In vivo two-photon microscopy study of short-term effects of microbeam irradiation on normal mouse brain microvasculature

Purpose: The purpose of this study was to assess the early effects of microbeam irradiation on the vascular permeability and volume in the parietal cortex of normal nude mice using two-photon microscopy and immunohistochemistry. Methods and Materials: The upper part of the left hemisphere of 55 mice was irradiated anteroposteriorly using 18 vertically oriented beams (width 25 fun, interdistance 211 mu m; peak entrance doses: 312 or 1000 Gy). At different times after microbeam exposure, the microvasculature in the cortex was analyzed using intravital two-photon microscopy after intravascular injection of fluorescein isothiocyanate (FITC)-dextrans and sulforhodamine B (SRB). Changes of the vascular volume were observed at the FITC wavelength over a maximum depth of 650 mu m from the dura. The vascular permeability was detected as extravasations of SRB. Results: For all times (12 h to I month) after microbeam irradiation and for both doses, the FITC-dextran remained in the vessels. No significant change in vascular volume was observed between 12 h and 3 months after irradiation. Diffusion of SRB was observed in microbeam irradiated regions from 12 h until 12 days only after a 1000 Gy exposure. Conclusion: No radiation damage to the microvasculature was detected in normal brain tissue after a 312 Gy microbeam irradiation. This dose would be more appropriate than 1000 Gy for the treatment of brain tumors using crossfired microbeams. (c) 2006 Elsevier Inc.