The imaging provided demonstrates a CT adrenal washout study. CT adrenal washout studies are performed for lesions that do not satisfy diagnostic criteria for demonstrating the presence of lipid rich adenoma either by non contrast CT or by MRI paired T1 in and out of phase imaging. Note that it is a common error to perform an MRI examination following a non contrast CT examination that demonstrates CT Hounsfield units greater >10 HU, or inversely to perform a non contrast CT following the MRI examination that does not demonstrate significant signal dropout. As these tests are both evaluating the same physical characteristic, namely the presence of intra voxel fat, a surrogate of microscopic fat, there is no additional diagnostic benefit in performing both examinations.
CT adrenal washout studies are performed by performing a non contrast examination to establish the baseline attenuation of the adrenal lesion. A subsequent portal venous phase acquisition is performed, supplemented by a 15-minute phase evaluation. In principle benign adrenal adenomas, including those that are lipid poor, measuring >10 HU at baseline, will demonstrates significant enhancement and subsequent de-enhancement (washout). Malignant tumours demonstrate enhancement but less washout. It is important that measurements are performed with a representative region of interest (ROI) that encompasses generally greater than 70% of the lesion on the sections best demonstrating the lesion, rather than utilising small ROIs that may selectively demonstrate one region. Within larger lesions, however, it is prudent to perform more than one confirmatory large ROI measurement.
Absolute washout is defined as the following (PV=Portal venous, Pre=Precontrast):
100 x (PV HU - Delayed HU)/ (PV HU - Pre HU).
>60% suggests adenoma (sens 86%, spec 92%).
It is possible to also to consider only the portal venous and delayed phase (e.g. this is sometimes helpful with incidnetal adrenal nodules in two phase CT colonography studies). This results in relative washout measurement which is defined as:
Relative washout: 100 x (PV HU - Delayed HU)/ PV HU.
>40% suggest adenoma (sens 82%, spec 92%).
Therefore, in this instance the lesion demonstrates an absolute washout of 67% and relative washout of 53%, both of these suggesting a benign adenoma. However, there are certain caveats to CT adrenal washout studies. Firstly the studies that evaluate both densitometry washout of adrenal nodules generally evaluated smaller lesions, measuring 3 cm or less. The probability of neoplasia in larger adrenal lesions is significantly higher, particularly as lesions exceed 5-6 cm. Heterogeneous lesions are also more likely to be neoplastic. Finally it is also important to consider that an adrenal lesion that measures more than 120HU on the portal venous phase acquisition is highly likely to reflect a malignant metastasis (particularly hepatocellular carcinoma or renal cell carcinoma) or phaeochromocytoma. There is also well recognised a minority phaeochromocytomas may also mimic the washout characteristics of lipid poor adenomas. In this case the marked heterogeneous enhancement and large size of the tumour with PET portal venous enhancement greater than 120 HU highly suggested phaeochromocytoma which was the ultimate diagnosis confirmed from urinary metanephrines.