The 49-year-old wife of rock star Ozzy Osbourne, who became a TV star thanks to MTV reality show The Osbournes, is to undergo three months of chemotherapy.
She said in an interview in People magazine that the prognosis was "very very good".
Osbourne underwent an operation for the disease on 3 July and her publicist said at the time that she was expected to make a full recovery.
But one of the two lymph nodes removed during surgery has since tested positive for cancerous cells, indicating that the cancer has spread.
"It was not the best news," she said. "You think nothing will ever happen to you. Then after you get over the shock and panic you realise how lucky you are to be alive."
"That's where I am right now. I simply have to take care of this thing."
The next series of the show, which begins filming in a few weeks, will include scenes of Osbourne undergoing treatment.
"It'll be interesting," she said.
"We need to get a different vibe going. I think I'll bring some music, we can sing some songs and cheer the place up."
The diagnosis came weeks after Osbourne signed a new $14m (£9m) deal for a second series of the show.
Her husband Ozzy became so hysterical when she told him the news of her illness, that he had to be sedated by a doctor.
"I've always had a plan that I'd get sick before she did," he told People.
"But my plan didn't work out."
"I've done a lot of praying, believe it or not," he added.
"She's my whole world - she's been my pillar of strength for many years."
The couple plan to renew their marriage vows before Sharon's 50th birthday in October.
Meanwhile, the couple's son Jack hurt his arm after falling from an empty building on Malibu's derelict pier.
The 16-year-old had been preparing to dive into the ocean. A lifeguard who treated him at the scene said he suffered only minor injuries.