PRODUCTION of the new fall series “Parenthood” was shut down last week because one of its stars, Maura Tierney, has breast cancer, the actress said yesterday.

“I have discovered a tumor in my breast which requires surgery,” Tierney said in a statement. ” I’m very optimistic as to the outcome and . . . look forward to going back to work soon.”

NBC, which said the series’ debut would now have to be pushed back until early next year, created some mystery when it refused to say last Friday what was wrong with Tierney.

“In an effort to guard my privacy, it seems that the wording of NBC’s press release has unfortunately caused some confusion and undue alarm about my health,” Tierney acknowledged yesterday.