Understanding Adrenal Incidentalomas
An adrenal incidentaloma is an adrenal mass, typically 1 cm or larger, discovered during a radiological examination for an unrelated reason [1.7.1, 1.7.3]. The prevalence of these incidentalomas increases with age, from less than 1% in individuals under 30 to as high as 7% in those over 70 [1.7.6]. While the majority (around 85%) of these tumors are benign and non-functional, they require careful evaluation to rule out malignancy or hormonal overproduction (functional tumors) [1.6.3, 1.7.2].
Computed tomography (CT) is a primary imaging modality for this evaluation. One of the most crucial techniques involves assessing how quickly a contrast agent is 'washed out' of the adrenal mass. This process helps characterize the lesion, as benign adenomas tend to wash out contrast more rapidly than malignant tumors or pheochromocytomas [1.4.3].
The Adrenal Washout Formulas
To calculate adrenal washout, attenuation measurements of the lesion are taken in Hounsfield Units (HU) at three different phases: unenhanced (pre-contrast), portal venous (approximately 60-70 seconds post-contrast), and delayed (10-15 minutes post-contrast) [1.2.6]. Two main formulas are used.
Absolute Percent Washout (APW)
The absolute washout calculation is used when an unenhanced (pre-contrast) CT scan is available. It compares the enhancement at the portal venous and delayed phases to the baseline unenhanced value [1.2.4].
Formula:
$$APW = \frac{(HU_{portal\,venous} - HU_{delayed})}{(HU_{portal\,venous} - HU_{unenhanced})} \times 100$$
Interpretation: An APW of 60% or greater is highly suggestive of a benign adrenal adenoma [1.2.4, 1.4.1]. An APW below 60% is considered indeterminate [1.2.7].
Relative Percent Washout (RPW)
The relative washout is calculated when a pre-contrast scan is not available. It relies only on the portal venous and delayed phase images [1.2.4].
Formula:
$$RPW = \frac{(HU_{portal\,venous} - HU_{delayed})}{HU_{portal\,venous}} \times 100$$
Interpretation: An RPW of 40% or greater is considered indicative of a benign adrenal adenoma [1.2.4, 1.4.1]. An RPW below 40% is indeterminate [1.2.7].
CT Protocol and Interpretation
A dedicated adrenal protocol CT is essential for accurate washout calculation. This involves:
- Unenhanced Phase: An initial scan without intravenous contrast to measure the baseline density of the lesion. A lesion with an attenuation value of ≤10 HU on this scan is almost certainly a benign, lipid-rich adenoma, and no further washout calculation is typically needed [1.8.5, 1.3.7].
- Portal Venous Phase: A scan performed approximately 60-75 seconds after contrast administration captures the peak enhancement of the adrenal lesion [1.2.7].
- Delayed Phase: A final scan taken 15 minutes after contrast administration measures how much contrast has washed out of the lesion [1.4.3].
Adrenal Adenoma vs. Other Lesions: A Comparison
Different adrenal lesions exhibit distinct characteristics on CT scans. Correctly interpreting these features alongside washout values is crucial for diagnosis.
| Feature | Benign Adenoma | Pheochromocytoma | Metastasis |
|---|---|---|---|
| Unenhanced HU | Typically ≤10 HU (lipid-rich) but can be >10 HU (lipid-poor) [1.8.5]. | Variable, but most are >10 HU [1.8.1]. Can appear heterogeneous [1.8.2]. | Usually >10 HU [1.8.5]. |
| Enhancement | Moderate enhancement [1.8.1]. | Typically avid and heterogeneous enhancement, often greater in the arterial phase [1.8.1, 1.8.2]. | Variable enhancement, often avid [1.4.3]. |
| Washout (APW/RPW) | Rapid washout. APW ≥60%, RPW ≥40% [1.4.1]. | Washout characteristics can overlap with adenomas, but many show slow washout [1.8.1, 1.4.3]. | Slow or poor washout [1.4.1]. |
| Size & Shape | Usually small (<4 cm), round, with smooth margins [1.6.4]. | Can be large and are often heterogeneous or cystic [1.8.1, 1.8.2]. | Size is variable; may have irregular borders [1.6.4]. |
Limitations and Pitfalls
While a powerful tool, adrenal washout calculation has several limitations:
- Lesion Heterogeneity: Masses with significant necrosis, hemorrhage, or calcification cannot be reliably assessed with washout formulas [1.5.3].
- Small Lesion Size: For lesions smaller than 1 cm, accurate HU measurement can be difficult due to partial volume effects [1.5.5].
- Overlapping Features: Some pheochromocytomas and hypervascular metastases (e.g., from renal cell carcinoma) can demonstrate rapid washout, mimicking an adenoma [1.4.3].
- Diagnostic Performance: Some recent studies suggest that while washout CT has a high negative predictive value, its overall sensitivity for excluding malignancy in certain populations can be poor, and its utility has been debated [1.5.2, 1.5.4, 1.4.2].
Conclusion
Calculating adrenal washout is a fundamental skill in the radiological assessment of adrenal incidentalomas. By applying the absolute and relative washout formulas to data from a properly executed adrenal protocol CT, clinicians can effectively differentiate benign adenomas from more concerning lesions. An absolute washout of ≥60% or a relative washout of ≥40% strongly suggests a benign etiology [1.4.1]. However, radiologists must remain aware of the technique's limitations, considering other imaging features like lesion size, homogeneity, and unenhanced attenuation to arrive at an accurate diagnosis.
For further reading, consider this authoritative source: Adrenal washout | Radiology Reference Article - Radiopaedia.org
