Background: Glioblastoma multiforme (GBM) is one of the deadliest cancers characterized by very limited sensitivity to chemo- and/or radiotherapy. The presence of GBM stem-like cells in the tumor might be relevant for GBM treatment resistance. Aim: To provide a proof-of-concept of the efficacy of photon activation therapy (PAT) using monochromatic synchrotron radiation (SR), in killing GBM stem cells pre-treated with cisplatin. Materials and Methods: Irradiation was performed using a 1-8 Gy dose range and energies just above or below the platinum K-shell edge (78.39 keV) or with a conventional X-ray source. Cells were exposed to drug concentrations allowing 90% cell survival, mimicking the unfavourable tissue distribution generally achieved in GMB patients. Results: a significant enhancement in cell lethality was observed using SR compared to conventional X-ray irradiation. Conclusion: PAT deserved to be further explored in in vivo models based on GBM stem-like cells.