Adrenal simple cyst (endothelial cyst) is a thin-walled, clear fluid-containing benign cystic lesion. It constitutes 45% of all adrenal cysts. Usually unilateral, solitary, and asymptomatic. On CT, appears as a water-density (0-20 HU), thin-walled, non-enhancing lesion. On MRI, shows T1 hypointense, T2 hyperintense fluid signal. Contains no internal septation, wall calcification, or solid component — presence of these features should suggest pseudocyst or cystic neoplasm. Size is usually <5 cm, diagnosis is made by imaging and no further workup is needed. Treatment may be considered for large cysts (>5 cm) due to mass effect or hemorrhage risk.
Age Range
20-80
Peak Age
50
Gender
Female predominant
Prevalence
Uncommon
Adrenal simple cysts are cystic structures lined by endothelium, most commonly classified as lymphangiomatous type (lymphatic endothelial origin) or vascular type (vascular endothelial origin). Although pathogenesis is not fully understood, it is thought to be related to dilation, obstruction, or congenital malformation of lymphatic or vascular channels. Cyst fluid is of serous transudate nature — low protein content, clear fluid. The pathophysiological basis of imaging findings: near-water density on CT (0-20 HU) results from cyst fluid being nearly pure water — low protein content attenuates X-ray photons as minimally as water. T1 hypointense signal on MRI reflects the long T1 relaxation time of free water protons; T2 hyperintense signal reflects the long T2 relaxation time. Absence of enhancement results from the thin, avascular cyst wall and the lack of vascular structures within the cyst contents.
A cystic lesion in the adrenal gland with 0-20 HU density, thin smooth wall, no enhancement, and no internal septation or solid component is diagnostic for simple cyst. Confirmed on MRI with T1 hypointense, T2 markedly hyperintense fluid signal. This combination provides near-100% diagnostic reliability and requires no additional workup.
On non-contrast CT, adrenal simple cyst appears as a homogeneous water-density (0-20 HU) lesion. Wall is thin (<2 mm) and smooth. Contains no internal septation, debris, or calcification. ROI measurement should be placed centrally, avoiding the wall. If protein content is increased, density may rise to 20-30 HU — this is considered a 'complicated cyst.'
Report Sentence
A cystic lesion measuring approximately ___ cm in the left/right adrenal gland with water density (___ HU), thin smooth wall, and no enhancement is seen, consistent with adrenal simple cyst.
In the arterial phase, cyst contents and thin wall show no enhancement. This reflects the avascular cyst contents and very thin wall. Cyst density remains unchanged from non-contrast phase.
Report Sentence
In the arterial phase, no enhancement is seen in the cystic lesion, supporting the diagnosis of simple cyst.
On T1-weighted images, simple cyst shows low signal (hypointense) — due to the long T1 relaxation time of free water protons. Isointense to CSF. If protein content is increased, T1 signal intensity may slightly increase. T1 hyperintense areas may be seen in the presence of hemorrhage (methemoglobin).
Report Sentence
On T1-weighted images, the cystic lesion shows hypointense fluid signal.
On T2-weighted images, simple cyst shows markedly hyperintense signal — isointense or slightly hyperintense to CSF. This reflects the long T2 relaxation time of free water protons. Homogeneous fluid signal confirms absence of solid component or debris.
Report Sentence
On T2-weighted images, the cystic lesion shows markedly hyperintense signal isointense to CSF, consistent with simple cyst.
On DWI, simple cyst does not show diffusion restriction. Signal disappears at high b values (T2 shine-through minimal). ADC values are high (>2.0 × 10⁻³ mm²/s) — reflecting free water diffusion. This finding is critical in differentiating simple cyst from abscess (diffusion restriction positive).
Report Sentence
On DWI, no diffusion restriction is seen in the cystic lesion, with ADC value of ___ × 10⁻³ mm²/s; this finding is consistent with simple cyst and argues against abscess.
On FDG PET-CT, simple cyst shows no FDG uptake. Cyst contents are metabolically inactive. The thin cyst wall also does not show significant FDG uptake. Presence of FDG uptake excludes simple cyst diagnosis and should suggest infectious or neoplastic process.
Report Sentence
On FDG PET-CT, no FDG uptake is observed in the cystic adrenal lesion.
Criteria
Lined with lymphatic endothelium. Most common adrenal simple cyst type. Immunohistochemistry: D2-40 positive.
Distinct Features
Cannot be distinguished from vascular type on imaging. Histopathological diagnosis.
Criteria
Lined with vascular endothelium. Second most common type. Immunohistochemistry: CD34 positive, CD31 positive.
Distinct Features
Cannot be distinguished from lymphangiomatous type on imaging. Histopathological diagnosis.
Criteria
Lined with epithelium. 9% of all adrenal cysts. Mesothelial or glandular epithelium.
Distinct Features
Cannot be distinguished from endothelial cyst on imaging. Some carry potential malignancy risk (cystadenoma). Histopathology required.
Distinguishing Feature
Pseudocyst shows thick fibrous wall, wall calcification (40%) and proteinaceous/hemorrhagic content (>20 HU). Simple cyst has thin wall, water density, no calcification.
Distinguishing Feature
Cystically degenerated pheochromocytoma contains solid enhancing component, high-density wall, markedly hyperintense on T2. Simple cyst is entirely cystic, no enhancement.
Distinguishing Feature
Adrenal hemorrhage shows high density (>40 HU acute), stage-dependent MRI signal, decreases over time. Simple cyst has water density, stable size.
Distinguishing Feature
N/A
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upAdrenal simple cyst is a benign lesion, and when typical imaging findings are met (water density, thin wall, no enhancement), diagnosis is definitive — no biopsy or follow-up needed. In large cysts (>5-6 cm), surgery or percutaneous aspiration may be considered for symptomatic discomfort or hemorrhage risk. When atypical features are present (thick wall, septation, solid component, >20 HU), pseudocyst, cystic pheochromocytoma, or cystic neoplasm should be excluded — MRI and/or further evaluation is needed in these cases.
Adrenal simple cyst is benign and requires no follow-up or treatment. Diagnosis can be confidently made when typical imaging features are present.