Use of Contrast-Enhanced Ultrasound in the Differential Diagnosis of Adrenal Tumors in Dogs
ABSTRACT
We evaluated the diagnostic accuracy of the contrast-enhanced ultrasonography (CEUS), using a second-generation microbubble contrast agent, in differentiating the different types of adrenal mass lesions in 24 dogs. At B-mode ultrasound, 9 lesions involved the right adrenal gland, 14 the left, and 1 was bilateral. Each dog received a bolus of the contrast agent into the cephalic vein, immediately followed by a 5-mL saline flush. The first contrast enhancement of each adrenal lesion was evaluated qualitatively to assess the degree of enhancement and its distribution during the wash-in and wash-out phases, as well as the presence of non-vascularized areas and specific vascular patterns. Pathological diagnoses were determined in all dogs by histopathology or by cytology. Combining enhancement degree and vascularity resulted in the best predictive model, allowing CEUS to differentiate adrenocortical adenoma (n=10), adenocarcinoma (n=7), and pheochromocytoma (n=7) with an accuracy of 91.7% (P < 0.001). Combining enhancement degree and vascularity, CEUS can discriminate malignant versus benign adrenal lesions with a sensitivity of 100.0%, a specificity of 80.0%, and an accuracy of 91.7% (P < 0.001). In conclusion, results of this study confirm that CEUS is useful for differentiating between the different types of adrenal tumors in dogs.
Introduction
The differential list for adrenal pathologies that cause adrenal mass lesions encompasses several types of benign and malignant tumors, including adrenocortical adenomas, adrenocortical adenocarcinomas, and pheochromocytomas, as well as metastatic cancers and other less frequent tumor types.1 In dogs, primary adrenal gland tumors have a relatively low incidence, representing only 0.17 to 0.76% of all the neoplasias, but account for 10 to 20% of dogs diagnosed with Cushing's syndrome.2,3 The adrenal masses can cause a large array of clinical signs ranging from hyperadrenocorticism to catecholamine excess depending on the cell type involved by the pathology; in some dogs, the adrenal tumors do not cause any obvious clinical signs at all.4,5 Often, these adrenal masses are incidental findings encountered during imaging screening for other diseases.6 All adrenal mass lesions, including incidentalomas, require clinical evaluation to reach a definitive diagnosis. Pituitary and adrenal tumors may coexist in dogs with hyperadrenocorticism, thus further complicating the diagnosis.7
Imaging of adrenal glands by ultrasound is a valuable technique for the detection of adrenal hyperplasia and adrenal masses in dogs.8,9 Overall, bilateral and/or unilateral adrenal gland enlargement, with loss of the normal shape and parenchymal structure, are frequently diagnosed in dogs.10 However, differentiating between benign and malignant adrenal masses (i.e., adrenocortical adenomas and carcinoma) or between tumors arising from the adrenal cortex or medulla (i.e., pheochromocytoma) based only on their morphological appearance on ultrasound remains inconclusive.11,12 In human medicine, computed tomography (CT) and magnetic resonance imaging (MRI) can differentiate benign and malignant adrenal masses in such a way that only a limited number of patients requires adrenal biopsy to confirm the diagnosis.13
Recently, contrast-enhanced ultrasonography (CEUS) has been employed to evaluate the perfusion patterns of the adrenal gland in normal dogs, as well as in dogs with pituitary-dependent hyperadrenocorticism.14–16 In humans, CEUS has been found to provide useful clinical information, being accurate enough to discriminate adrenal adenomas and non-adenomatous masses with a sensitivity comparable to that of CT and MRI techniques.17
Our hypothesis was that the contrast enhancement patterns found with CEUS might be specific to each different type of adrenal pathology. Therefore, the purpose of this study was to evaluate whether CEUS can reliably differentiate dogs with benign or malignant adrenal lesions, as well as between the different types of adrenal tumors.
