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- Most non-functional adrenal masses can be definitively characterized with CT; functional
masses are best diagnosed by clinical examination and laboratory testing
- Adrenal protocol CT has a high sensitivity and specificity for differentiating between benign
and malignant masses
- Non-functional masses indeterminate on CT may be further characterized by MRI, PET, or
masses are found incidentally in about 3% of patients undergoing CT or
MRI examinations for other purposes. Unless the patient is known to
have cancer, the vast majority of lesions <4 cm in diameter are
benign adenomas; but other possibilities must be considered, including
metastasis, primary cancer, or a functional mass. As many as 15% of
incidentally detected adrenal masses are functional, some of which are
sub-clinical. Endocrine work-up should be obtained to evaluate these
Adrenal imaging is an important diagnostic tool
for differentiating benign from metastatic non-functional adrenal
masses. On the other hand, clinical examination and laboratory tests
are much more sensitive than radiology for the diagnosis of functional
masses, such as pheochromocytoma, or aldosteroma, and adenomas
associated with Cushing’s syndrome.
Non-Functional Adrenal Masses
If clinical and laboratory findings indicate that an adrenal mass is
non-functional, fine collimation non-contrast multi-detector CT is the
best initial imaging examination. If a recent non-contrast CT scan is
available, it may be unnecessary to perform an additional scan. If the
attenuation of the mass measures <10 Hounsfield units (HU), it is
highly unlikely to be malignant and non-functional lesions <0 HU are
considered benign. All lesions that are not hemorrhagic or calcified
but have an attenuation of >43 HU should be regarded as non-adenomas
and suspicious for malignancy.
If the density of the adrenal mass is >10 HU, an adrenal protocol CT
examination should be performed. In this protocol, images are obtained
before and at 75 seconds and 10 minutes after administration of
contrast. The relative percentage washout (RPW) and absolute percentage
washout (APW) of contrast agent is then calculated. At a threshold of
38% for RPW, the sensitivity and specificity of detecting malignant
lesions has been shown to be 100% and 95%, respectively and that for
APW, at a threshold of 52%, is 100% and 98%, respectively. In this
analysis, lesions that have pre-contrast attenuations <0 are
considered benign and those over 43 HU as malignant.
Non-Functional Adrenal Lesions Indeterminate on Adrenal Protocol CT
|Reported Accuracy of Imaging for Distinguishing Malignant from Benign Adrenal Masses
|Contrast CT with delayed washout images2
|PET (maximum SUV*)3
|PET/CT with delayed washout CT images (preliminary data)3
*SUV, standard uptake values
Data from 2Blake et al, 2006a; 1Mayo-Smith, et al, 2001; 3Blake et al, 2006b.
If the adrenal protocol CT examination is indeterminate and the patient
has a known extra-adrenal neoplasm, then the adrenal mass may be a
metastasis and a CT-guided biopsy should be considered if needed for
treatment planning. If there is no history of known extra-adrenal
neoplasm, further imaging may help determine the identity of the mass.
Chemical shift imaging is the most accurate MRI method for
distinguishing between adenomas and malignancies. In this method, T1
weighted images are acquired at echo times that are in phase and out of
phase. In adenomas, out-of-phase signal intensity is lower than that on
in-phase images. The sensitivity of chemical shift MRI varies with CT
attenuation and is 89% for masses in the range of >10 to <30 HU
and 67% for all the adenomas in the study, all of which had an
attenuation >10. The specificity for detecting adenomas is 100%
across the full range of attenuations. MRI is generally not used
to characterize small (<1 cm) masses because of its lower resolution
compared to CT.
FDG PET has excellent sensitivity for detecting adrenal malignancy and
may be used to evaluate indeterminate adrenal lesions. Fusion PET-CT
has the advantage of better image co-registration than separate PET and
CT images acquired separately and preliminary findings indicate that
both the sensitivity and specificity of using PET-CT and determining
both FDG uptake and CT contrast agent APW is near 100%.
