BACKGROUND AND PURPOSE: The differentiation between cerebral GBM and solitary MET is clinically important and may be radiologically challenging. Our hypothesis is that routine MR imaging with qualitative and quantitative analysis is helpful for this differentiation. MATERIALS AND METHODS: Forty-five GBM and 21 solitary metastases were retrospectively identified, with their preoperative routine MR imaging analyzed. According to the comparison of the area of peritumoral T2 prolongation with that of the lesion, the tumors were classified into grade I (prolongation area tumor area <= tumor area) and grade II (prolongation area > tumor area). The signal intensities of peritumoral T2 prolongation were measured on T2WI and normalized to the values of the contralateral normal regions by calculating the ratios. The ratio (nSI) of both types of tumors was compared in grade I, grade II, and in tumors without grading. The best cutoff values to optimize the sensitivity and specificity were determined for optimal differentiation. RESULTS: The nSI of GBM was significantly higher than that of MET without T2 prolongation grading (P < .001), resulting in AUC = 0.725. The difference was significant (P = .014) in grade I tumors (GBM, 38; MET, 9), with AUC = 0.741, and in grade II tumors (GBM, 7; MET, 12), with AUC = 0.869 (P = .017). Both types of tumors showed a different propensity in T2 prolongation grading (chi(2) = 12.079, P = .001). CONCLUSIONS: Combined with qualitative and quantitative analysis of peritumoral T2 prolongation, routine MR imaging can help in the differentiation between brain GBM and solitary MET.