Materials and Methods
Clinical Cases
All dogs included were required to have informed owner consent, and procedures were performed in accordance with Italian laws on animal care. In this study, we enrolled 24 dogs (13 females and 11 males) over a period of 33 mo (from October 2010 to June 2013), all of which were found to have either unilateral (23 dogs) or bilateral (1 dog) adrenal mass lesions identified during an abdominal ultrasound examination. Once the adrenal mass lesions were identified, the dogs recruited in this study underwent a standardized protocol that included B-mode sonography imaging followed by CEUS. Additional inclusion criteria required a cytologic or histologic confirmation of an adrenal gland tumor and no treatment for hyperadrenocorticism (Cushing's syndrome). No other dogs with adrenal mass lesions were diagnosed during this 33-mo study period, other than two cases of suspected adrenal tumors without histopathologic confirmation and seventeen cases of bilateral adrenal hyperplastic nodules; none of these cases were included in this study.
B-Mode Ultrasonography Examination
Following IV injection of 0.1 mg/kg butorphanola, all scanning procedures were performed by two operators on mildly sedated dogs in lateral recumbent position using a machine equipped with a contrast tuned imaging technologyb. The dogs were first examined using B-mode ultrasound with linear (4–13 MHz) and microconvex transducers (6.6–8.0 MHz) to obtain scans of adrenal glands and other abdominal organs, including liver and ovaries of intact females. The adrenal mass lesions were evaluated for size, margins, echogenicity, echotexture, and extension to adjacent vessels (caudal vena cava and/or aorta). During the B-mode examination, adrenal glands were also measured in the longitudinal plane: the greatest cranio-caudal dimension was defined as the adrenal length, whereas the greatest dorso-ventral dimension perpendicular to the longitudinal axis was defined as adrenal thickness. An adrenal gland was considered atrophic when its thickness was less than 5 mm.18
Contrast-Enhanced Ultrasonography
For CEUS examination, the probe used was a linear transducer (3–8 MHz) with low-mechanical index (0.04–0.05) to avoid the rupture of microbubbles; the presets of the machine were modified as follows: Dynamic Range, 7, and Persistence, between 6 and 12. The ultrasound contrast agent employed was the sulphur-hexafluoride echo-signal enhancerc. The average diameter of the microbubbles was 2.5 μm when dissolved in physiological solution. Microbubbles consist of phospholipid capsules that are stabilized by surfactant substances and filled with sulphur hexafluoride. Once prepared, the aqueous solution of the contrast agent remains stable for 6–8 hr before use, but its lifespan decreases to 5 min following IV injection. In each dog, a bolus dose (0.03 mL/kg of body weight) of the freshly prepared contrast agent was rapidly infused via an 18G catheter, provided with a three-way-valve inserted in the cephalic vein, immediately followed by a 2.5- or 5-mL saline flush (NaCl 0.9%), depending on dog size.
The contrast examination was performed by two experienced operators: the first injected the contrast medium through the catheterized vein, while the second performed the ultrasound scans of the adrenal glands by holding the transducer as still as possible on the selected position during the contrast study. The left and right adrenal glands were examined on right and left lateral recumbent position, respectively, following separate injection of ultrasound contrast agent bolus 10 min apart, starting from the gland with the adrenal lesions previously identified by B-mode ultrasound. The adrenal target lesions were scanned continuously for 5 min and the dynamic process of contrast enhancement was examined in real time. The adrenal glands were imaged in their longitudinal axis. For the left adrenal gland, the phrenicoabdominal vessels, left renal artery, cranial mesenteric, and celiac arteries (when possible) were maintained in the image to serve as an in vivo reference; for the right adrenal gland, the in vivo reference vessels included in the image were caudal vena cava and right renal artery. During each examination, a digital video clip of 1-min in duration was taken during the injection of the contrast agent and stored in the hard disk of the ultrasoundscanner.
Evaluation of B-Mode US and CEUS Imaging
The images of each dog were analyzed by two radiologists. Neither had performed the ultrasound examination and both were blinded to the clinical diagnosis and to the result of the histological examination.