If no conclusive categorization of adrenal mass has been obtained,
follow-up unenhanced CT imaging is recommended at 2, 6, and 18 months
after the initial discovery of the adrenal lesion. These times are
based on the expected growth rate of an adrenal carcinoma.
Figure 1. CT
images of a 1.3 cm left adrenal nodule (arrows) in a 42 year old
woman representing a typical adrenal adenoma with (A) unenhanced
attenuation of 4HU, (B) IV contrast enhanced (performed for other
indications) attenuation of 64 HU and (C) 10 minute delayed attenuation
of 30 HU. The adenoma has a relative percentage washout of 53% and
an absolute percentage washout of 57%.
|General CT Features of Benign and Malignant Adrenal Lesions
Often contain lipid (Unenhanced HU often <10)
Smooth border, round
Rapid washout of contrast
Not lipid-containing (Unenhanced HU usually >10)
Irregular border and shape
Slow washout of contrast
Large size (>4 cm)
With the exception of pheochromocytoma, no additional imaging is
generally necessary for diagnosis of functional masses.
Pheochromocytomas are rare catecholamine-secreting tumors that can
cause cardiovascular crises and are usually diagnosed with clinical
evaluation and laboratory testing. However, approximately 10-40% of
pheochromocytomas are clinically silent and can mimic other adrenal
lesions on both CT and MR imaging. Pheochromocytomas have been
reported to represent 1.5-9% of adrenal masses incidentally detected on
cross-sectional imaging in patients with no history of cancer.
There is some theoretical concern when administering iodinated contrast
material to a patient with a clinically suspected pheochromocytoma but
intravenous use of current non-ionic intravenous agents has been
reported as acceptable practice in such cases. The most common
appearance of a pheochromocytoma in MR images is a mass with high T2
and low T1 signal intensity, which avidly enhances with contrast
material. However, many do not fit this description and the diagnostic
accuracy of MRI for pheochromocytoma is about 65%. If needed, nuclear
scintigraphy using 123I-metaiodo-benzylguanidine
(MIBG) can be used to confirm pheochromocytoma. This imaging technique
has a specificity of 100% but a low sensitivity. MIBG scans can also be
used to search for a clinically suspected extra-adrenal
Radiology examinations may be ordered through ROE (http://mghroe
) or by
telephone 617-724-XRAY (9729) for all locations. CT is performed at the
main campus as well as Mass General West Imaging, Waltham and Mass
General Imaging, Chelsea. Nuclear imaging is performed at the MGH Main
Campus and Mass General West Imaging, Waltham.
For further questions, please contact Michael A. Blake, M.R.C.P.I, F.R.C.R., F.F.R., R.C.S.I.
, Abdominal and Interventional Radiology, 617-726-8396.
We would like to thank Dr. Blake as well as Giles W. Boland, M.D.,
Abdominal and Interventional Radiology, and Paul M. Copeland, M.D.,
Endocrine Unit, Department of Medicine, for their assistance and advice
for this issue.
This article provided useful information about the appropriate use of imaging studies:
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Blake, MA, Kalra, MK, Sweeney, AT, Lucey, BC, Maher, MM, Sahani, DV,
Halpern, EF, Mueller, PR, Hahn, PF and Boland, GW. (2006) Distinguishing benign from malignant adrenal masses: multi-detector row CT protocol with 10-minute delay. Radiology: 578-585
Blake, MA, Kalra, MK, Maher, MM, Sahani, DV, Sweeney, AT, Mueller, PR, Hahn, PF and Boland, GW. (2004) Pheochromocytoma: an imaging chameleon. Radiographics 24: S87-99
Blake, MA, Slattery, JM, Kalra, MK, Halpern, EF, Fischman, AJ, Mueller, PR and Boland, GW. (2006) Adrenal lesions: characterization with fused PET/CT image in patients with proved or suspected malignancy--initial experience. Radiology 238: 970-7
Copeland, PM. (1999) The incidentally discovered adrenal mass: an update. The Endocrinologist 9: 415-423
Mayo-Smith, WW, Boland, GW, Noto, RB and Lee, MJ. (2001) State-of-the-art adrenal imaging. Radiographics 21: 995-1012