Michael Tilson Thomas conducts a performance of Copland's Appalachian Spring by The Boston Symphony Orchestra in Lenox, Mass. in 2018. Thomas, who led the San Francisco Symphony for 25 years, was diagnosed with a brain tumor in 2021. Paul Marotta/Getty Images hide caption toggle caption Paul Marotta/Getty Images Conductor, composer and educator Michael Tilson Thomas, who led the San Francisco Symphony for 25 years, establishing its reputation as a world-class orchestra, died on Wednesday at his home in San Francisco, according to a statement from his publicist, Constance Shuman. He was 81 years old. In 2021, Thomas announced that he was being treated for a brain tumor. In 2022, he acknowledged that he had been battling glioblastoma multiforme, an aggressive brain cancer. Thomas held the position of music director at the San Francisco Symphony [SFS] from 1995 to 2020 and was by far the orchestra's longest-serving music director.…