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Discussion of Adrenal adenoma
- Adrenal adenoma is a common tumor, with a prevalence of 3% in autopsy series.
- As the majority of lesions are small and nonfunctional, most adenomas are incidental findings.
- Hyperfunctional adenomas also occur and are responsible for important endocrine disorders such as Cushing syndrome and Conn syndrome.
- Lipid-rich adenomas (approximately 80% of adenomas) are easily identified at both CT and MR imaging.
Imaging Findings for Adrenal adenoma
- Adrenal adenomas appear as small (<3 cm), well-defined homogeneous masses that are typically hypoattenuating relative to the liver.
- At an attenuation value of less than 0 HU at unenhanced CT, the diagnosis of an adenoma can be made with 100% confidence; however, this threshold has only 47% sensitivity.
- At cutoff of 18 HU, a diagnosis of adenoma was made with 100% specificity and 85% sensitivity, compared to the specificity:sensitivity ratio of 68%:100% with a more conservative cutoff of 10 HU.
- A rational approach advocated by some authorities is to choose the CT number threshold on the basis of the patient’s risk for metastatic disease. For example, a threshold of 10 HU could be applied to older patients or to those with known primary malignancies. A threshold of 18 HU could be applied to younger patients without underlying cancer.
- Lipid-poor adenomas are more difficult to diagnose because the CT numbers increase and approach those of soft tissue.
- Contrast-enhanced imaging with 10-minute-delayed CT scans may be helpful in these cases.
- By using a threshold of 30 HU, the sensitivity and specificity for delayed contrast-enhanced CT in the characterization of benign disease are 80% and 100%, respectively.
- A relative percentage washout of more than 50% in the delayed study represents a sensitivity and specificity of 98% and 100%, respectively, for the detection of adenoma.
- Chemical shift MR imaging can be used for further characterization when CT results are indeterminate.
- Because of the high sensitivity of chemical shift MR imaging to minute amounts of intravoxel fat, MR imaging demonstrates signal intensity loss on opposed-phase images in the majority of adenomas, with a sensitivity of 89% for lesions with an attenuation of 10–30 HU and 100% for lesions with an attenuation of 10–20 HU with a maintained specificity of 100%.
References for Adrenal adenoma
- Jose M. Pereira, Claude B. Sirlin, Pedro S. Pinto, and Giovanna Casola. CT and MR Imaging of Extrahepatic Fatty Masses of the Abdomen and Pelvis: Techniques, Diagnosis, Differential Diagnosis, and Pitfalls. RadioGraphics 2005 25: 69-85.