Permeability of Brain Tumor Vessels Induced by Uniform or Spatially Microfractionated Synchrotron Radiation Therapies.

PURPOSE: To compare the blood-brain barrier permeability changes induced by synchrotron microbeam radiation therapy (MRT, which relies on spatial fractionation of the incident x-ray beam into parallel micron-wide beams) with changes induced by a spatially uniform synchrotron x-ray radiation therapy. METHODS AND MATERIALS: Male rats bearing malignant intracranial F98 gliomas were randomized into 3 groups: untreated, exposed to MRT (peak and valley dose: 241 and 10.5 Gy, respectively), or exposed to broad beam irradiation (BB) delivered at comparable doses (ie, equivalent to MRT valley dose); both applied by 2 arrays, intersecting orthogonally the tumor region. Vessel permeability was monitored in vivo by magnetic resonance imaging 1 day before (T-1) and 1, 2, 7, and 14 days after treatment start. To determine whether physiologic parameters influence vascular permeability, we evaluated vessel integrity in the tumor area with different values for cerebral blood flow, blood volume, edema, and tissue oxygenation. RESULTS: Microbeam radiation therapy does not modify the vascular permeability of normal brain tissue. Microbeam radiation therapy-induced increase of tumor vascular permeability was detectable from T2 with a maximum at T7 after exposure, whereas BB enhanced vessel permeability only at T7. At this stage MRT was more efficient at increasing tumor vessel permeability (BB vs untreated: +19.1%; P=.0467; MRT vs untreated: +44.8%; P<.0001), and its effects lasted until T14 (MRT vs BB, +22.6%; P=.0199). We also showed that MRT was more efficient at targeting highly oxygenated (high blood volume and flow) and more proliferative parts of the tumor than BB. CONCLUSIONS: Microbeam radiation therapy-induced increased tumor vascular permeability is: (1) significantly greater; (2) earlier and more prolonged than that induced by BB irradiation, especially in highly proliferative tumor areas; and (3) targets all tumor areas discriminated by physiologic characteristics, including those not damaged by homogeneous irradiation.

Assessment of multiparametric MRI in a human glioma model to monitor cytotoxic and anti-angiogenic drug effects.

Early imaging or blood biomarkers of tumor response is needed to customize anti-tumor therapy on an individual basis. This study evaluates the sensitivity and relevance of five potential MRI biomarkers. Sixty nude rats were implanted with human glioma cells (U-87 MG) and randomized into three groups: one group received an anti-angiogenic treatment (Sorafenib), a second a cytotoxic drug [1,3-bis(2-chloroethyl)-1-nitrosourea, BCNU (Carmustine)] and a third no treatment. The tumor volume, apparent diffusion coefficient (ADC) of water, blood volume fraction (BVf), microvessel diameter (vessel size index, VSI) and vessel wall integrity (contrast enhancement, CE) were monitored before and during treatment. Sorafenib reduced tumor CE as early as 1 day after treatment onset. By 4 days after treatment onset, tumor BVf was reduced and tumor VSI was increased. By 14 days after treatment onset, ADC was increased and the tumor growth rate was reduced. With BCNU, ADC was increased and the tumor growth rate was reduced 14 days after treatment onset. Thus, the estimated MRI parameters were sensitive to treatment at different times after treatment onset and in a treatment-dependent manner. This study suggests that multiparametric MR monitoring could allow the assessment of new anti-tumor drugs and the optimization of combined therapies.

Preferential effect of synchrotron microbeam radiation therapy on intracerebral 9L gliosarcoma vascular networks.

