Story highlights Missouri Sen. Claire McCaskill said Monday she's been diagnosed with breast cancer

The Missouri Democrat said she'll be in St. Louis for treatment the next three weeks

Washington (CNN) Sen. Claire McCaskill has been diagnosed with breast cancer, she announced Monday afternoon.

The Missouri Democrat announced her diagnosis on social media. She said in a post on Tumblr that "my prognosis is good and I expect a full recovery."

I very recently learned that I have breast cancer... - https://t.co/VYOi5pohqP — Claire McCaskill (@clairecmc) February 22, 2016

She said she'll be in St. Louis for the next three weeks receiving treatment -- and will post on her Senate website how she would have voted on any matters before the Senate.

"During this time my staff will continue to assist Missourians and I will be posting on my Senate website (McCaskill.senate.gov) 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," McCaskill said on Tumblr . "Additionally, I'll be submitting questions in writing for any missed Senate hearings."

She added, "Thank you for the honor of serving you in the Senate."

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