What is CT Washout?
CT washout is a specialized imaging technique that measures how rapidly intravenous contrast material leaves an adrenal lesion over time, calculated from attenuation values obtained during a three-phase dedicated adrenal protocol CT scan. 1
Technical Protocol
CT washout requires a dedicated three-phase adrenal protocol CT that includes: 1
- Noncontrast phase to establish baseline attenuation in Hounsfield units (HU)
- Contrast-enhanced venous phase at 60-70 seconds post-injection
- Delayed phase typically at 15 minutes post-injection
Standard single-phase abdominal CT protocols are inadequate for adrenal lesion characterization and should not be used. 1
Calculation Methods
Two washout formulas are used: 1
- Relative washout = [(venous HU - delayed HU) / (venous HU - noncontrast HU)] × 100%
- Threshold: >40% indicates adenoma
- Absolute washout = [(venous HU - delayed HU) / venous HU] × 100%
- Threshold: >60% indicates adenoma
Clinical Rationale
The physiologic basis for washout analysis is that benign adenomas demonstrate marked early enhancement with rapid contrast loss due to normal vascular architecture, while malignant lesions show slower washout due to abnormal blood vessels. 1
When to Use CT Washout
CT washout should only be performed when noncontrast CT shows attenuation >10 HU in a homogeneous adrenal lesion. 1 If noncontrast attenuation is ≤10 HU, the lesion is definitively a benign lipid-rich adenoma and no contrast study is needed. 1, 2
Critical Limitations and Pitfalls
Recent evidence has revealed significant limitations that clinicians must understand: 1
False Positives (Benign Lesions Appearing Malignant)
- Approximately one-third of benign adenomas fail to meet washout thresholds and would be misclassified as indeterminate 3
- In one large pathology-confirmed series, 61.6% of lesions with <60% absolute washout were actually benign adenomas 3
False Negatives (Malignant Lesions Appearing Benign)
- Up to 35% of pheochromocytomas demonstrate washout characteristics identical to adenomas 1, 4
- Hypervascular metastases (particularly renal cell carcinoma and hepatocellular carcinoma) can mimic adenoma washout patterns 1
- In a multi-institutional study of true incidentalomas, 20% of malignant lesions showed ≥60% absolute washout 5
Performance Data from Contemporary Studies
In a 2022 multi-institutional study of 336 incidental adrenal nodules <4 cm, washout ≥60% had only 77.5% sensitivity and 70.0% specificity for benign lesions, with a notably poor 9.2% negative predictive value. 5 This means that among nodules failing to meet washout criteria, only 9.2% were actually malignant—highlighting that washout has limited utility in true incidentalomas. 5
A 2021 surgical series found that CT washout ≥60% had 67.6% sensitivity, 77.8% specificity, and only 38.4% negative predictive value for detecting adenomas. 3 The accuracy was 69.7%, meaning nearly one-third of lesions were misclassified. 3
Current Guideline Recommendations
Recent endocrine guidelines from the American College of Endocrinology and American Association of Clinical Endocrinologists now recommend MRI with chemical shift imaging over contrast-enhanced CT washout when additional imaging is required after noncontrast CT. 1, 2 This shift reflects growing recognition of washout CT's limitations. 1
Specific Scenarios Where Washout CT May Still Be Used
Despite limitations, washout CT can be incorporated to "rule-in" benign tumors when: 3
- The lesion is homogeneous
- Noncontrast attenuation is between 10-30 HU
- There is low clinical suspicion for pheochromocytoma
However, clinicians must recognize that pheochromocytoma cannot be excluded by imaging alone, and biochemical testing with plasma or urinary fractionated metanephrines is mandatory before any adrenal biopsy or intervention. 4, 2
Optimal Delayed Imaging Timing
The 15-minute delayed phase provides optimal diagnostic performance, with sensitivity of 93.9% and specificity of 95.8% using a 55% washout threshold in pathology-confirmed cases. 6 Earlier studies using 10-minute delays showed slightly lower accuracy. 7, 6
Alternative Approach: MRI Chemical Shift Imaging
MRI with chemical shift imaging (CSI) detects intracellular lipid without ionizing radiation and is now preferred over CT washout for indeterminate lesions. 1, 2 CSI is particularly valuable for heterogeneous lesions where CT washout performs poorly. 1