The Hall of Famer learned Thursday that he has "several new spots/tumors" on his brain, Carter's daughter, Kimmy Bloemers, wrote on the family website.
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"I write these words with tears because I am so sad for my dad," Bloemers wrote. "Dr. Jimmy Harris will be coming to my parent's house this evening to talk to the family about the next step."
Carter, 57, was diagnosed last May with four small tumors on his brain. His battle against the illness has largely kept him out of the public eye.
Carter spoke at his charity golf event this week in Florida, but he admitted he was not feeling well. Bloemers, in her latest blog entry, detailed how Carter's condition had deteriorated.
At a doctor's appointment on Jan. 12, Carter "had a very bad fall in the hallway," Bloemers wrote. He received immediate care, and further tests showed that Carter had torn his rotator cuff. It requires surgery to repair, Bloemers wrote, "but all Dad can do right now is rehab to heal."
Carter managed in the Mets' minor league system in 2005 and 2006, hoping to one day manage the major league team. He quit after the 2006 season rather than accept a job at the Double-A level. Carter managed the Long Island Ducks for one season in 2009.
Bloemers is scheduled to accept an award on behalf of her father at the Baseball Writers Association of America New York chapter's dinner Saturday night in Manhattan.
The Mets broadcast a "Get Well, Kid" message on their scoreboard during home games last season, referring to Carter by his well-known nickname. SNY broadcaster Ron Darling has been particularly involved in Carter's cause. The former Mets pitcher filmed a public-service announcement for Stand Up To Cancer and held a sign reading, "I stand up for my catcher."
Carter, elected to the Hall of Fame in 2003, played five seasons for the Mets and was a key member of the 1986 world championship team.