Gamma knife radiosurgery compared to whole brain radiation therapy enhances immunity via immunoregulatory molecules in patients with metastatic brain tumours

Background: There is lack of data on the effect of stereotactic radiosurgery in modulation of the immune system for cancer patients with metastatic brain tumours. Therefore, we investigated the change in levels of immunoregulatory molecules after Gamma Knife radiosurgery (GKR) and whole brain radiation therapy (WBRT) in patients with brain metastases. Methods: Peripheral blood samples were collected from 15 patients who received GKR, nine patients who received WBRT for brain metastases and 10 healthy controls. Samples were obtained at three time points such as before, 1h after and 1 week after the index procedure for patients treated with GKR or WBRT. All patients' demographic data and radiosurgical parameters were retrospectively reviewed. We analyzed the change in the levels of T-lymphocyte-associated antigen 4 (CTLA-4) and programmed cell death ligand-1 (PD-L1), and cytokines such as IL-2, IL-10, IFN-gamma, TNF-alpha after GKR and WBRT using Enzyme-linked immunosorbent assays (ELISA). Results: Baseline level of IFN-gamma was found to be lower and that of PD-L1 was higher in the GKR group compared to WBRT group and healthy controls (p < 0.05 and p < 0.01, respectively). Levels of IFN-gamma and IL-2 were increased (p < 0.01 and p < 0.01, respectively), while CTLA-4 and PD-L1 were decreased (p = 0.05 and p = 0.01, respectively) after GKR compared to pre-GKR levels, while there was no change after WBRT. Conclusion: GKR regulates immunoregulatory molecules towards enhancing the immune system, while WBRT did not exert any effect. These findings suggested that treatment of metastatic brain lesion with GKR might stimulate a systemic immune response against the tumour.