Conductor and composer Michael Tilson Thomas, who led the San Francisco Symphony for 25 years and founded Miami’s New World Symphony, announced Friday his withdrawal from several public appearances after undergoing surgery for a brain tumor that “required immediate operation.”

Among the conductor’s canceled appearances is a hosting turn at the Kennedy Center’s 50th Anniversary Celebration Concert scheduled for Sept. 14, a multidisciplinary program that the 2019 Kennedy Center honoree curated and modeled after his mentor Leonard Bernstein’s 1962 “An American Pageant for the Arts.”

“I deeply regret missing projects that I was greatly anticipating,” Tilson Thomas, 76, said in a statement Friday. “I look forward to seeing everyone again in November.”

“I can’t begin to express my emotions in learning of Michael’s diagnosis,” Kennedy Center President Deborah Rutter said in a statement. “In addition to being a close personal friend, he is arguably one of the most important and influential artists of our time. We will miss him at our 50th Anniversary Celebration Concert but the essence and design of the program remains his intent. We offer him our love and best wishes for the next months on his journey to health.”

Tilson Thomas will undergo therapy expected to last several months at the UCSF Medical Center in San Francisco. The conductor was also forced to cancel dates with the Indianapolis Symphony Orchestra, the New World Symphony and the Chicago Symphony Orchestra.