Differentiation of adrenal adenoma and nonadenoma in unenhanced CT: new optimal threshold value and the usefulness of size criteria for differentiation

Abstract

Objective: To determine the optimal threshold for the attenuation values in unenhanced computed tomography (CT) and assess the value of the size criteria for differentiating between an adrenal adenoma and a nonadenoma.

Materials and methods: The unenhanced CT images of 45 patients at our institution, who underwent a surgical resection of an adrenal masses between January 2001 and July 2005, were retrospectively reviewed. Forty-five adrenal masses included 25 cortical adenomas, 12 pheochromocytomas, three lymphomas, and five metastases confirmed by pathology were examined. The CT images were obtained at a slice thickness of 2 mm to 3 mm. The mAs were varied from 100 to 160 and 200 to 280, while the 120 KVp was maintained in all cases. The mean attenuation values of an adrenal adenoma and nonadenoma were compared using an unpaired t test. The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy at thresholds of 10 HU, 20 HU, and 25 HU were compared. The diagnostic accuracy according to the size criteria from 2 cm to 6 cm was also compared.

Results: The twenty-five adenomas showed significantly lower (p < 0.05) attenuation values (mean+/-SD; 16.3+/-14.9) than the nonadenomas (38.1+/-6.8). Nineteen (90%) of the 20 nonadenomas had attenuation values ranging from 30 to 50 HU. The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy for diagnosing adenomas were 36%, 100%, 100%, 56%, and 64%, respectively, at a threshold of 10 HU; 60%, 100%, 100%, 67%, and 78%, respectively, at a threshold of 20 HU; and 72%, 95%, 95%, 73%, and 82%, respectively, at a threshold of 25 HU. The adenomas had a significantly (p < 0.05) smaller diameter (2.44+/-1.24 cm) than the nonadenomas (5.09+/-2.37 cm). The size criteria using a diameter of 4-6 cm showed a sensitivity > 90% but a specificity < 70%. Size criteria of 2 or 3 cm had a high specificity of 100% and 80% but a low sensitivity of 20% and 60%.

Conclusion: The threshold attenuation values of 20 or 25 HU in the unenhanced CT appear optimal for discriminating an adrenal adenoma from a nonadenoma. The size criteria are of little value in differentiating adrenal masses because of their low specificity or low sensitivity.

Fig. 1

Scattergram of the attenuation values in the unenhanced CT of 25 adenomas and each group of 20 nonadenomas (NA-L: lymphoma, NA-M: metastasis, NA-P: pheochromocytoma). Note that all masses with a HU < 20 were adenomas. Among the 20 nonadenomas, none had an attenuation value < 20, and eighteen (90%) of the nonadenomas had attenuation values > 30 HU.

Fig. 2

Scattergram of the adenomas and nonadenomas according to their size.

Fig. 3

Receiver operating characteristic curve for unenhanced CT attenuation value in enabling differentiation of adenomas from nonadenomas (A) and receiver operating characteristic curve for the size of adrenal lesions (B). The area under the receiver operating characteristic curve for CT attenuation values (0.904±0.044) was larger than that for size (0.860±0.060), indicating a greater ability of unenhanced CT attenuation value for distinguishing adenomas from nonadenomas.

Fig. 4

Example of a typical adrenal adenoma in the noncontrast image. A 1.6 × 1.2 cm, well-defined, low attenuated nodule (arrow) is shown in the left adrenal gland with a HU value of -13.6. This nodule was confirmed to be an adrenal adenoma by a pathological examination.

Fig. 5

Non-contrast CT image of a 57 year-old female adrenal gland, showing a 2.9 × 2.7 cm, round-shaped nodule (arrow) in the left adrenal gland with a HU value of 38.7. The nodule was confirmed to be a benign adrenal adenoma by a pathological examination.

Fig. 6

Noncontrast abdominal CT scan of 17-year-old male, showing a 5.4 × 3.3 cm right adrenal mass (arrow) with a HU value of 33.4, which was suggestive of a nonadenoma rather than an adrenal adenoma. This mass was confirmed by a pathological examination to be a right adrenal metastasis from a renal cell carcinoma.