MIAMI BEACH, Fla. -- Conductor Michael Tilson Thomas is resigning as artistic director of the New World Symphony after 34 seasons with the orchestra, wanting to lessen administrative responsibilities as he deals with a brain tumor.The 77-year-old had surgery last August for the tumor. He has conducted 20 concerts since then with the New York Philharmonic, the Los Angeles Philharmonic and the San Francisco Symphony.“Making music with these great artists and for so many friends in the audience felt like coming home, like coming back to life.