Can Adrenal Adenomas Be Differentiated From Adrenal Metastases at Single-Phase Contrast-Enhanced CT?

Objective: The purpose of this study is to evaluate whether adrenal metastases can be reliably differentiated from adenomas at single-phase contrast-enhanced CT.

Materials and methods: Sixty-one consecutive patients from a single-institution lung cancer registry (40 metastases and 36 adenomas) who underwent single-phase contrast-enhanced CT at baseline diagnosis were retrospectively studied by two radiologists (blinded to the diagnoses) who independently evaluated four features previously described in adenomas: smooth margin, rim enhancement, central vein sign (preserved adrenal vein), and homogeneity (using a 5-point Likert scale). A third radiologist measured size and attenuation and performed quantitative texture analysis. Comparisons were performed using chi-square, logistic regression, and ROC analysis.

Results: Metastases were larger than adenomas (mean [± SD] 24 ± 11 mm [range, 11-66 mm] vs 19 ± 5 mm [range, 11-34 mm]; p = 0.012), with overlap between groups. Attenuation of metastases and adenomas did not differ significantly (58.2 ± 21.0 HU [range, 21.0-108.0] vs 55.5 ± 21.5 HU [range, 14.0-105.0]; p = 0.582). Skewness and kurtosis did not differ between groups (p = 0.612 and 0.978, respectively), whereas entropy was higher in metastases (p = 0.013). The AUC for entropy to diagnose metastases was 0.65 (95% CI, 0.52-0.77). Tumor margin, rim enhancement, and the central vein sign did not differ between groups (p > 0.05). Metastases were considered more heterogeneous by both radiologists (p = 0.001 and 0.011, respectively), and agreement was satisfactory (κ = 0.51). Likert scores of 4 or 5 (mostly or completely heterogeneous) yielded sensitivity and specificity for diagnosis of metastases of 32.5% and 97.2%, respectively, for radiologist 1 and 22.5% and 97.2%, respectively, for radiologist 2.

Conclusion: Adrenal metastases cannot be reliably differentiated from adenomas at single-phase contrast-enhanced CT. Increased tumor size and heterogeneity were specific findings but showed unacceptably low sensitivity.