MIAMI BEACH, Fla. (AP) — 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. Tilson Thomas says he was diagnosed with a glioblastoma multiforme, an aggressive tumor and that it currently is in check. Tilson Thomas becomes the New World Orchestra's artistic director laureate and intends to work with orchestra fellows and alumni.