The Pennsylvania Democrat says that while the news "came as a shock," his outlook for the future is "excellent."
The 62-year-old said his doctors expect him to make a full recovery after undergoing surgery in "coming months." He does not expect his illness to impact his work in the Senate and predicted "minimal disruption" to his office.
His fellow Pennsylvanians and Senate colleagues took to social media Thursday to lend moral support.
A Scranton native, Sen. Casey is the son of the late Gov. Bob Casey Sr. Before his election to the Senate in 2006, Casey served two terms as state Auditor General and one term as state Treasurer.
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