In a Tumblr post titled "I very recently learned that I have breast cancer," McCaskill discusses being diagnosed, getting treatment and continuing to serve her constituents.
"It was detected through a regular mammogram," she writes. "It's a little scary, but my prognosis is good and I expect a full recovery."
McCaskill, 62, says that she'll receive treatment in St. Louis for the next three weeks.
"During this time my staff will continue to assist Missourians and I will be posting on my 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," she writes. "Additionally, I'll be submitting questions in writing for any missed Senate hearings.
"Thank you for the honor of serving you in the Senate," she finishes.
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Many politicians and political reporters took to Twitter soon after McCaskill's announcement to express their support for the senator, including Hillary Clinton, Massachusetts Senator Elizabeth Warren, Speaker of the House Paul Ryan, MSNBC reporter Rachel Maddow and New York Senator Chuck Schumer.
.@clairecmc 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— Hillary Clinton (@HillaryClinton) February 22, 2016
@clairecmc We are all with you, the sisterhood we never wanted to join. Together we are strong. Thinking of you on this journey— Andrea Mitchell (@mitchellreports) February 22, 2016
Wishing my very best for a speedy recovery to one of the Senate’s toughest fighters, @clairecmc. The whole Twitterverse is here for you!— Elizabeth Warren (@SenWarren) February 22, 2016
@clairecmc From one survivor to another, I know you’ll beat this with the same fighting spirit you give to Missouri each and every day.— D Wasserman Schultz (@DWStweets) February 22, 2016
Janna's & my thoughts are with @clairecmc during this trying time.— Paul Ryan (@SpeakerRyan) February 22, 2016
From one breast cancer survivor to another soon-to-be survivor – you’re a fighter and I’m cheering for you all the way, @clairecmc— Sen. Heidi Heitkamp (@SenatorHeitkamp) February 22, 2016
I don’t know anyone who’d choose to battle with @clairecmc. You’re going to win this fight, my friend.— Chuck Schumer (@SenSchumer) February 22, 2016
McCaskill, a Democrat, has served as a felony courtroom prosecutor, sat in the Missouri State Legislature and was the first female county prosecutor in Jackson County, the largest county in Missouri. She became the first female Missouri senator in 2006 and won re-election in 2012.