For evaluation of the B-mode ultrasound, echogenicity of the adrenal mass lesions was compared to normal-surrounding adrenal or contralateral gland and categorized as hypoechoic, isoechoic, hyperechoic, or mixed hypo/hyperechoic. Echotexture of the adrenal lesion was defined as homogeneous or heterogeneous and its margins were characterized as well- or poorly defined. Position and number of the adrenal lesions, as well as the eventual atrophy of the contralateral gland, were also evaluated. For the CEUS images, the first contrast enhancement within the adrenal mass lesion was evaluated qualitatively to assess the following: (1) the degree of enhancement (hypo-, iso-, and hyper-enhancement) during the arterial phase; (2) the internal distribution of contrast enhancement (homogeneous, heterogeneous, nodular); (3) the presence of non-vascularized areas; (4) specific vascularization patterns (centripetal, peripheral, disordered); and (5) the time of contrast enhancement (fast/synchronous/slow) during the early wash-in and wash-out phases. Signal intensity of the adrenal mass lesion was compared with the surrounding normal adrenal tissue (when present) or with the contralateral adrenal gland.14,15,16
Reference Diagnosis
Definitive diagnosis of the adrenal mass lesions was made by cytopathologic or histopathologic evaluation reviewed by a veterinary pathologist. The adrenal specimens were obtained by fine-needle aspiration (n = 3), at the time of elective surgery (n = 6), or at necropsy (n = 15) within 3 mo from CEUS examination. The biopsy specimen, taken at surgery or at necropsy, were fixed in formalin, processed according to routine histological techniques, and stained with hematoxylin and eosin, whereas the fine-needle aspirates were smeared onto slides, air dried, and stained with Romanovsky stain or fixed in 95% ethanol for formal cytopathological review. The tumors were diagnosed histopathologically, while the cytological diagnosis of the other three tumors was made using the following features: presence of cells loosely aggregated together with many other single cells, characterized by numerous naked nucleoli, high nuclear pleomorphism, and basophilic cytoplasm with sparse eosinophilic granules for the pheochromocytoma; and the presence of cells predominantly arranged in groups, characterized by marked anisokaryosis with severe nuclear dysmetria, aberrant chromatin pattern, and multiple atypical nucleoli for the adrenal adenocarcinoma.19,20
Statistical Analysis
The statistical analysis was performed using a software packaged. The Shapiro-Wilk test was used to evaluate normality of continuous data that were expressed as mean (± standard deviation)and median. The single dog with a diagnosis of adrenocortical lipoma was excluded from the statistical analysis. Due to nonparametric distribution, the significance of differences in sizes between each adrenal tumor type was examined using the Kruskal-Wallis test, while that of tumors diagnosed as benign and malignant was calculated with the Mann Whitney U tests. The histological/cytological diagnosis of each adrenal lesion was used as the gold standard of reference for all dogs. The association between different adrenal lesions and B mode/CEUS features was assessed using Fisher's exact test. Receiver-operating characteristic curves and logistic regression analysis were used for evaluating the diagnostic performance (i.e., sensitivity, specificity, and diagnostic accuracy, defined as % of correct prediction with reference diagnosis) of qualitative B-mode and CEUS enhancement patterns for differentiating malignant versus benign adrenal lesions and between the tumor types, respectively.21 In the simple logistic regression analysis, all independent variables were included as covariates. In the multiple logistic regression analysis, correlated variables were not included simultaneously in the model, and the stepwise selection method was used. Differences were considered significant at P < 0.05.
