Maura Tierney - who played Dr Abby Lockhart during ten series of the medical drama - is due to have surgery to remove a tumour found in her breast.
The 44-year-old actress has postponed filming of her new TV project Parenthood to receive the treatment.
"I will not know either my exact diagnosis or course of treatment until that surgery is performed. I'm very optimistic as to the outcome," the Emmy-nominated star said in a statement.
Doctors have told Maura that her condition is "very treatable."