New Imaging Techniques in Characterization of Adrenal Neoplasms

Abstract

Most adrenal lesions are discovered during radiological examinations performed for unrelated reasons. The clinically silent adrenal masses incidentally detected are commonly referred to as adrenal ‘‘incidentalomas’’ (Işık et al., 2012). Adrenal gland can harpor a wide spectrum of pathology. Most incidental adrenal masses are benign in patients without malignancy, but in oncologic patients, adrenal glands are common site of metastasis. Contemporary CT, MRI, and PET are excellent imaging tools in the detection and characterization of adrenal masses (Song & Mayo-Smith 2014). Differentiating adenomas from metastases has been the major objective in the radiological evaluation of adrenal masses, especially in patients with a known primary malignancy (Işık et al., 2012). Various imaging modalities have been investigated for their ability to characterize adrenal lesions. In the context of lipid-rich adenomas, Low-density (Hounsfield unit) values less than 10 HU are characteristically observed on CT images. The metastatic adrenal masses usually measure greater than 10 HU. At CT washout analysis, metastatic masses usually show delayed washout with APW less than 60% and RPW less than Summary and conclusion 129 40%, while a value of APW greater than 60% and RPW greater than 40% is diagnostic of adenoma (Song & Mayo-Smith 2014). The CT histogram method with a threshold value of >10% negative pixels can identify many benign adrenal nodules with attenuation values >10 HU on unenhanced CT with extremely high specificity (Bae et al., 2003). Magnetic resonance imaging (MRI) using a chemical shift technique, adenoma is diagnosed when an adrenal mass becomes dark on out-of-phase compared with in-phase images, using the spleen as the reference. Lack signal loss on opposed-phase is observed in metastatic masses (Song & Mayo-Smith 2014). However, drawing a clear distinction between metastases and adenomas is not always possible, especially for lipid-poor adenomas. Recent advances in image acquisition and post processing techniques on both CT and MR are helping to expand the research tools available for adrenal imaging (Holalkere& Blake, 2009). DWI has been shown to be helpful in characterization of tumors on the basis of diffusion effects using apparent diffusion coefficient (ADC) measurements, restriction of diffusion of water molecules may provide strong evidence for malignancy in an adrenal lesion. On the other hand, the lack of restricted diffusion Summary and conclusion 130 may correlate with a benign or borderline neoplastic tumor. however, there are limited studies on characterization of adrenal lesions using DWI (Blake et al., 2009). In vivo MRS is a noninvasive technique for evaluating the various biochemical processes. MR spectroscopy may prove to be useful in identifying fat in benign adrenal lesions. The presence of a lipid peak at 1.3ppm on the MR spectra in an adrenal lesion could confidently characterize adrenal lesions into a benign category. Quantification of choline (Cho) containing compounds at 3-3.3ppm is also of great interest because such compounds have shown to be increased in malignancy. Thus, MRS is as a promising tool in adrenal imaging with the continuing advances in MR scanners and post processing techniques (Blake et al., 2010). Numerous studies have evaluated the efficiency of positron emission CT (PET-CT) in differentiating benign and malignant adrenal lesions, in which lesions with low or no FDG activity are mostly benign. Although this technique is reported to have high sensitivity and specificity, it is well known that a subset of adenomas demonstrates slightly increased uptake, and a subset of metastatic lesions may not demonstrate any uptake (Blake et al., 2010).