Abstract Methylation of the O6-methylguanine-DNA methyltransferase (MGMT) promoter predicts favorable response to temozolomide in adult-type diffuse gliomas. Accurate MGMT results require adequate tumor cellularity. Too many admixed non-neoplastic cells (which are always unmethylated) may cause false-negative results. Currently, ~70% cellularity, based on estimation by microscopy, is recommende...