The attended effects of Gamma Knife radiosurgery (GKR) on brain tumors are mainly found only several months to years after irradiation. However, some early clinical and/or radiological changes after treatment have sporadically been observed.

     In this study we reviewed modifications in tumor size and contrast enhancement on MRI of brain tumors treated by GKR and that had a control MRI performed within the 3 weeks following irradiation.

     A series of 48 tumors irradiated by GKR and controlled by an MRI within the 3 weeks were studied.

     We co-registered 3D-sequences of the control MRI with the MRI of the planning in GammaPlan, and we created a tumor volume of the same lesion on both imaging. We noted 1) apparition or significant increase of a loss of contrast enhancement in the center of the tumor (often interpreted as a necrotic area); and 2) a significant (i.e. >25% of the initial volume) modification in tumor size.

     The tumors included 40 metastasis, 6 hemangiopericytomas and 2 meningiomas. Time interval between irradiation and the control MRI ranged from 1 to 19 days (mean 5 days ; median 4 days). No change was observed for 33 lesions (69%). An apparition or a significant increase of a loss of contrast enhancement in the center of the tumor occurred in 8 patients (17%). A significant reduction in tumor size was observed in 9 patients (19%) and a significant increase in tumor size in 1 patient (2%). These early radiological changes were not related to the radiation dose used or other dosimetric parameters but could be related to the tumor size.

     This is a retrospective study.

     Despite the main effects of GKR on brain tumors are visible several months to years, early radiological changes can occur in the following days after irradiation.

     This observation suggests presence of early effects of GKR on tumor cells and/or vascularization.