A: BENIGN CONDITIONS

1. Adenoma: the most common benign adrenal tumor, characterized by well-demarcated round or oval masses with homogeneous density. Typically, these adenomas exhibit low attenuation values (<10 HU) (Figure 1) on CT scans, distinguishing them from other adrenal lesions. A 15-minute post-contrast CT protocol is often employed to differentiate adenomas from non-adenomas, particularly in cases where lipid content is poor, in which is calculated the absolute percentage washout (APW), that above 60% shows a high probability of adenoma (EDEU Formula):

• APW = (60s Enhanced HU – 15 min Delayed/ 60s Enhanced HU – Unenhanced) x 100% (Figure 2)
• APW = E-D/E-U x 100%
• APW > 60% -> ADENOMA

At MRI, T1W out of phase sequence shows a signal “drop-out” lipid rich mass. (Figure 3). 

2. Hyperplasia: characterized by bilateral adrenal enlargement, resulting in an increase in gland volume (Figure 4). On imaging studies, adrenal hyperplasia typically presents with glands measuring greater than 10 mm in size, with smooth margins and a homogeneous structure. Additionally, nodules of variable size may be observed within the enlarged glands.

3. Myelolipoma: appears as a round mass with well-defined borders. They contain a mixture of mature fat and hematopoietic cells, resulting in characteristic imaging features. They demonstrate low attenuation values (< -30 HU) on CT scans due to the presence of adipose tissue. Additionally, they exhibit T1-weighted imaging (T1WI) hyperintensity and signal loss with fat suppression. (Figure 5).

4. Pheochromocytoma: originating from chromaffin cells within the adrenal medulla, is a neuroendocrine tumor renowned for its excessive secretion of catecholamines, chiefly adrenaline and noradrenaline. They can manifest as solid or cystic masses, occasionally exhibiting calcific or necrotic components. They enhance vividly during the portal or arterial phase and have hyperintense T2-weighted signal. While generally considered benign, approximately 10% of cases may harbor malignant potential. (Figure 6)

5. Hemorrage: presents as a hyperdense lesion on imaging studies, exhibiting high attenuation ranging between 50 to 90 Hounsfield units (UH) (Figure 7). Trauma represents a common etiology for adrenal hemorrhage, although it can also occur as a complication of tumors or coagulopathies. MRI reveals varied signal intensities depending on the age of the hematoma, with different stages of clot evolution exhibiting distinct patterns. 

6. Simple cyst: characterized by a well-defined, nonenhancing mass with water density. Typically, it appears hyperintense on T2-weighted imaging (T2WI). The absence of contrast enhancement and its distinct hyperintense signal on T2WI contribute to its straightforward identification. (Figure 8).

7. Angiomyolipoma: a distinctive mesenchymal tumor arising from the adrenal gland. These tumors typically consist of a heterogeneous mixture of blood vessels, smooth muscle cells, and adipose tissue. Due to their unique composition, adrenal angiomyolipomas can exhibit varied imaging characteristics, often appearing as solid masses with areas of fat attenuation. (Figure 9).

8. Epitelial cyst: presents as a well-defined, nonenhancing mass on imaging studies. These cysts, typically lined by epithelial cells, are distinguished by their lack of contrast enhancement and clear demarcation from surrounding tissues. (Figure 10).

9. Other less common: Lipomas typically appear as well-defined masses with fat attenuation, exhibiting homogeneous signal intensity. Hemangiomas, composed of vascular tissue, may present as hypervascular masses with contrast enhancement on imaging, often demonstrating characteristic flow voids. Lymphangiomas, comprised of lymphatic vessels, manifest as cystic masses with fluid attenuation and may exhibit septations or internal debris. 

B: INFECTIOUS DISEASES

1. Tuberculosis: commonly presents with caseous necrosis, characterized by central low attenuation and peripheral enhancement within the affected adrenal gland. Chronic tuberculosis can lead to atrophy of the adrenal glands along with the development of calcifications (Figure 11). 

2. Histoplasmosis: often presents with calcifications on imaging studies. These calcifications may appear as punctate or coarse deposits within the adrenal tissue (Figures 12,13). However, these calcifications can pose a challenge in the differential diagnosis, as they may resemble calcifications seen in other conditions such as tuberculosis, lymphoma, or metastasis. 

C: BENIGN TUMORS

1. Schwannoma: typically originate from the sheath of the nerves surrounding the gland. These tumors often appear hypodense with inhomogeneous contrast enhancement, reflecting their neurogenic origin and variable tissue composition. 

2. Adenomatoid Tumor:  typically present as hypoattenuating lesions on imaging studies, exhibiting heterogeneous contrast enhancement. These tumors may also demonstrate calcifications and septations, further adding to their imaging complexity.

3: Ganglioneuroma: typically originate from sympathetic nerves and present as well-circumscribed solid lesions on imaging studies. These tumors often exhibit characteristic imaging features, appearing hypoattenuating on early post-contrast phases and hyperattenuating on delayed phases (Figure 14).

4. Oncocytoma: manifest as well-demarcated masses on imaging studies, featuring heterogeneous contrast enhancement attributable to degenerative changes within the tumor. 

D: MALIGNANT TUMORS

1. Carcinoma: typically exhibit a heterogeneous appearance on imaging studies, attributed to the presence of necrosis, calcifications, and hemorrhage within the tumor. Often demonstrate heterogeneous and predominantly peripheral enhancement on contrast-enhanced imaging. (Figure 15).

2: Lymphoma: In primary lymphoma, imaging typically reveals a single complex hypoattenuating mass without calcifications, often showing minimal contrast enhancement (Figure 16). Conversely, in secondary lymphoma, the adrenal glands may appear enlarged but maintain a relatively normal morphology on imaging studies. 

3. Metastases:  adrenal metastases from various primary cancers such as lung cancer (LC) (Figure 17), breast, colon, melanoma, kidney, and hepatocellular carcinoma (HCC) (Figure 18) often present as bilateral lesions, a characteristic feature distinguishing them from primary adrenal tumors. These lesions typically manifest as focal masses exhibiting strong and prolonged enhancement on the portal venous phase. However, their washout pattern is slower compared to adrenal adenomas.