Unexpectedly discovered adrenal masses are found in about 5% of all abdominal CT examinations done in patients not known to have malignancy or endocrine abnormality. The incidence of AIs increases to 9–13% in patients imaged for a known malignancy .
The most common AI is benign adrenal adenoma . The aim of this work is to study the role of MRI, specifically chemical shift imaging (CSI), against various MDCT scans (non-enhanced, enhanced, and delayed) in the characterization of incidentally discovered adrenal masses to offer a way for the patients to avoid unnecessary time and money-wasting imaging modalities used to reach a diagnosis of their incidentally discovered adrenal lesions.
In previous studies, a considerable overlap in size between the suprarenal adenomas and the non-adenomatous lesions, especially metastases was demonstrated. This was also noted in our study. We found that the size of < 4 cm has 100.00% sensitivity and 66.6% specificity for being a benign suprarenal adenoma. This is keeping with Angelelli et al. 2013 stating that the size criterion yielded 98% sensitivity, but a specificity of 53%, which was too low to be accepted as a discriminator . We concluded that the differentiation between benign and malignant suprarenal lesions cannot rely on the lesions size alone. The size could be a helpful factor to reach the precise diagnosis but still we need more reliable diagnostic criteria.
In our study, we demonstrated that unenhanced CT (UCT) attenuation value of adrenal lesions is a useful imaging modality to characterize lipid-rich adenomas that measure 10 HU or less. A threshold of 10 HU on UCT can be used. The sensitivity and specificity for adenoma characterization were 72.7% and 77.78%, respectively.
This partially agrees with Park et al. 2016 and Kamiyama et al. 2009 who came out with that an attenuation value of a suprarenal mass of less than 10 HU has a sensitivity and specificity for adenoma of 71% and 98%, respectively [10, 11].
In this study, the unenhanced value is considered significant to distinguish between adenomas and non-adenomas. This is keeping with Angelelli et al. 2013 stating that no case of non-adenoma lesion with the exception of myelolipomas and cysts has a basal density of less than 10 HU .
Previous investigators have demonstrated that the washout attenuation is much larger in adenomas than in non-adenomas [12, 13]. In our study, washout values were also significantly larger in lipid-rich adenomas. We concluded that relative percentage washout (RPW) is a good indicator of the lipid-rich content and benignity of adenomas with area under the curve (AUC) 1.000, sensitivity and specificity of 100% (Table 2) in comparison to absolute percentage washout (APW) with (AUC) 0.883, sensitivity and specificity of 90% and 85%, respectively.
This agrees with Park et al. 2016 who stated that RPW is more accurate than APW for diagnosing an adrenal adenoma. If an adenoma measures less than 0 HU in unenhanced CT images, RPW becomes higher than APW. Therefore, it is not uncommon for APW to be below 60% in lipid-rich adenoma .
The best MR imaging technique for to characterize suprarenal adenoma is found to be chemical shift imaging (CSI). Its sensitivity and specificity in diagnosis of indeterminate adrenal lesions are reported as 67% and 89–100%, respectively . Subgroup analysis reveals that for adrenal lesions measuring 10–30 HU in unenhanced CT studies, the sensitivity of CSI is as high as 89%, which is comparable to the data retrieved from washout CT. However, its sensitivity declines markedly to be as low as 13% for suprarenal lesions measuring more than 30 HU in unenhanced CT images. This concludes that MR imaging is not a reliable tool for these lesions, although the specificity can be as high as 100% . On that account, adrenal lesions measuring 10–30 HU can be further evaluated with either washout CT or chemical shift MR imaging according to personal and institutional circumstances; lesions with CT attenuation measuring higher than 30 HU are best imaged with washout CT .
In our study, CSI, also referred to by dual phase imaging, has proven to be the best indicator for evaluating adenomas from non-adenomas, showing loss of signal in all benign lesions except for adrenal cysts with a total of 14/15 benign lesions. Drop of signal in the out phase images showing the highest (AUC) 0.944 a standard error of 0.0556 with sensitivity and specificity of 100% and 88.89%, respectively.
This agrees with El-Kalioubie et al. 2016 who stated that diagnosis of malignant lesions was accurately reached by observing the lack of signal drop on CS MRI in all malignant lesions, by showing the absence of signal drop on out-of-phase (OP) images and showing signal loss in all lipid-rich lesions . There was no signal loss in any of the malignant masses. Some adrenocortical carcinomas have been reported to contain intracellular lipid ; however, no cases of lipid-rich adrenocortical carcinomas were included in our study population.
CT versus CS MRI
No statistically significant difference was seen in our study regarding the preference of using CT over CSI (Table 4) unless in certain situations when the use of one diagnostic modality may be superior.
This is keeping with literature stating that the sensitivity and specificity of CSI for the differentiation of incidental adrenal lesion are similar to those of unenhanced CT densitometry, at 81–100% and 94–100%, respectively [14, 17, 18].
Studies have shown that for lipid-rich adenomas, there is effectively no difference between CECT and MRI, as in our study, but CSI might be superior when evaluating lipid-poor adenomas . However, Park et al. 2016 showed that CSI might be useful only when the unenhanced CT attenuation is less than 30 HU and that delayed enhanced CT can characterize additional hyper-attenuating adrenal masses that cannot be identified with chemical shift MRI .
Some limitations that were encountered in our study included our study design which depended on accidentally discovered adrenal masses in patients undergoing abdominal studies for irrelevant causes which limited our sample size. Also, other limitations included the wide varieties of pathologies that cause incidentally discovered adrenal masses. This variation affected the sample size in each pathological category. Further study on a larger number of patients may be of value in retrieving better statistical results.