She told interviewer Anderson Cooper: "I was advised to [do so]. I was contracted to go into a movie at that time, 'Little Black Book' with Brittany Murphy, who I miss very much. My doctors at the time, they had to get insurance approval and all of that so I was very quiet about it and had to go back to work right away."
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Kathy, 63, says she also wanted to go through the experience on her own despite having a supportive network of family and friends.
She said: "Nobody else really knows what you're going through except another cancer patient. Even though your family's supportive and surrounds you, I just got to the point where I would go to chemo by myself and just really go through it on my own."
However, the 'Misery' star - who successful beat the disease - says she would do things differently if it were to happen again.
She added: "I admire people who have been open, like Melissa Etheridge and women I see walking around facing it without wigs and all of that stuff. I think I'd be more courageous next time."