Results
Clinical Findings and Reference Diagnosis
The signalment (age, breed, sex) and clinical signs for the 24 dogs are summarized in Table 1. Affected dogs ranged in age from 6 to 15 yr (median, 10.5 yr) and weighed 7 to 36 kg (median, 12 kg). Only six dogs had clinical signs attributable to hyperadrenocorticism, whereas three dogs reported episodes of collapse and one retinal detachment. Fourteen dogs did not have any clear clinical signs related to adrenal pathology, and the adrenal lesions were found incidentally during abdominal ultrasound examination for workup of unrelated diseases. The 25 adrenal lesions classified on the basis of histopathological (n=22) or cytological (n=3) evaluation (Table 1) included adrenocortical adenomas (10; 40.0%), adrenocortical adenocarcinomas (7; 28.0%), pheochromocytomas (7; 28.0%), and lipoma (1; 4.0%). Dog #4 in Table 1 was found to have bilateral adrenocortical adenoma. All pheochromocytomas here examined were diagnosed as malignant.
B-mode Ultrasound
Upon B-mode ultrasonography examination, the adrenal lesions involved the right adrenal gland in 9 dogs (37.5%), the left adrenal gland in 14 dogs (58.3%), and both adrenal glands in 1 dog (4.2%). In most dogs, the adrenal lesions were not precisely localizable within cortical or medullary region. The mean size of adrenocortical adenomas was 1.5 ± 0.8 cm (median, 1.2 cm; range, 0.7–3.1 cm), whereas that of adenocarcinomas was 2.5 ± 1.4 cm (median, 2.9 cm; range 0.8–4.6 cm) and that of pheochromocytomas was 2.4 ± 1.3 cm (median, 2.8 cm; range 0.9–4.0 cm). The size of the adrenocortical lipoma was 1.7 cm. There was no statistical difference in the mean size of the adrenal mass lesions among the diagnosis groups or between malignant and benign adrenal tumors. Of the six dogs with clinical signs of Cushing's syndrome, atrophy of the contralateral adrenal gland (0.3 ± 0.04 cm) was found in all three dogs with adrenocortical adenoma and in two out of three dogs with adenocarcinoma (Table 1).
The ultrasonographic appearances of the adrenal lesions are provided in Table 2. Adrenocortical adenomas had well-defined margins (Figure 1A), while most adenocarcinomas showed non-homogeneous parenchyma characterized by anechoic areas, hyperechoic spots, septa, and areas of different echogenicity (Figure 1B). The pheochromocytomas had variable echotexture and poorly defined margins (Figure 1C). In two of the seven dogs with adenocarcinoma, the tumors invaded the surrounding tissue, and in four cases these altered the profile and surface of the adrenal gland. Metastases were detected in surrounding renal and lumbar aortic lymph nodes of one dog and in the lung of another. In four out of seven dogs, the pheochromocytoma invaded the caudal vena cava (Figure 1C) and, in these cases, the diagnosis could be made by B-mode ultrasound. In the dog with adrenal lipoma, ultrasound scanning characterized a well-defined mass in the caudal pole of left adrenal gland with hyperechoic margins and hypoechoic core; the adrenal surface was deformed by this mass, but there was no apparent invasion of surrounding tissues (Figure 1D). Adrenal masses with irregular margins were found more frequently (P < 0.05) in malignant than in benign lesions (Table 2). By using irregular adrenal margins of the lesions as the criteria, the B-mode accuracy in differentiating malignant versus benign tumors was 75%, with a sensitivity of 71.4% and a specificity of 80%. There were no statistical differences in the other qualitative ultrasound variables among the diagnosis groups or between benign and malignant adrenal lesions (Table 2).
Citation: Journal of the American Animal Hospital Association 52, 3; 10.5326/JAAHA-MS-6363
Contrast-Enhanced Ultrasound
In none of our cases did we observe adverse effects associated with the IV injection of the contrast agent. The enhancement patterns of the adrenal lesions after contrast injection are shown in Table 3. The degree of contrast enhancement greatly differed (P < 0.001) among the diagnostic groups, with all adenomas and adenocarcinomas having a reduced enhancement and all pheochromocytomas having an increased enhancement (Figure 2). The distribution of the ultrasound contrast agent within the adrenal lesions was also different (P < 0.05), showing either homogenous, heterogeneous, or nodular patterns, depending on tumor type. In two adrenal adenomas, local perfusion defects were evidenced by the absence of contrast enhancement. Non-vascularized areas were identified in all adrenal adenocarcinomas and also in four pheochromocytomas (Figure 3).
