Incidentaloma CT scan

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:

Overview

[Location] CT scan may be helpful in the diagnosis of [disease name]. Differentiation between benign and malignant incidentaloma is important. Malignancy is suggested on CT by a large diameter more than 6 cms, irregular border, inhomogeneity, a “washout” of contrast after 15 min of less than 40%, and calcifications. Contrast-enhanced washout CT utilizes the unique perfusion pattern of adenomas. Adenomas take up intravenous CT contrast rapidly, but also have a rapid loss of contrast – a phenomenon termed ‘contrast enhancement washout’. It is assumed that malignant adrenal lesions usually enhance rapidly but demonstrate a slower washout of contrast medium.

CT scan

• On CT scanning, differentiation between benign and malignant incidentaloma is important. Malignancy is suggested on CT by a large diameter more than 6 cms, irregular border, inhomogeneity, a “washout” of contrast after 15 min of less than 40%, and calcifications.
• However, many malignant lesions look benign. A large diameter is not diagnostic; only about 25% of masses larger than 4 cm were adrenal cancer.

The Hounsfield unit differentiation

• CT has a high quantitative contrast resolution, which allows assessment of tissue density by measuring X-ray absorption of tissues.
• This allows calculation of tissue attenuation or tissue density values, which are measured in Hounsfield units (HU).

• The Hounsfield unit (HU) is a quantity commonly used in computed tomography (CT) scanning to express CT numbers in a standardized and convenient form.
• Hounsfield units are obtained from a linear transformation of the measured attenuation coefficients.

• The intracytoplasmic fat in adenomas results in low attenuation on nonenhanced CT in contrast to nonadenomas have higher attenuation in nonenhanced CT.

• Typical pre-contrast Hounsfield unit (HU) values are adipose tissue = -20 to -150 HU and kidney = 20 to 150 HU.

• If an adrenal mass measures <10 HU on unenhanced CT, the likelihood that it is a benign adenoma is nearly 100 percent.

• Up to 30 percent of adenomas do not contain large amounts of lipid.

• A homogeneous adrenal mass with a smooth border and an attenuation value <10 HU on unenhanced CT is likely to be a benign adenoma.

• The only patients in the nonadenoma groups with a noncontrast CT HU <10 were those with myelolipomas.

Contrast-enhanced washout CT

• Contrast-enhanced washout CT utilizes the unique perfusion pattern of adenomas. Adenomas take up intravenous CT contrast rapidly, but also have a rapid loss of contrast – a phenomenon termed ‘contrast enhancement washout’.
• It is assumed that malignant adrenal lesions usually enhance rapidly but demonstrate a slower washout of contrast medium.
• Fifteen minutes after administration of contrast, an absolute contrast medium washout of more than 40 percent was reported to be 100 percent sensitive and specific for adenoma when patients with adenomas were compared with carcinomas, pheochromocytomas, and metastases.

CT without contrast showing adrenal adenoma with HU less than 10, source: Case courtesy of A.Prof Frank Gaillard, Radiopaedia.org, rID: 26

CT shows cortical mass fungating from the lower pole of the left kidney, predominantly of fat density, with areas of soft tissue component and prominent vascularity within. most ommonly angiomyolipoma, source: Case courtesy of Dr Abdallah Khateeb , Radiopaedia.org, rID: 44940