BACKGROUND: Patients with tumors that recur from a prior LGG have quite different outcomes depending on their histological subtype, grade, and molecular/cytogenetic features. Non-invasive biomarkers that accurately reflect the biological properties of the lesion are urgently needed to make informed decisions about patient care. The primary objective of this study was to evaluate the role of noninvasive imaging parameters as biomarkers of malignant progression in diffuse LGG (WHO II) and to use these parameters to select the most metabolically active regions of tumors for characterization of their histological features. METHODS: After obtaining informed consent, 125 patients undergoing surgical resection of tumors that recurred from a prior LGG were prospectively enrolled from 2008-2012 in an institutional review board approved protocol. Preoperative MR metabolic and physiological images were used to select image guided tissue sample targets for histological status, IDH1 mutation and ex vivo metabolic profiles. H&E and immunohistochemistry of the tissue samples provided the following parameters: tumor score, necrosis, CA9 status (measure of hypoxia), MIB-1 status, cellularity, SMI-31 status and vascular status. RESULTS: 44% of the 125 tumors were grade II at recurrence, with 56% having upgraded in histology (44% grade III and 12% grade IV). Significant differences were observed for in vivo measures of maximum choline to N-acetylasparate index (CNI) and lactate, normalized apparent diffusion coefficient <1.5, and normalized cerebral blood volume >2 for patients with upgraded versus non-upgraded lesions. CONCLUSIONS: The results from this study highlight tumor heterogeneity and underline the use of metabolic and physiologic imaging for non-invasively predicting upgrade status for patients with recurrent LGG who are not surgical candidates and also for directing the surgeon to the most malignant region of the lesion to get representative tissue for evaluation. These enhanced parameters will be utilized in a new cohort of 100 patients with recurrent LGG and their tissue will be used to characterize the genomic properties of non-upgraded versus upgraded samples. SECONDARY CATEGORY: n/a.