Glioblastoma multiforme (GBM) is the most aggressive type of primary brain tumor. GBM growth dynamics vary widely across patients, making it difficult to accurately gauge their response to treatment. We developed a model-based metric of therapy response called Days Gained that accounts for this heterogeneity. Here we demonstrate in 63 newly diagnosed GBM patients that Days Gained scores from a simple GBM growth model computed at the time of the first post-radiation therapy MRI scan are prognostic for time to tumor recurrence and overall patient survival. After radiation treatment, Days Gained also distinguished patients with pseudoprogression from those with true progression. Since Days Gained scores can be easily computed with routinely available clinical imaging devices, it offers immediate potential to be used in ongoing prospective studies.