(CNN) -- Juanita Castro Ruz, sister of former Cuban leader Fidel Castro and Cuban President Raul Castro, is undergoing radiation treatment in Miami, Florida, for lung cancer, her spokeswoman told CNN.
Castro underwent surgery more than a month ago in a Miami hospital to remove a tumor in her lung, her biographer and spokesperson, journalist Maria Antonieta Collins, said.
Collins said Castro is recovering well and her diagnosis is very good.
"I am in the best medical hands, receiving the proper treatment," Castro said on her website. "It's a serious condition, but I'm not at the side of my grave."
Castro released an autobiography last year titled "My Brothers Fidel and Raul, the Secret History."
The notoriously private Castro decided to go public about her condition unlike her ailing brother Fidel Castro.
Fidel Castro fell ill in 2006 of a as yet unspecified stomach ailment. Raul Castro formally took over as President in 2008.
CNN's Robert Lenz contributed to this report.