A specimen from the tumor site that subsequently manifested rapid clinical progression contained a <i>PIK3CA</i> mutation E542K, and yielded propagating xenografts that retained the OD/AOD-defining genomic alterations (<i>IDH1</i><sup>R132H</sup> and 1p/19q codeletion) and <i>PIK3CA</i><sup>E542K</sup>, and displayed characteristic sensitivity to alkylating chemotherapeutic agents.