Citation: Journal of the American Animal Hospital Association 52, 3; 10.5326/JAAHA-MS-6363
Citation: Journal of the American Animal Hospital Association 52, 3; 10.5326/JAAHA-MS-6363
The enhancement patterns varied (P < 0.01) among adrenal lesions (Table 3). In most adenomas, the enhancement arose simultaneously from different small arteries, which maintained a ramified but centripetal direction (Figure 2A). The vascularization showed a disordered pattern in most adrenocortical adenocarcinomas and in all pheochromocytomas. In one dog with adenocarcinomas, the enhancement originated from the periphery of the lesion with small arteries directed centripetally, whereas, in two dogs with adenocarcinomas, only periphery vessels were seen causing an annular enhancement pattern (Table 3 and Figure 2B).
In addition, CEUS examination allowed a detailed view of those adrenal lesions that invaded the surrounding tissues and/or the caudal vena cava. In these latter cases, the microbubbles were clearly visualized in the arterial vessels of the pheochromocytoma inside the caudal vena cava during the early wash-in phase; only later did they appear within the venous system, allowing the illumination of the entire caudal vena cava. Compared to the normal adrenal parenchyma, the perfusion pattern of all adrenal adenomas and adenocarcinomas was characterized by slow wash-in and fast wash-out phases, whereas that of pheochromocytomas had a slow wash-out phase, regardless of their size (Figure 2). The perfusion pattern of the lipoma was characterized by greatly reduced enhancement of the contrast agent during the different phases of its vascular distribution. During the arterial phase, two vessels entering the lesions from the periphery were visualized.
The area under the receiver-operating characteristic curve (AUC) for the enhancement pattern was higher (AUC = 0.84, P < 0.01) than those for the other categories (vascularity, enhancement degree, wash-in, wash-out, AUC = 0.75, P < 0.05) and significantly different from the true area (AUC = 0.5) of the null hypothesis of the reference line in discriminating malignant vs. benign adrenocortical tumors (Figure 4). Of all the CEUS variables here studied by simple logistic analysis, the enhancement pattern had the highest sensitivity (92.9%, P < 0.01) and accuracy (83.3%, P < 0.01) in predicting the nature of the adrenal lesions (malignant versus benign), while the highest specificities (100%, P < 0.01) were achieved by enhancement degree and both the wash-in and wash-out phases (Table 4). Including both enhancement degree and vascularity in the model, the predictive power of CEUS in discriminating malignant versus benign adrenal lesions increased (P < 0.001) achieving a sensitivity of 100, a specificity of 80, and an accuracy of 91.7%.
Citation: Journal of the American Animal Hospital Association 52, 3; 10.5326/JAAHA-MS-6363
In a multinomial logistic regression analysis, use of either contrast enhancement, wash-in, or wash out alone had an accuracy of 70.8% (P < 0.001) in differentiating adrenocortical adenomas, adrenocortical adenocarcinomas, and pheochromocytomas. When several CEUS parameters were simultaneously included (stepwise selection), the best model (P < 0.001) resulted by combining two enhancement categories, such as contrast enhancement degree and vascularity (or vascularity and either wash-in or wash-out characteristics). Using either one of these combinations, all adrenocortical adenocarcinomas and pheochromocytomas, as well as 80% of adrenocortical adenomas, were correctly diagnosed; the overall predictive accuracy of CEUS was 91.7% congruent with the reference diagnosis (P < 0.001).
