Recent advances in the imaging of adrenal and neuroendocrine tumours
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 tumours
Computed 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.1With 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. 18F-fluoro-deoxy-glucose positron emission tomography (18F-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.5–7 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 tumours
Functioning pancreatic neuroendocrine tumours are typically small at the time of presentation and their detection is often challenging. As they are characteristically intensely vascular, multidetector CT, which allows the acquisition of excellent arterial-phase imaging, has undoubtedly improved the sensitivity of CT for their detection. Similarly, modern MRI with faster gradients and improved radiofrequency coils has improved their detection. Studies comparing CT and MRI are often difficult to evaluate as the cohorts studied are small and the modalities are not at comparable stages of development. We believe, however, that their sensitivities are similar and exceed 80%.8CT and MRI have the advantage of the detection of small liver metastases although they may be difficult to identify on contrast-enhanced CT and gadolinium-enhanced MRI.9The use of liver-specific contrast agents such as mangafodipir-DPDP may improve their detection.10
Endoscopic ultrasound (EUS) also enables visualisation and localisation of small pancreatic neuroendocrine tumours, particularly those in the pancreatic head. It has a sensitivity of between 79–100%.11The wide variation in reported sensitivities is likely to be due to the fact that it is an operator-dependent technique. Recently, a study into the use of contrast-enhanced EUS in the characterisation of pancreatic tumours found that hypovascularity as a sign of malignancy had a sensitivity of 92% with a 100% specifity. FDG-PET has not been shown to be of benefit in the imaging of neuroendocrine tumours except for those that are dedifferentiated or have high proliferative activity.13Other PET tracers such as 18F-L-Dopa and 68Gaoctreotide have shown some promise13 but the role of PET in neuroendocrine tumours has yet to be defined.
- © 2009 Royal College of Physicians
References
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- Haider MA
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- Khan TS
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- Ichikawa T
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- Bader TR
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