U.S. Sen. Claire McCaskill, a top surrogate for Democratic presidential candidate Hillary Clinton and a mainstay on national talk shows, announced Monday she has breast cancer.

The second-term Missouri Democrat said in her announcement she expects a �full recovery� after three weeks of upcoming treatment in St. Louis. She and her staff didn�t offer additional details about her diagnosis.

McCaskill, 62, known for her acumen with social media, made the announcement via Tumblr on Monday afternoon. Her statement said she �very recently learned that I have breast cancer,� detected through a regular mammogram.

�It�s a little scary, but my prognosis is good and I expect a full recovery,� she said in the statement.

McCaskill has long been a leading voice in Washington on sexual assault and other issues and has emerged as a surrogate for Clinton�s presidential campaign, most recently kicking off the opening of Clinton�s St. Louis campaign office last week.

Clinton tweeted of McCaskill on Monday: �If ever cancer picked the wrong person to mess with, it�s you. Stay strong and take care of yourself. I�m in your corner. -H�

McCaskill�s announcement gives no details about what stage cancer it is or what type of treatment she faces, and her staff said no further information was immediately available.

But a Washington University Medical School physician said the fact that it was detected through a mammogram and that she is being treated for just three weeks are positive signs.

�Most women with cancer detected mammographically are going to have Stage 1,� said Mike Naughton, associate professor of medicine at Washington University�s Siteman Cancer Center. He said that would normally entail surgery and radiation treatment, which is short term with relatively quick recovery time, as opposed to chemotherapy, which would entail months of treatment and a longer recovery.

�With surgery, obviously, it would have a short-term effect � but most women go on to a normal life.�

McCaskill�s statement said that during her treatment, she will post on her Senate website �how I would have voted on any matters that come before the Senate during my absence � which I�ll also enter into the Congressional record.