Discussion
As far we know, this is the first report that characterizes the vascular perfusion patterns of different adrenal tumor types by use of CEUS in dogs. Our results indicate that this technique is useful in distinguishing between benign and malignant adrenal mass lesions, allowing the differential diagnosis between adrenocortical adenoma, adenocarcinoma, and pheochromocytoma with accuracy comparable to that found in humans using CEUS or reference CT scan and MRI.22,23
With the widespread use of ultrasound screening in dogs, the number of unexpected discoveries of adrenal lesions is greatly increasing, as has also been reported in human medicine.6 Once detected, the management of dogs with either adrenal mass or incidentalomas, which are often not showing any or only vague clinical signs, requires an appropriate diagnostic approach to differentiate between benign and malignant tumors for directing proper treatment. Although most incidental adrenal masses in humans are benign, their characterization (i.e., benign versus malignant) remains a challenging task.6 This is true also in veterinary medicine, where the clinical and economical resources are often limited.
In the present study, adrenal mass lesions included adenoma, adenocarcinoma, pheochromocytoma, and lipoma, but more than half were indeed incidentalomas and not related to any specific clinical signs of adrenal disease. No difference in the prevalence between left- or right-sided adrenal masses was evident, in agreement with previous findings.10
Although large adrenal masses associated with clinical signs of adrenal dysfunction are more often indicative of malignancy, no definitive differentiation between benign and malignant adrenal lesions can be made using B-mode ultrasonographic criteria alone.1,3,9,10,24 Our results partly confirm this limitation of B-mode ultrasonography. In fact, only the findings of irregular margins due to invasion of surrounding tissue allowed the differentiation of malignant versus benign adrenal tumors with an accuracy of 75%. Other ultrasound morphological features such as vascular invasion and/or metastasis may predict the nature and/or type of the adrenal lesions.25,26 The use of CT and/or MRI can better characterize adrenal masses to help differentiate benign and malignant lesions in humans as well as dogs.27,28,29 Fine-needle aspiration of adrenal masses do not always lead to a definite diagnosis, and this procedure is certainly not without risk.30 In addition, the adrenocorticotropic hormone stimulation and/or dexamethasone suppression tests may be inconclusive, as both benign and malignant adrenocortical masses can be nonfunctional or functional, secreting excess amounts of glucocorticoids or other adrenocortical hormones.9,31 Thus, for all these reasons, a non-invasive and safe diagnostic tool such as CEUS, capable of discriminating between the long differential list of primary versus secondary and benign versus malignant adrenal lesions, would be of great value for the clinician.
In dogs, CEUS has already been used to characterize the vascular patterns of hyperplastic adrenal glands and focal lesions in the liver and spleen.15,16,32,33 In humans, the potential validity of CEUS using a second-generation contrast agent as a tool for differentiating between benign and malignant adrenal mass lesions has been evaluated only in a few published studies, but with conflicting results.22,34 Some studies have failed to find any useful vascular pattern for diagnosing the different types of adrenal mass lesions.34 In contrast, other studies, by using time-intensity curves, were able to identify malignant adrenal masses with a sensitivity of 100% and specificity of 82%, on the basis of early arterial contrast enhancement followed by fast wash-out.22
By CEUS, all of the qualitative enhancement patterns examined in our dogs differed significantly between benign and malignant adrenal lesions as well as from that of normal adrenal glands.14,15 Adrenal adenomas commonly showed hypo-enhancement, a homogenous distribution, and a centripetal pattern with an abnormal flow rate during the wash-in and wash-out phases. Most adenocarcinoma were characterized by small arterial vessels entering the adrenal lesion during the wash-in phase, disordered distribution of the enhancement pattern, and the presence of non-vascularized areas likely due to necrotic and/or calcified regions. The presence of non-vascularized areas, however, do not represent typical landmarks that differentiate malignant from benign adrenal tumors because they can also occur in adrenal adenomas. In addition, adrenal malignant lesions might be too small to have hypovascularized areas. Pheochromocytomas were mainly characterized by increased degree of enhancement, disordered distribution pattern, synchronous wash-in phase, and slow wash-out phase. By B-mode, adrenal lipoma appears as a well-defined lesion with hyperechoic margins and hypoechoic core that can be easily misdiagnosed with nonfunctional adenoma or adenocarcinoma, based on size and echotexture. Although it may seem difficult to differentiate lipoma from other adrenal tumors by B-mode criteria alone, CEUS imaging may help in diagnosing this type of lesion. In our case, the lipoma was characterized by a marked hypo-enhancement pattern during all the distribution phases compared to normal glandular parenchyma. In addition, the lesion appeared well-defined, without infiltration of surrounding tissues.
