Imaging an adrenal mass

  • Incidental masses in 5% of abdominal scans
  • 6% of people >60 years carry an adrenal adenoma. 70-80% of these are non-functioning
  • Prevalence of 9-13% in patients with an extra-adrenal malignancy
  • In a patient with known carcinoma, only 26 – 36% are metastatic 
European Society of Endocrinology Clinical Practice Guidelines for adrenal masses
  1. Unenhanced CT should be performed to confirm a lipid-rich adenoma (HU<10)
  2. If unenhanced CT is indeterminate, and clinically non-functioning, 3 options
    1. Immediate imaging with another technique (contrast-enhanced adrenal protocol CT, MRI with chemical shift imaging or FDG PET). FDG PET is only done in the setting of extra-adrenal malignancy
    2. Interval imaging in 6-12 months to assess lesion growth
    3. Surgical resection
  3. MRI recommended in children and adults <40 years, in pregnant women
  4. In setting of extra-adrenal malignancy, if CT/MRI confirms benign nature, no further follow-up is needed
  5. Biopsy recommended in an extra-adrenal malignancy setting if 
    1. Lesion is hormonally inactive
    2. Lesion is indeterminate on imaging
    3. Management will be altered by knowing histology of the lesion
Intracellular fat
  • >70% adenomas are rich in intracellular fat
  • If lesion is 10 HU or less, specificity for adenoma is 98%, sensitivity 71%
  •  <10 HU lesions in an unenhanced CT warrant no further imaging – diagnostic for lipid rich adenoma
Contrast enhancement and washout
  • Adenoma enhance rapidly and contrast washout is also rapid
  • Malignancy: lesion enhances rapidly, with slower contrast washout
  • 3 phases: unenhanced, 60 sec and 15 minutes post contrast
  • Electronic cursor should encompass ⅔ of lesion
  • Absolute contrast enhancement washout (ACEW) = (60 sec – 15 min) / (60 sec – unenhanced) x 100
  • ACEW > 60% favors adenoma. Sensitivity 86-88%, specificity 92-96%
  • Relative contrast enhancement washout (RCEW) = (60 sec – 15 min) / 60 sec x 100
  • RCEW > 40% favors adenoma. Sensitivity 96%, specificity 100%
  • This rule does not hold good for pheochromocytomas – they can have a benign or malignant pattern and has to be correlated with clinical/biochemical tests
  • Plain and contrast enhanced imaging do not contribute much to distinguishing adrenal masses
  • Chemical shift imaging is key
  • In and out of phase images can be used to identify intracellular lipid. Lipid rich adenomas demonstrate signal drop in out of phase imaging 
  • 90% adenomas show homogeneous or ring enhancement
  • 60% malignant lesions have heterogeneous enhancement
  • Visual assessment of signal drop in out of phase imaging is sufficient but quantitative techniques are also available
  • Adrenal mass to spleen signal intensity ratio or the signal intensity index may be used
  • Adrenal lesion to spleen ratio (ASR) of 70 or less is 100% specific for adenomas, 78% sensitive
  • Signal intensity index  >5% suggest adenoma. Less than 5% suggest metastasis 

Out of phase adrenal lesion signal / out of phase spleen signal
ASR   = ———————————————————————————————-    x 100
                      In phase adrenal lesion signal / in phase spleen signal 

In phase lesion signal – out of phase lesion signal
SII        =  ————————————————————————————  x 100
        In phase lesion signal

  • Subtraction MRI: out phase images subtracted from in phase images. Adenomas demonstrate higher signal intensities. Quantitatively, mean signal intensity from ROI calculated. Cut off value above 106 for adenomas and below 36 for metastasis 
  • Inconclusive in distinguishing benign and malignant lesions
  • C-11 metomidate (MTO) -marker of 11 beta hydroxylase, can trace adrenocortical tissue lesions 
Adrenal scintigraphy
  • Iodocholesterol-labelled analogues (I-131 6 beta iodomethyl 19 norcholesterol NP-59 and Se-75 6 beta selenomethyl cholesterol – picks up adrenocortical tumors as does I-123 MTO
  • I-123 MIBG localizes pheochromocytoma