Tierney's spokeswoman Christina Papadopoulos says she "has finished treatment and is doing very well and is happy to move forward with the next phase of her life."
She is filming four episodes scheduled to air in 2011, the series' seventh and final season.
Tierney said in August after filming the pilot of "Parenthood" that she was leaving to continue treatment. Lauren Graham then joined the series, which premiered in February.
The 45-year-old Tierney is best known as a longtime cast member of the NBC drama "ER."