Distinguishing benign from malignant adrenal masses
© International Cancer Imaging Society 2003
Accepted: 5 November 2002
Published: 5 May 2015
The approach to the radiological and clinical evaluation of adrenal masses in the oncologic and non-oncologic patient is discussed. In addition, the value of unenhanced and enhanced CT densitometry with emphasis on the washout features to distinguish between lipid-rich and lipid-poor adenomas and malignant lesions is detailed. The roles of magnetic resonance imaging and positron emission tomography in distinguishing benign from malignant adrenal masses will also be discussed.
KeywordsAdrenal gland CT MR neoplasms PET
Silent or incidental adrenal tumors
a known underlying malignancy
clinically overt or biochemical evidence for adrenal dysfunction.
In patients with no known malignancy, the most important differentiation is between an adrenal carcinoma and an incidental non-functioning adrenal adenoma. Most adrenal cancers do ‘function’, and can be diagnosed by elevated biochemical markers and therefore distinguished from incidental adenomas.
Most surgeons will surgically remove all masses larger than 5 cm in size, irrespective of their imaging appearances (except for classical adrenal cysts and adrenal myelolipomas), as the incidence of adrenal carcinoma tends to be higher in masses of this size. Masses ranging in size between 3 and 5 cm are in the ‘grey zone’. They require additional imaging such as unenhanced/chemical-shift MRI to determine whether or not the lesion is a lipid-rich adenoma, in addition to biochemical evaluation to exclude a functioning lesion/mass. Smaller lesions (1–3 cm) with a non-specific appearance are usually presumed, in the absence of adrenal dysfunction, to represent adenomas, although a confirmatory imaging exam will usually be used to prove their benign nature.
Metastatic disease to the adrenal glands without the detection of a primary neoplasm is very rare.
enhanced CT and washout calculations
Enhanced CT densitometry and washout features of adenomas and non-adenomas
Washout features of lipid-poor adenomas
Magnetic resonance imaging
Comparison of unenhanced CT densitometry and chemical-shift MRI
In two studies, these two techniques were used to evaluate the same group of patients with adrenal adenomas. It was shown that there was linear correlation between unenhanced CT numbers of the adrenal masses, and relative loss of SI on chemical-shift MRI[13,14]. This suggests that both techniques evaluate the same tissue composition of the adenoma, i.e. its lipid component. In instances where unenhanced CT numbers were indeterminate, chemical-shift MRI was not helpful and vice versa. So whilst one of the two tests could be used to determine whether or not an adrenal mass is a lipid-rich adenoma, the two tests used in conjunction are not complementary.
Histological correlation of unenhanced CT densitometry and chemical-shift changes on MR in adrenal adenomas
University of Michigan adrenal mass characterisation protocol using CT
The optimal threshold value for a percentage enhancement washout of greater than or equal to 60% has a specificity of 95% and sensitivity of 79–89% for adenoma diagnosis.
Although adrenal biopsy is a very valuable tool in the differentiation between metastases and adrenal adenoma, it is less frequently used than in days prior to the use of unenhanced CT densitometry and chemical-shift MRI. CT and MR are now being used to triage which patients will require biopsy or operation. So at our institution, adrenal biopsies are performed only when imaging studies are equivocal and a malignant lesion is strongly suspected. References
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