The two-time Emmy winner, who plays grumpy neighbor Mrs. McCluskey on the hit TV series, discovered she had the disease after a routine check with doctors last week.
And the former smoker, who also appeared on The West Wing, has been left devastated by the shocking news -- because she beat lung cancer eight years ago.
She tells People.com, "I felt like the legs had been pushed out from under me. I completely did not expect it, and was devastated. I was crying for nearly five days straight."
But Joosten insists she won't be beaten by the illness, and has the full backing of her bosses.
She adds, "They're totally supportive. I said, 'If you want to put it in the storyline, do it! Tell anybody you want, because the public's going to know.'
"I've got a little hang-up here but we're going to handle it and move forward."
Joosten revealed all about her latest cancer setback during a speaking engagement in Oklahoma, where she was booked to talk about surviving the disease.
She said, "To get back in control, I have to attack. I have to make this something that I can have an effect on."
And she's keen to erase "the stigma of lung cancer," adding, "The first thing everyone says is, 'Did you smoke?' Yeah, I smoked. I smoked because Bette Davis said it was very glamorous. I smoked because it was seen everywhere and done everywhere. I got addicted because the tobacco companies add additives to their tobacco to make it more addictive.
"I'm damned mad at all of them. But that stigma has to go away... No one deserves lung cancer."
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