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Letters to the Editor |
Department of Radiology, Rhode Island Hospital, Brown University School of Medicine, 593 Eddy Street, Providence, RI 02903. e-mail: wmayo-smith@lifespan.org
Editor:
Dr Caoili and colleagues are to be congratulated for their article about computed tomographic (CT) characterization of adrenal masses in the March issue of Radiology (1). The past decade has been an exciting era of research on the adrenal gland, as recent findings have demonstrated that noninvasive CT techniques can help to reliably differentiate the majority of benign adrenal adenomas from adrenal metastases. This is clinically important because benign adrenal adenomas are common, and the adrenal gland is also a common site of metastases. The presence of adrenal metastases affects the staging of many cancers (particularly lung carcinoma), and reliable staging determines effective treatment options.
As the authors point out, there are two main features of adrenal lesions that are useful in the differentiation of adenomas from metastases: lipid content and enhancement characteristics. The majority of adenomas (approximately 80%) contain lipid and result in low attenuation on CT scans and signal drop-off on chemical shift magnetic resonance (MR) images. Thus, unenhanced CT or chemical shift MR imaging can be used to reliably characterize adrenal adenomas. For the remaining 20% of lipid-poor adenomas, which have similar attenuation as the metastases on unenhanced CT scans, CT enhancement washout calculations have recently been shown by Dr Caoili and colleagues and others (3–6) to help differentiate adenomas from metastases.
I have found that radiologists in general, and myself in particular, are not fond of memorizing calculations. To this end, I point out that the calculation of "relative percentage of enhancement washout" as used by Dr Caoili and colleagues is the same calculation as that used by Pena et al (2) to calculate the relative percentage washout in the characterization of adrenal masses. The calculation of the percentage of enhancement washout, which requires a preenhancement attenuation value, was not specifically addressed by Pena et al.
On the basis of the current literature, perhaps the following algorithm is useful for the practicing radiologist to adopt.
First, for patients undergoing dedicated CT for evaluation of a known adrenal mass, the first step is an unenhanced CT examination. If the attenuation value of the lesion is less than or equal to 10 HU, no further imaging is required because the lesion is diagnostic of an adrenal adenoma. If the attenuation value of the adrenal mass is greater than 10 HU, contrast material–enhanced CT should be performed and delayed imaging should be used to calculate the percentage of enhancement washout. To calculate this washout, use the following equation: percentage of enhancement washout = (attenuation value at dynamic CT - attenuation value at delayed CT)/(attenuation value at dynamic CT - attenuation value at unenhanced CT) x 100. On the basis of findings of the Caoili et al study, a 60% or greater enhancement washout value is diagnostic of an adenoma.
Second, for patients undergoing unenhanced CT for other reasons, an attenuation measurement of the adrenal mass should be performed. If the attenuation value of the adrenal mass is less than 10 HU, no further work-up is required as the finding is diagnostic of an adenoma. If the attenuation value is greater than 10 HU, patients with a known malignancy should be brought back and a dedicated adrenal protocol as just described should be performed.
Third, for a patient in whom an adrenal mass was discovered at dynamic enhanced CT, a delayed scan should be obtained (if the patient is still on the CT table) along with a relative percentage of enhancement washout. To calculate this washout, use the following equation: relative percentage of enhancement washout = (attenuation value at dynamic CT - attenuation value at delayed CT)/(attenuation value at dynamic CT) x 100. For this calculation, on the basis of the results of Caoili and colleagues, a relative percentage of enhancement washout greater than or equal to 40% is diagnostic of an adenoma and no further work-up is required.
Three points remain for Dr Caoili and colleagues when one compares their results with those of Pena et al. First, Pena et al used a delay of 10 minutes, whereas Dr Caoili and colleagues used a 15-minute delay. Second, Pena et al found that an attenuation value of less than or equal to 30 HU at delayed imaging was diagnostic of an adenoma. Third, Dr Caoili and colleagues used a relative percentage of enhancement washout value of 40%, whereas Pena et al used a value of 50%. As the radiologic literature is "zeroing in" on a standard protocol, it would be helpful for Dr Caoili and colleagues to compare their results with those of Pena et al and specifically comment on point 1, the optimal time for delayed imaging; point 2, by using their data, estimate the accuracy of a 30-HU cutoff to differentiate adenomas from metastases; and point 3, analyze their data by using the relative percentage of contrast enhancement washout value of 50%, as advocated by Pena et al.
REFERENCES
Department of Radiology, University of Michigan Medical Center, 1500 East Medical Center Drive, Ann Arbor, MI 48109-9723. e-mail: caoili@umich.edu
We thank Dr Mayo-Smith for his insightful comments. The use of CT techniques to identify an adrenal adenoma is certainly clinically relevant in that many more adrenal masses are being discovered with the increased use of cross-sectional imaging. Also, adrenal adenomas are common. As described in our article (1), our protocol is similar to the one described by Dr Mayo-Smith. Patients suspected of having an adrenal mass first undergo unenhanced CT examination. If the attenuation value of the lesion is less than or equal to 10 HU, no further imaging is required. If the attenuation value is greater than 10 HU, contrast-enhanced and delayed contrast-enhanced imaging is performed to determine the lesion’s washout characteristics. For those adrenal masses that are initially discovered at contrast-enhanced CT, we recommend the dedicated adrenal adenoma protocol as well. This protocol has a sensitivity of 98% and a specificity of 92%.
Dr Mayo-Smith referenced the work of Pena et al (2) and raised several issues. First, the choice for an imaging delay is arbitrary; however, the threshold value used with a chosen delay needs to be validated. Korobkin et al (3) found that a 15-minute delay provides a better combination of sensitivity and specificity (96% for both) compared with a 10-minute delay. Second, by using our data, the accuracy of a 30-HU cutoff to differentiate adenomas from metastases is not optimal. With this criterion, only two (9%) of 22 lipid-poor adenomas in our study population would have been detected. The sensitivity of the protocol would have decreased to 84%. Third, by using a threshold of 50% for the relative percentage enhancement washout to distinguish an adenoma is also not sufficient. With this criterion, only nine (41%) of 22 lipid-poor adenomas in our study population would have been detected. The sensitivity of the protocol would have decreased to 90%. In fact, several of Dr Pena’s colleagues (Drs Boland and Mueller) have more recently concluded (4) that benign lesions frequently demonstrate less than 50% relative washout (10 of 39 [26%]), which suggests that the optimal threshold is probably closer to 40% than to 50%.
In summary, although alternatives exist for identification of an adrenal adenoma, a combination of unenhanced CT and delayed contrast-enhanced CT is most effective. The use of this protocol takes advantage of two independent properties of adrenal adenomas: lipid content and enhancement washout characteristics and allows nearly all of the adrenal masses to be diagnosed with high sensitivity and specificity.
REFERENCES
This article has been cited by other articles:
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  M. A. Haider, S. Ghai, K. Jhaveri, and G. Lockwood Chemical Shift MR Imaging of Hyperattenuating (>10 HU) Adrenal Masses: Does It Still Have a Role? Radiology, June 1, 2004; 231(3): 711 - 716. [Abstract] [Full Text] [PDF]
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