Provided herein are methods for using a thrombin peptide derivative to treat a medulloblastoma. In one embodiment, the method includes administering to a subject having a medulloblastoma an effective amount of a thrombin peptide derivative, and exposing cells of the medulloblastoma of the subject to a cancer treatment after administering the thrombin peptide derivative to the subject. An example of a cancer treatment is radiation therapy. In one embodiment, viability of cells of the medulloblastoma is decreased compared to viability of cells of the medulloblastoma before the administering and the exposing. The cells of the medulloblastoma having decreased viability can be cancer stem cells. In one embodiment, shrinkage of the medulloblastoma in the subject is increased compared to shrinkage of the medulloblastoma before the administering and the exposing.