Adrenal adenoma and metastasis from clear cell renal cell carcinoma: can they be differentiated using standard MR techniques?

BACKGROUND Chemical-shift magnetic resonance imaging (MRI) has been known to successfully differentiate adenomas from metastases. However, there has been concern that metastasis from extra-adrenal primary malignancies which contain high lipid content such as clear cell renal cell carcinoma (RCC) could mimic adrenal adenomas. PURPOSE To evaluate the ability of MR to differentiate adrenal adenoma from metastasis using chemical-shift imaging and MR feature analysis in patients with clear cell RCC. MATERIAL AND METHODS This study was institutional review board-approved; informed consent was waived. Eleven patients with 13 metastases and 13 patients with 15 adrenal adenomas in patients with clear cell RCC for evaluation of an adrenal mass underwent MR. Signal intensity on in- and opposed-phases, signal intensity index (SII), size, T2 SI, cystic change, necrosis, and hemorrhage were evaluated. Statistical analyses included Student t-test and Fisher exact test. If available, precontrast CT attenuation of the adrenal adenomas was measured. SII was correlated with attenuation using Pearson correlation coefficient. RESULTS Mean size of adenomas was smaller than that of metastases (P < 0.002). Mean SII of adenomas (45.0% ± 24.6) was significantly greater than that of metastases (6.6% ± 4.7; P < 0.001). With a threshold of 16.5% for SII, the sensitivity, specificity, and accuracy for adenomas were 80%, 100%, and 89.2%, respectively. All six lipid-rich adenomas were diagnosed as adrenal adenoma. Three of eight (37.5%) lipid-poor adenomas were misdiagnosed as metastases. While up to 53.8% (7/13) of the metastases demonstrated cystic change, necrosis, or hemorrhage, only one (6.7%) adenoma exhibited cystic change or necrosis (P < 0.05 for all). Precontrast attenuation and SII were significantly correlated: r = -0.810 (P < 0.001). CONCLUSION In patients with clear cell RCC who underwent MR for adrenal masses, SII and MR features such as cystic change, necrosis, and hemorrhage were helpful in differentiating adenomas from metastases.