Tierney, 44, will undergo surgery to remove a tumor found on her breast and said doctors assured her the condition is "very treatable."
"I have discovered a tumor in my breast which requires surgery," the Emmy-nominated star said in a statement. "I'm very optimistic as to the outcome and ... look forward to going back to work soon."
Production of the new fall NBC series "Parenthood" was halted after Tierney was diagnosed with the condition.
Tierney said she but chose to release a clarifying statement on her condition because NBC's announcement about the show's delay last Friday "has unfortunately caused some confusion and undue alarm," she said.
Tierney played Dr. Abby Lockhart on 10 seasons of the medical drama "ER," which ended in March after 15 years. She has also appeared in films "Liar Liar" and "Baby Mama" and starred in the TV hit series "News Radio."