Adrenal Adenoma and Differentiation from Other Adrenal Masses
Incidental discovery of adrenal masses is a common clinical problem as a result of the widespread use of imaging procedures, occurring in up to 5% of patients who have undergone abdominal CT.57 Most adrenal masses are benign nonhyper-functioning adenomas even in patients with a known extra-adrenal malignancy (Figure 31.9).57 Primary adrenal carcinoma is rare, with a reported incidence of 2 cases per million (Figure 31.10). Differentiation of benign and malignant adrenal masses is critical to determine appropriate treatment. With advances in CT and MR imaging with dedicated imaging protocols, characterization of these adrenal masses has greatly increased in accuracy.
CT is generally considered the modality of choice for initial characterization of the adrenal mass. A noncontrast scan should be first performed, and a contrast-enhanced CT scan may be necessary if the noncontrast scan is not con-clusive.58 MRI is reserved for cases that have indeterminate findings on CT.58,59
Generally, larger adrenal lesions have a greater likelihood of being malignant (see Figure 31.10). In 39 patients with extraadrenal malignancy, 87% of lesions smaller than 3 cm were benign and 95% of lesions greater than 3 cm were malignant.60 In another study, among 45 adrenal masses greater than 5 cm found by imaging studies, 33% were malignant.61 Because of significant overlap of benign and malignant lesions based on size criteria alone, most authorities recommend that masses greater than 4 or 5 cm should be biopsied or surgically excised. Increase in size of the lesion during follow-up can be helpful in predicting malignancy. Adenomas tend to have smooth margins and a homogeneous density, whereas metastases can be heterogeneous and have an irregular shape. However, these are not specific signs of malignancy.58
The presence and amount of lipid in many adrenal adenomas accounts for their low attenuation on unenhanced CT scans and their loss in relative signal intensity on chemical shift MR images (see Figure 31.9). There is an inverse linear relationship between the percentage of lipid-rich cortical cells
figure 31.10. Adrenal carcinoma. (A) Axial T2-weighted breath-hold half Fourier single-shot fast spin-echo image shows 8-cm mass in the right adrenal gland (large arrow). There are gallstones (small arrows). (B) Axial T1-weighted gradient echo image with fat suppression obtained after gadolinium intravenous contrast material shows enhancement of the adrenal mass. (C) Coronal T1-weighted gradient echo image with fat suppression obtained after gadolinium intravenous contrast agent injection shows the mass clearly above the right kidney (arrow), displacing the right kidney medially and laterally.
in adrenal adenomas and the unenhanced CT attenuation number and a similar inverse linear relationship to the relative change in MR signal intensity on chemical shift images. Metastases, on the other hand, have little intracytoplasmic fat and therefore do not have low attenuation at nonenhanced CT.58 Similarly, metastases do not lose signal on chemical shift MR images.
Studies have been performed to determine an adequate threshold to differentiate benign versus malignant adrenal masses on nonenhanced CT. Boland et al. performed a meta-analysis of 10 studies to determine an optimal density threshold to differentiate benign from malignant lesions, and reported that using 10 HU as a threshold, noncontrast CT has 71% sensitivity and 98% specificity for characterizing adrenal masses.62
If the adrenal mass is more than 10 HU at noncontrast CT, CT with intravenous contrast material should be performed to evaluate enhancement washout characteristics. Adenomas lose enhancement more rapidly than nonadenomas, and delayed CT attenuation value was used to differentiate adenomas from nonadenomas. Using an attenuation measurement of less than 30 to 40 HU at 10 to 30 minutes after contrast injection, the adrenal mass is almost always an adenoma.63 A relative percentage enhancement washout of greater than 50% calculated by (1 - delayed enhanced HU value/initial enhanced HU value) x 100 has a sensitivity of 98% and specificity of 100% for benign disease.64 Other investigators used percentage enhancement washout calculated by (initial enhanced HU value - delayed enhanced HU value)/ (initial enhanced HU value - unenhanced HU value) with a threshold of 60% at 50 to 80 seconds after administration of contrast material for the initial scan and 15 minutes for the delayed scan, and achieved 86% to 98% sensitivity and 92% to 96% specificity.65,66
When CT with intravenous contrast is equivocal, chemical shift MR imaging should be performed as chemical shift MR imaging is the most sensitive technique for differentiating adenomas from metastases of the adrenal gland.58 With the chemical shift MR technique, the sensitivity and specificity for differentiating adenomas from metastasis ranges from 81% to 100% and 94% to 100%, respectively.67-70 Reported cases of nonadenomas that met these CT or MRI criteria for adenoma, included pheochrcomocytoma,71 metastasis from renal cell carcinoma, and adrenocortical carcinoma.66,72
Pheochromocytoma is usually benign, but approximately 10% of these lesions are malignant. CT is the study of choice to confirm the diagnosis when a pheochromocytoma is suspected on clinical and laboratory grounds.58 On CT, pheochromocytoma appears as a well-defined mass with marked enhancement after intravenous contrast administration. MR findings may enable characterization of pheochromocytoma because the signal intensity of these tumors may be very high on T2-weighted images, probably caused by cystic components. However, there is considerable overlap between the MR appearance of pheochromocytoma and other adrenal lesions. The sensitivity and specificity of MR imaging for diagnosing pheochromocytoma were 64.7% and 88.0%, respectively.73
Adrenal biopsy is required when imaging studies cannot accurately characterize an adrenal mass. Harisinghani et al. analyzed 225 oncologic patients who had undergone CT-guided biopsies of an adrenal mass that were indeterminate at CT or MRI and reported that a negative or benign pathology can be regarded as a true-negative evaluation with no necessity to repeat the biopsy.59
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