Recent advances in the imaging of adrenal and neuroendocrine tumours

Abstract

The recent rapid development of imaging technology has revolutionised the detection of adrenal and neuroendocrine tumours by allowing more precise lesion characterisation and improving the staging of malignant disease. These developments are briefly reviewed below. Adrenal tumoursComputed tomography (CT) remains the cornerstone of adrenal imaging. Masses are initially assessed without contrast medium to measure their attenuation value (Hounsfield Unit, HU). About 70% of benign cortical adenomas will contain sufficient intracellular lipid to lower the attenuation value to 10 HU. If the attenuation value is greater than 10 HU, immediate and delayed scans are performed following administration of contrast for the calculation of contrast medium washout. In general, adenomas will demonstrate greater washout than a nonadenomatous lesion. This technique has a sensitivity of 98% and a specifity of 92% in differentiating an adenoma from other adrenal tumours. 1 With the advent of multidetector CT, it is now possible to obtain exquisite reformatted images of the adrenal glands in any plane. This has proven helpful determining whether a large mass is of renal or adrenal origin, and also in depicting possible invasion into adjacent organs.Chemical shift magnetic resonance imaging (MRI) is another powerful tool for the characterisation of adrenal lesions. It does not involve the use of ionising radiation or contrast medium and relies on the difference in resonance frequencies of protons in water and intracytoplasmic lipid. Adenomas typically lose signal on the out-of-phase images compared with the in-phase images and the loss of signal can be assessed visually. It therefore also depends on the presence of intracellular lipid and although more sensitive than unenhanced CT it is less sensitive than the CT washout techniques. The specificity of this technique is comparable to washout CT. 2 Newer functional imaging techniques of the adrenal gland such as positron emission tomography (PET)-CT and single photon emission computed tomography (SPECT) combine anatomic and functional information. 18 F-fluoro-deoxy-glucose positron emission tomography ( 18 F-FDG-PET) exploits the increased metabolism of glucose in malignant lesions. PET-CT with this tracer has a high specificity for the detection of malignant adrenal lesions but there have been reports of FDG uptake in benign lesions such as adrenal adenomas and myelolipomas. 3,4 FDG-PET is positive in the majority of phaeochromocytomas, whether benign or malignant. It is useful for the localisation of phaeochromocytomas that fail to concentrate MIBG. There have been several recent studies evaluating the use of PET using 11C-metomidate as a tracer. [5][6][7] This technique is highly sensitive and specific for differentiating between adrenocortical lesions (both carcinomas and adenomas) and non-adrenocortical masses. It remains to be seen if the cost and limited availability of metomidate (MTO)-PET will justify its widespread use in future. Neuroendocrine tumoursFunctioning pancreatic ne...