A 28 year old man underwent an abdominal CT for suspicion of appendicitis and was found to have an incidental right adrenal nodule.
He underwent an adrenal protocol CT, here are the arterial phase images:
Here are the multiphase images through the enhancing right adrenal nodule:
The attenuation of the nodule measures:
Unenhanced: 28 HU
Arterial: 150 HU
Venous: 134 HU
Delay: 52 HU
This nodule is a pathology-proven pheochromocytoma.
Absolute adrenal washout is calculated using unenhanced, venous, and 15 minute delayed acquisitions.
- (Venous – Delay) / (Venous – Unenhanced) * 100
An absolute washout of >60% has been reported as diagnostic of adrenal adenoma (Caoili EM, et al. AJR 2000;175:1411-5), and this nodule has washout of 77%.
However, pheochromocytomas may also have significant washout on delayed phase, though they are significantly more hyperenhancing in the arterial/venous phases. A venous phase attenuation of >110 HU has been described as suspicious for pheochromocytoma. Additionally, up to 25% of pheochromocytomas enhance most avidly in arterial phase, but this is rare in adenomas. Northcutt BG, et al. AJR 2014;201:834-9. Northcutt BG, et al. MDCT of the Indeterminate Adrenal Mass: Identification of a Venous Enhancement Level to Distinguish Pheochromocytoma from Adenoma. Presented at RSNA 2013. Note that conflicting data does exist. Patel G, et al. AJR 2013;201:122-7.
A more specific approach to imaging pheochromocytoma may be provided by molecular imaging. An established method is using Iodine-123 Meta-iodo-benzyl-guanidine (MIBG), a guanethidine analog similar to norepinephrine which localizes in presynaptic adrenergic neurons.
Here is this patient’s I-123 MIBG scan:
There is focal intense radiotracer uptake in the patient’s right adrenal pheochromocytoma.
Molecular imaging has an established role in identifying lymph node and distant metastases in patients with malignant pheochromocytoma, and tumor-specific radiotracers provide opportunity for targeted radiotherapy.