What characteristics of suprarenal (adrenal gland) nodules suggest infection or cancer?

Characteristics of Suprarenal (Adrenal) Nodules Suggesting Infection or Cancer

Malignancy should be strongly suspected when adrenal nodules are larger than 4-5 cm, demonstrate inhomogeneous appearance with irregular margins, show high attenuation (>10 Hounsfield units) on unenhanced CT, or exhibit poor contrast washout (<60% at 15 minutes). 1

Size Criteria for Malignancy Risk

Size is the single most important predictor of malignancy in adrenal nodules:

• Nodules >5 cm: Malignancy should be strongly suspected, particularly if combined with other suspicious features 1
• Nodules >4 cm: Warrant comprehensive evaluation and are considered high-risk for adrenocortical carcinoma in nonfunctioning tumors 1
• Nodules <3 cm: Have 87% probability of being benign in patients without cancer history 1

Imaging Characteristics Suggesting Malignancy

CT Findings

On unenhanced CT, the following features suggest malignancy:

• Hounsfield units >10: A threshold value distinguishing benign from malignant tumors 1
• Inhomogeneous appearance: Internal heterogeneity with irregular solid components 1
• Irregular margins: Infiltrative or microlobulated borders rather than smooth contours 1
• Poor contrast washout: Enhancement washout <60% at 15 minutes on delayed contrast-enhanced CT suggests malignancy (benign adenomas typically show >60% washout) 1

MRI Findings

Chemical-shift MRI is highly sensitive and specific for differentiation:

• Absence of signal loss: Most malignant tumors do not contain intracellular lipid, whereas benign adenomas do 1, 2
• Lack of fat content: Malignant lesions typically lack the lipid content seen in adenomas on opposed-phase imaging 1, 2
• Better visualization of local invasion: MRI more clearly documents invasion into adjacent structures and inferior vena cava involvement than CT 1

Features Suggesting Metastatic Disease vs Primary Malignancy

Additional imaging findings that increase suspicion for malignancy:

• Adjacent lymphadenopathy: Presence of enlarged regional lymph nodes 1
• Liver metastases: Concurrent hepatic lesions on contrast-enhanced imaging 1
• Local invasion: Extension into kidney, liver, pancreas, spleen, or diaphragm 1
• Vascular involvement: Tumor extension into the inferior vena cava 1

PET/CT Findings

FDG-PET/CT provides additional diagnostic value:

• High FDG uptake: Adrenal masses with standardized uptake value (SUV) greater than liver suggest malignancy with 83.3% sensitivity and 85.4% specificity 3
• Negative predictive value of 93%: Non-FDG avid masses are likely benign 3
• False-negative rate: Patients receiving therapy may have false-negative PET results despite malignancy 3

Characteristics Suggesting Infection (Particularly Histoplasmosis)

Infectious etiologies, especially granulomatous infections, can mimic malignancy:

• Bilateral adrenal enlargement: Mass-like enlargement of both glands suggests infection (particularly histoplasmosis) or lymphoma 4
• Intense FDG uptake: Granulomatous infections can demonstrate intense uptake similar to malignancy 4
• Associated pulmonary nodules: Multiple lung nodules with mild uptake alongside adrenal masses suggest disseminated infection 4
• Clinical context: Immunocompromised state, endemic area exposure, or systemic symptoms favor infectious etiology 4

Critical Diagnostic Algorithm

When evaluating suspicious adrenal nodules, follow this approach:

1. Obtain unenhanced CT first: Measure Hounsfield units—if <10 HU, likely benign adenoma 1
2. If HU >10: Proceed with contrast-enhanced CT with 15-minute delayed washout imaging 1
3. If washout <60%: Consider chemical-shift MRI for further characterization 1
4. If imaging remains indeterminate: FDG-PET/CT may help differentiate benign from malignant lesions 1, 3
5. Consider biopsy only after excluding pheochromocytoma: Biopsy carries risk of hypertensive crisis and tumor spillage 1

Common Pitfalls to Avoid

Critical errors in adrenal nodule evaluation:

• Never biopsy before biochemical workup: Always exclude pheochromocytoma with plasma metanephrines before considering biopsy to avoid potentially fatal hypertensive crisis 1
• Don't rely on size alone: Even smaller nodules can be malignant if they demonstrate irregular margins, heterogeneity, or high attenuation 1
• Avoid laparoscopic approach for suspected malignancy: Open adrenalectomy is preferred for high-risk lesions due to increased risk of local recurrence and peritoneal spread with laparoscopy 1
• Don't assume bilateral involvement is always metastatic: Bilateral adrenal masses can represent infection (histoplasmosis, tuberculosis) or lymphoma 4