San Francisco Symphony Music Director Laureate Michael Tilson Thomas announced he has been diagnosed with glioblastoma multiforme, an aggressive form of brain cancer, according to an open letter released Wednesday, March 2.

This is the first time the longtime Symphony conductor has spoken publicly about the cancer that led him towithdraw from public appearancesfor three months in 2021.

In August, 77-year-old Thomas had surgery, followed by a course of radiation and chemotherapy.

“Currently, the cancer is in check,” he writes. “But the future is uncertain as glioblastoma is a stealthy adversary. Its recurrence is, unfortunately, the rule rather than the exception.”

The letter continues, announcing that Thomas has decided to step down as artistic director of New World Symphony, the Miami training orchestra that he co-founded and has headed for 34 years. He will assume the title of artistic director laureate, stepping back from administrative duties to concentrate on musical endeavors.

Since returning to the stage in November, Thomas has conducted 20 concerts with theSan Francisco Symphony, the New York Philharmonic and the Los Angeles Philharmonic, an experience he describes as “heartwarming.”

“Making music with these great artists and for so many friends in the audience felt like coming home, like coming back to life,” he writes. “I couldn’t have been happier.”

But he adds that these performances have made demands on his strength, and that it’s time “to consider what level of work and responsibilities I can sustain in the future.”

Thomas last conducted in Davies Symphony Hall in January, and he has no further appearances there that have been announced publicly though he said that he is planning to go forward with his appearances with orchestras in the U.S. and abroad.His websitelists imminent engagements in Montreal, Washington, D.C., Cleveland, London and Prague, among others.

He also singles out his husband, Joshua Robison, for gratitude. “He has always been by my side and is, as ever, my hero,” he writes.

“I will continue to compose, to write, and to mull over your thoughts and mine,” Thomas says in closing. “I’m planning more time to wonder, wander, cook, and spend time with loved ones — two-legged and four. Life is precious.”