In a therapeutic approach using the suicide gene HSV-1-tk fused to eGFP, we demonstrated a highly significant therapeutic effect for both lentiviral vectors compared to control groups. Using MRI to follow tumor growth, we detected complete remission in 7 out of 8 animals for LCMV-GP and VSV-G pseudotyped vectors after 30 days of ganciclovir treatment. HSV-1-tk has been reported to be an effective therapeutic gene in previous studies [19, 41] . The limited success in clinical studies has been a result of inefficient gene delivery systems rather than lack of efficacy of the suicide mechanism [11] . However, there is still space for improvement of the prodrug delivery such as application length, treatment intervals and route of delivery. Tai et al. demonstrated that multiple cycles of prodrug application are superior over one cycle of prodrug [42] . In our study, we still detected GFP-positive tumor cells after one cycle (30 days) of ganciclovir administration in treated animals indicating that application in cycles might also improve the therapeutic effect in this setting. Further, these results demonstrate that vector-transduced tumor cells retain the ability to invade brain tissue and migrate even to distant brain regions.