In the present study, the most specific qualitative trait for differentiating between benign and malignant adrenal lesions was the contrast enhancement pattern with a sensitivity, specificity, and accuracy of 92.9, 70, and 83.3%, respectively. By combining the degree of contrast enhancement with vascularity, adrenal malignant lesions are evidenced with greater sensitivity, specificity, and accuracy (100, 80, and 91.7%, respectively). Contrast enhancement degree, together with vascularity (or vascularity together with either wash-in or wash-out characteristics), were also the most effective qualitative variables for differentiating between adrenal adenomas, adrenal adenocarcinomas, and pheochromocytomas with an overall accuracy in predicting the correct diagnosis of 91.7%. Interestingly, using either one of these two combinations, all adrenocortical adenocarcinomas and pheochromocytomas were correctly diagnosed, being congruent with the reference diagnosis. The pathophysiologic bases of these findings are not clear, but likely reflect the different angiogenic activity of these tumors, as well as their vascular architecture and blood flow velocity. In addition, the blood perfusion patterns characterizing the different adrenal tumor types were also much different from the nodular vascular pattern found in dogs with pituitary-dependent hyperadrenocorticism.15
One limitation of the present study was that our dogs with adrenal mass lesions did not have a complete endocrine workup for hyperadrenocorticism or pheochromocytoma (e.g., low- or high-dose dexamethasone suppression testing, plasma adrenocorticotropic hormone concentration, serum sex hormone values, aldosterone, or plasma or urine catecholamine levels). Such endocrine testing likely would have helped better define the secretory nature of the adrenal masses identified in our dogs. However, the main purpose of this study was not to determine the secretory nature of these adrenal masses, but, rather, to differentiate between their different types (e.g., adrenocortical adenomas, adrenocortical carcinoma, pheochromocytoma, and other lesions). One must remember that some adrenal mass lesions will be nonsecretory, whereas others will secrete one or more adrenocortical sex hormones not routinely analyzed in the workup for dogs with hyperadrenocorticism or pheochromocytoma. This may be especially true for dogs with incidentalomas, which made up most of the cases in this study.
The relatively small number of dogs enrolled in this study necessarily limits the estimation on the diagnostic accuracy of CEUS from a statistical point of view, but all our CEUS diagnosis were confirmed histopathologically. However, the diagnostic performance of qualitative CEUS analysis was similar to that reported in much larger studies obtained in human patients, suggesting that CEUS characteristics of adrenal lesions of dogs are quite similar to those in man.22,35 Therefore, we believe that CEUS should be considered in the diagnostic workup for the detection of adrenal lesions, as an aid in differentiation and evaluation of different tumor types, especially in those cases in which the adrenal lesion has overlapping features at B-mode imaging. There is little doubt that the differential diagnosis of the adrenal lesions with CEUS in dogs can rely also on the analysis of quantitative parameters derived from time-intensity curves. However, in our experience, qualitative evaluation of perfusion patterns is more reliable than quantification of blood flow parameters in arriving at a functional diagnosis sufficient to serve clinical needs. In fact, actual ultrasound enhancement measurements are subject to several physiological conditions (i.e., injection protocol, cardiac output, pharmacological sedation) and individualized ultrasound settings (i.e., machine gains, dynamic range, gray scale, actual dimension of the region of interest), all of which are potential sources of variability.
