Cluster versus ROI analysis to assess combined antiangiogenic therapy and radiotherapy in the F98 rat-glioma model.

For glioblastoma (GBM), current therapeutic approaches focus on the combination of several therapies, each of them individually approved for GBM or other tumor types. Many efforts are made to decipher the best sequence of treatments that would ultimately promote the most efficient tumor response. There is therefore a strong interest in developing new clinical in vivo imaging procedures that can rapidly detect treatment efficacy and allow individual modulation of the treatment. In this preclinical study, we propose to evaluate tumor tissue changes under combined therapies, tumor vascular normalization under antiangiogenic treatment followed by radiotherapy, using a voxel-based clustering approach. This approach was applied to a rat model of glioma (F98). Six MRI parameters were mapped: apparent diffusion coefficient, vessel wall permeability, cerebral blood volume fraction, cerebral blood flow, tissue oxygen saturation and vessel size index. We compared the classical region of interest (ROI)-based analysis with a cluster-based analysis. Five clusters, defined by their MRI features, were sufficient to characterize tumor progression and tumor changes during treatments. These results suggest that the cluster-based analysis was as efficient as the ROI-based analysis to assess tumor physiological changes during treatment, but also gave additional information regarding the voxels impacted by treatments and their localization within the tumor. Overall, cluster-based analysis appears to be a powerful tool for subtle monitoring of tumor changes during combined therapies.

Evaluation of the relationship between MR estimates of blood oxygen saturation and hypoxia: effect of an antiangiogenic treatment on a gliosarcoma model.

PURPOSE: To assess the reproducibility of the magnetic resonance (MR) estimate of blood oxygen saturation (sO(2)) in the rat brain, to evaluate the relationship between low MR estimate of sO(2) values and tissue hypoxia in a hypoxic and necrotic glioscarcoma model (9L gliosarcoma cells), and to evaluate the capability of the MR estimate of sO(2) parameter to help identify modifications induced by an antiangiogenic treatment (sorafenib) in 9L gliosarcoma tumors. MATERIALS AND METHODS: Experiments were performed with permits from the French Ministry of Agriculture. Forty-eight male rats bearing a 9L gliosarcoma were randomized in untreated and treated (sorafenib) groups. MR blood volume fraction and MR estimate of sO(2) parameters were estimated 1 day before and 1, 3, 5, and 8 days after the start of the treatment. The in vivo MR estimate of sO(2) measurement was correlated with the ex vivo hypoxia assessment by using pimonidazole staining. Paired and unpaired t tests, as well as parametric Pearson tests, were used for the statistical analyses. RESULTS: In healthy tissues, MR estimate of sO(2) measurements were comparable to literature values and were reproducible (mean across all animals, 68.0% ± 6.5 [standard deviation]). In untreated tumors, MR estimate of sO(2) and immunohistochemical analysis yielded correlated fractional hypoxic-necrotic areas (R(2) = 0.81). In tumors treated with antiangiogenic therapy, tumor MR estimate of sO(2) was decreased with respect to the healthy tissue (P< .001). CONCLUSION: Results of this study suggest that the MR estimate of sO(2) is a reproducible estimate that could be used as an in vivo probe of hypoxia in brain tumors and as a sensitive reporter of the hypoxic effects of antiangiogenic therapies.

Monitoring blood-brain barrier status in a rat model of glioma receiving therapy: dual injection of low-molecular-weight and macromolecular MR contrast media.

PURPOSE: To evaluate the sequential injection of a low-molecular-weight (gadoterate meglumine [Gd-DOTA], 0.5 kDa) and a macromolecular (P846, 3.5 kDa) contrast media in monitoring the effect of antitumor therapies (antiangiogenic therapy and/or microbeam radiation therapy [MRT]) on healthy brain tissue and implanted tumors. MATERIALS AND METHODS: Animal use was compliant with official French guidelines and was assessed by the local Internal Evaluation Committee for Animal Welfare and Rights. Eighty male rats bearing 9L gliosarcoma were randomized into four groups: untreated, antiangiogenic (sorafenib) therapy, MRT, and both treatments. Magnetic resonance (MR) imaging was performed 1 day before and 1, 5, and 8 days after the start of the treatment. At all time points, vascular integrity to a macromolecular contrast medium (P846) and, 11 minutes 30 seconds later, to low-molecular-weight contrast medium (Gd-DOTA) was evaluated by using a dynamic contrast material-enhanced MR imaging approach. To quantify vessel wall integrity, areas under the signal intensity curves were computed for each contrast medium. Unpaired t tests and one-way analysis of variance were used for statistical analyses. RESULTS: Tumor vessels receiving antiangiogenic therapy became less permeable to the macromolecular contrast medium, but their permeability to the low-molecular-weight contrast medium remained unchanged. Healthy double-irradiated vessels became permeable to the low-molecular-weight contrast medium but not to the macromolecular contrast medium. CONCLUSION: Antiangiogenic therapy and MRT generate different effects on the extravasation of contrast medium in tumoral and healthy tissues. This study indicates that the use of a low-molecular-weight contrast medium and a macromolecular contrast medium provides complementary information and suggests that the use of two contrast media within the same MR imaging session is feasible.