PURPOSE: Synchrotron microbeam radiation therapy (MRT) relies on spatial fractionation of the incident photon beam into parallel micron-wide beams. Our aim was to analyze the effects of MRT on normal brain and 9L gliosarcoma tissues, particularly on blood vessels. METHODS AND MATERIALS: Responses to MRT (two arrays, one lateral, one anteroposterior (2 × 400 Gy), intersecting orthogonally in the tumor region) were studied during 6 weeks using MRI, immunohistochemistry, and vascular endothelial growth factor Western blot. RESULTS: MRT increased the median survival time of irradiated rats (×3.25), significantly increased blood vessel permeability, and inhibited tumor growth; a cytotoxic effect on 9L cells was detected 5 days after irradiation. Significant decreases in tumoral blood volume fraction and vessel diameter were measured from 8 days after irradiation, due to loss of endothelial cells in tumors as detected by immunochemistry. Edema was observed in the normal brain exposed to both crossfired arrays about 6 weeks after irradiation. This edema was associated with changes in blood vessel morphology and an overexpression of vascular endothelial growth factor. Conversely, vascular parameters and vessel morphology in brain regions exposed to one of the two arrays were not damaged, and there was no loss of vascular endothelia. CONCLUSIONS: We show for the first time that preferential damage of MRT to tumor vessels versus preservation of radioresistant normal brain vessels contributes to the efficient palliation of 9L gliosarcomas in rats. Molecular pathways of repair mechanisms in normal and tumoral vascular networks after MRT may be essential for the improvement of such differential effects on the vasculature.

High-precision radiosurgical dose delivery by interlaced microbeam arrays of high-flux low-energy synchrotron X-rays.

Microbeam Radiation Therapy (MRT) is a preclinical form of radiosurgery dedicated to brain tumor treatment. It uses micrometer-wide synchrotron-generated X-ray beams on the basis of spatial beam fractionation. Due to the radioresistance of normal brain vasculature to MRT, a continuous blood supply can be maintained which would in part explain the surprising tolerance of normal tissues to very high radiation doses (hundreds of Gy). Based on this well described normal tissue sparing effect of microplanar beams, we developed a new irradiation geometry which allows the delivery of a high uniform dose deposition at a given brain target whereas surrounding normal tissues are irradiated by well tolerated parallel microbeams only. Normal rat brains were exposed to 4 focally interlaced arrays of 10 microplanar beams (52 microm wide, spaced 200 microm on-center, 50 to 350 keV in energy range), targeted from 4 different ports, with a peak entrance dose of 200Gy each, to deliver an homogenous dose to a target volume of 7 mm(3) in the caudate nucleus. Magnetic resonance imaging follow-up of rats showed a highly localized increase in blood vessel permeability, starting 1 week after irradiation. Contrast agent diffusion was confined to the target volume and was still observed 1 month after irradiation, along with histopathological changes, including damaged blood vessels. No changes in vessel permeability were detected in the normal brain tissue surrounding the target. The interlacing radiation-induced reduction of spontaneous seizures of epileptic rats illustrated the potential pre-clinical applications of this new irradiation geometry. Finally, Monte Carlo simulations performed on a human-sized head phantom suggested that synchrotron photons can be used for human radiosurgical applications. Our data show that interlaced microbeam irradiation allows a high homogeneous dose deposition in a brain target and leads to a confined tissue necrosis while sparing surrounding tissues. The use of synchrotron-generated X-rays enables delivery of high doses for destruction of small focal regions in human brains, with sharper dose fall-offs than those described in any other conventional radiation therapy.

Assessment of blood volume, vessel size, and the expression of angiogenic factors in two rat glioma models: a longitudinal in vivo and ex vivo study.

Assessment of angiogenesis may help to determine tumor grade and therapy follow-up. In vivo imaging methods for non-invasively monitoring microvasculature evolution are therefore of major interest for tumor management. MRI evaluation of blood volume fraction (BVf) and vessel size index (VSI) was applied to assess the evolution of tumor microvasculature in two rat models of glioma (C6 and RG2). The results show that repeated MRI of BVf and VSI - which involves repeated injection of an iron-based MR contrast agent - does not affect either the physiological status of the animals or the accuracy of the MR estimates of the microvascular parameters. The MR measurements were found to correlate well with those obtained from histology. They indicate that microvascular evolution differs significantly between the two glioma models, in good agreement with expression of angiogenic factors (vascular endothelial growth factor, angiopoietin-2) and with activities of matrix metalloproteinases, also assessed in this study. These MRI methods thus provide considerable potential for assessing the response of gliomas to anti-angiogenic and anti-vascular agents, in preclinical studies as well as in the clinic. Furthermore, as differences between the fate of tumor microvasculature may underlie differences in therapeutic response, there is a need for preclinical study of several tumor models.