicinal Chinese tea to slow the disease's progress.
But Rudnick explained the cancer will probably "develop resistance to the therapy" and "it's not a matter of if, it's a matter of when."
"Going from having three months to live, or less; we're into our sixth month, and now there's even hope beyond right now we're looking at ... " said Harper's husband Tony Cacciotti.
"We're looking at Christmas!" added Harper.
Since announcing her diagnosis, Harper has published a memoir, taped an episode of "Hot in Cleveland" with her former "Mary Tyler Moore Show" cast members and starred in a TV movie.
TMZ reported Wednesday she also is to compete in the upcoming season of "Dancing with the Stars," although neither Harper nor the show's producers have confirmed her casting.
"Life is about buying time," Rudnick said in the documentary.
"Exactly," said Harper. "That's what I always say."
Harper is best known for her work on the sitcoms "The Mary Tyler Moore Show," "Rhoda" and "Valerie."