How this instrument works
Adrenal masses are often found by accident on a scan ordered for something else, and CT contrast washout is one way to characterize such a mass without a biopsy. The idea rests on a real physiologic difference: benign adrenal adenomas tend to take up contrast dye and then clear it, or wash it out, faster than other adrenal masses such as metastases, pheochromocytomas, or adrenocortical carcinoma. Comparing HU (Hounsfield unit, the CT attenuation measure) values across three timed phases — before contrast, an enhanced phase shortly after, and a delayed phase 10-15 minutes later — turns that difference into two percentages. StatPearls' reference on adrenal adenoma cites an absolute washout above 60% and a relative washout above 40% as figures reported to be highly sensitive and specific for an adenoma over other adrenal masses.
Absolute washout compares the drop from the enhanced phase to the delayed phase against the full rise from the unenhanced baseline, so it requires all three phases to have been scanned. Relative washout skips the unenhanced phase and compares the same drop against the enhanced value alone, which makes it usable when a true pre-contrast scan was never obtained — a common situation, since many adrenal masses are first noticed on a scan that was never planned with adrenal characterization in mind.
Washout thresholds are a well-established piece of the adrenal workup, not an infallible one. The same StatPearls reference notes that more recent studies have found this measurement carries lower sensitivity and specificity for distinguishing adenomas than the original figures suggested, and lipid-poor adenomas in particular can wash out more slowly and blur the line with other masses. A result above or below these thresholds is one input into a radiologist's overall read of the mass — size, growth over time, and unenhanced density all factor in alongside washout, and further imaging or follow-up can still be warranted either way.
- Enter Enhanced HU — the CT attenuation reading from the contrast-enhanced phase, taken shortly after contrast injection.
- Enter Delayed HU — the attenuation reading from a delayed scan, typically 10-15 minutes after contrast.
- Enter Unenhanced HU — the attenuation reading from a true pre-contrast scan, if one was obtained.
- Read both Absolute washout % and Relative washout %; use relative washout when no unenhanced phase is available.
Worked example — a clear washout and a borderline one
Enhanced 120 HU, delayed 60 HU, unenhanced 20 HU. Absolute washout is (120 − 60) ÷ (120 − 20) × 100 = 60 ÷ 100 × 100 = 60%, above the commonly cited 60% threshold suggestive of a benign adenoma. The relative figure is (120 − 60) ÷ 120 × 100 = 60 ÷ 120 × 100 = 50%, likewise above its own commonly cited 40% threshold — both figures point the same direction here.
Enhanced 100 HU, delayed 90 HU, unenhanced 10 HU. Absolute washout is (100 − 90) ÷ (100 − 10) × 100 = 10 ÷ 90 × 100 ≈ 11.11%, well under the benign-suggestive threshold — a small, slow drop like this would prompt further characterization rather than reassurance. The relative figure comes out to (100 − 90) ÷ 100 × 100 = 10%, also low, telling the same story from the enhanced value alone.
Enhanced 150 HU, delayed 50 HU, unenhanced 10 HU. Absolute washout is (150 − 50) ÷ (150 − 10) × 100 = 100 ÷ 140 × 100 ≈ 71.43%, comfortably above the benign-suggestive threshold. The relative figure is (150 − 50) ÷ 150 × 100 = 100 ÷ 150 × 100 ≈ 66.67%, also comfortably above its threshold — a large, fast drop consistent with a benign adenoma.
Questions
What counts as a benign-suggestive washout result?
Commonly cited thresholds are an absolute washout above 60% or a relative figure above 40%, numbers StatPearls describes as reported to be highly sensitive and specific for an adenoma over a metastasis, pheochromocytoma, or carcinoma. These are statistical thresholds from studied populations, not a guarantee for any one mass — more recent research has found somewhat lower accuracy than the original figures suggested.
What's the difference between absolute and relative washout?
Absolute washout measures the drop from the enhanced to the delayed phase against the full climb from a true unenhanced baseline, so it needs all three scan phases. The relative figure measures the same drop against the enhanced value alone, without needing an unenhanced phase at all — useful when a mass is spotted on a scan that was never planned to include a pre-contrast pass.
Why would a radiologist use relative washout instead of absolute?
Mainly necessity: many adrenal masses turn up incidentally on a CT that was ordered for an unrelated reason and never included a true unenhanced phase. Relative washout only needs the enhanced and delayed readings, so it can still be calculated in that situation, at some cost to accuracy compared with the absolute figure when a genuine unenhanced scan is available.
Does a benign-suggestive washout result rule out cancer completely?
No single washout figure rules anything out completely. It shifts the probability toward a benign adenoma and is one part of a fuller picture that includes the mass's size, its growth over time, its unenhanced density, and the person's overall clinical context — and recent studies have found these thresholds somewhat less reliable than once thought, which is why radiologists weigh it alongside everything else rather than in isolation.
What does HU mean in these calculations?
HU stands for Hounsfield unit, the standard scale CT scanners use to express how much a tissue attenuates, or blocks, the X-ray beam — water is defined as 0 HU and air as −1000 HU. A higher HU on the enhanced phase, followed by a bigger drop by the delayed phase, is the pattern that produces a high washout percentage and points toward a benign adenoma.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.