Conclusion
In conclusion, results of this study confirm that CEUS is a useful, non-invasive, and cost-effective tool for differentiating the types of benign and malignant adrenal tumors in dogs. The diagnostic accuracy of CEUS relies on specific vascularization patterns that characterize and distinguish between nodular hyperplasia, adrenocortical adenoma, adenocarcinoma, and other types of adrenal tumors, such as pheochromocytoma and lipoma. Wider application of CEUS in the diagnostic approach for the differentiation of adrenal lesions will likely reduce the number of dogs requiring unnecessary diagnostic procedures such as fine-needle aspiration and core biopsy and/or other imaging techniques such as CT and MRI without compromising accurate diagnosis.
Representative B-mode images of different adrenal mass lesions. (A) Adrenal adenoma (arrows). (B) Adrenal adenocarcinoma (arrows), which appeared as an heterogeneous nodular lesion with irregular margins. (C) Pheochromocytoma (white arrows) infiltrating tissues (black arrows) and invading the caudal vena cava (red arrow). (D) Adrenal lipoma (arrows) showing a well defined mass in the caudal pole of left adrenal gland with no invasion of surrounding tissues.
Representative sagittal images of the adrenal glands of three dogs showing different vascular patterns associated with adenoma (A), adenocarcinoma (B), and pheochromocytoma (C) adrenal lesions. Each image shows the contrast-enhanced ultrasound (right) and B-mode aspects (left) of the adrenal gland acquired at comparable times after contrast agent injection. (A1) The initial wash-in (17sec) shows a slow distribution of contrast enhancement within the adrenal adenoma (white arrows) compared to that of adjacent adrenal parenchyma (black arrow). (A2) Recorded 28 sec after the appearance of the ultrasound contrast agent, shows the typical vascular hypo-enhancement pattern of the adenoma also during the peak of enhancement. (A3) (36 sec) shows the wash-out period of the contrast, which, although progressive, remains hypo-enhanced compared to adjacent normal parenchyma. (B1–B3) Shows the blood flow patterns of an adenocarcinoma. In the early stages of wash-in (19 sec), large arterial vessels with disorderly distributions of contrast agent (B1 arrows) are visualized within the adrenal mass lesion. (B2) During the distribution phase (23 sec), the adenocarcinoma is constantly hypo-enhanced as a result of heterogeneous vascular areas and large necrotic areas that are not vascularized (in the dorsal part of mass lesion). (B3) Recorded 29 sec after ultrasound contrast injection; shows an adenocarcinoma characterized by a marked hypo-enhancement pattern with only the margins of the mass lesion being highlighted by the contrast. (C) Shows representative blood flow patterns of a pheochromocytoma. (C1) In the early stages of wash-in (14 sec), the pheochromocytoma (white arrows) has a very pronounced vascular enhancement. During the arterial phase of contrast distribution, vascularized tissue inside the central vena cava confirms the infiltration and diffusion of the lesion within the cava; it remains without contrast medium (blue arrow). (C2) During the venous phase (29 sec), the contrast is present in the central vena cava (blue arrow) that, at this stage, appears nearly iso-enhanced with the adrenal lesion. (C3) the wash-out of the contrast from the mass lesion is homogeneous, but slow compared to normal adrenal parenchyma (53 sec).
Representative contrast-enhanced ultrasonography image of a pheochromocytoma recorded 17 sec after the injection of contrast medium showing hyper-enhancement due to high perfusion pattern. Small, non-vascularized areas (arrows) are present within the mass, a characteristic frequently found in malignant lesions.
Graph of receiver operating characteristic curve of different qualitative enhancement variables found on contrast-enhanced ultrasonography examination of 24 adrenal mass lesions in 23 dogs. Positive state was malignant lesions (i.e., adenocarcinomas and pheochromocytomas); negative state was benign lesions (i.e., adenoma). Data from the dog with a diagnosis of lipoma were not included in the statistical analysis.
Contributor Notes
