Utility of MRI to Differentiate Clear Cell Renal Cell Carcinoma Adrenal Metastases From Adrenal Adenomas

OBJECTIVE. The purpose of this study is to compare MRI features of clear cell renal cell carcinoma (RCC) adrenal metastases and adenomas. MATERIALS AND METHODS. Fifteen clear cell RCC adrenal metastases imaged with MRI were compared to 29 consecutive adenomas between 2006 and 2015. Two blinded radiologists assessed homogeneity (homogeneous vs heterogeneous), signal intensity (SI) decrease on chemical-shift MRI, and T2-weighted SI (isointense, mildly hyperintense, or markedly hyperintense) relative to muscle. A third blinded radiologist measured the chemical-shift SI index, adrenal-to-spleen SI ratio, T2-weighted SI ratio, AUC for contrast-enhanced MRI, and histogram analysis. Analyses were performed using chi-square, linear regression, ROC, and logistic regression modeling. RESULTS. Clear cell RCC metastases were larger than adenomas (mean [+/- SD], 5.0 +/- 4.2 cm [range, 1.1-15 cm] vs 2.0 +/- 0.7 cm [range, 1.2-3.7 cm]; p < 0.0001). Subjectively, 33.3% (5/15) of metastases and 68.9% (20/29) of adenomas showed an SI decrease on chemical-shift MRI (p = 0.0421; kappa = 0.76). Chemical-shift SI index (mean, 9.2% +/- 20.6%; range, -30.0% to 57.9%) and adrenal-to-spleen SI ratio (0.94 +/- 0.23 [range, 0.44-1.33]) for metastases differed significantly from those for adenomas (47.3% +/- 27.8% [range, -9.4% to 86%] and 0.52 +/- 0.28 [range, 0.13-1.11], respectively) (p < 0.0001). Twenty percent (3/15) of metastases had chemical-shift SI index in the adenoma range (> 16.5%). Metastases had higher T2-weighted SI than did adenomas, both quantitatively (5.1 +/- 3.0 [range, 1.5-10.6] vs 1.8 +/- 0.8 [range, 0.5-3.8]; p < 0.0001) and subjectively (p < 0.0001; kappa = 0.89). Metastases had higher entropy than did adenomas (6.76 +/- 0.61 vs 6.1 +/- 0.74; p = 0.0051) and were subjectively more heterogeneous (p < 0.0001; kappa = 0.86). The contrast-enhanced MRI AUC, skewness, and kurtosis did not differ between groups (p > 0.05). The ROC AUCs were 0.91 (95% CI, 0.79-1.0) for T2-weighted SI ratio and 0.85 (95% CI, 0.68-1.0) for entropy. The logistic regression model of T2-weighted SI ratio plus entropy improved accuracy (ROC AUC, 0.97; 95% CI, 0.93-1.0]) compared with either feature alone (p = 0.0215). CONCLUSION. Increased T2-weighted SI and heterogeneity are features that can differentiate clear cell RCC adrenal metastases from adenomas using quantitative and subjective analysis.