Adrenal Washout Calculator - APW and RPW from CT Hounsfield Units

An adrenal washout calculator is a clinical radiology decision support tool that reads the three Hounsfield unit measurements from a triple-phase adrenal CT and returns the absolute and relative percentage washout values used in adrenal mass interpretation. It is useful for radiologists triaging adrenal incidentalomas, residents comparing washout patterns before sign-out, endocrine and surgical clinicians, and patients who want a structured summary for a follow-up visit.

• • Adrenal incidentaloma triage: Decide whether a newly found adrenal mass shows the washout pattern of a benign lipid-poor adenoma or one needing further workup.
• • Oncology staging review: Compare the washout of an adrenal lesion in a cancer patient against the adenoma threshold.
• • Protocol planning: Plan the unenhanced, portal venous, and 15-minute delayed phases of an adrenal CT and document the HU values.
• • Patient follow-up: Bring a structured washout summary to a follow-up visit so the next step is clear.

Adrenal masses are common incidental findings, and most are benign lipid-rich or lipid-poor adenomas. The washout CT protocol is the standard way to separate those adenomas from lesions that look more like metastases, pheochromocytomas, or primary adrenal malignancies. The calculator turns the three HU values into the percentage washout numbers a radiologist reads out in the report.

Use the output as a clinical reference, not a diagnosis. Confirm any borderline pattern with the radiology report, the unenhanced attenuation, MRI or chemical shift imaging when needed, and the broader endocrine and oncologic picture.

For other threshold-driven imaging categories, the same washout-style logic appears in the TIRADS Calculator, which sorts thyroid nodules by their own point and size thresholds.

The calculator runs two short formulas on the entered Hounsfield unit values. The first uses all three phases to compute the absolute percentage washout. The second uses the enhanced and delayed phases to compute the relative percentage washout, which is the right number to trust when the unenhanced phase was not obtained.

• Unenhanced HU: Pre-contrast attenuation of the adrenal mass. Optional; leave blank to skip the absolute washout. An unenhanced value at or below 10 HU already points to a lipid-rich adenoma.
• Enhanced HU: Portal venous phase attenuation, 60 to 80 seconds after contrast injection. Used in both washout formulas.
• Delayed HU: Delayed phase attenuation, 15 minutes after contrast injection. Used in both washout formulas.

The relative washout is the right number when the unenhanced phase was not obtained, the patient was imaged on a scanner without a pre-contrast series, or the mass is being compared against a reference set that used the same two-phase protocol. The absolute washout is the more specific test, and the triple-phase protocol is preferred whenever the unenhanced series is available.

The 15-minute delayed phase is the standard timing used to define both thresholds. An unenhanced value of 10 HU or below already points to a lipid-rich adenoma regardless of the washout numbers, while an unenhanced value above 10 HU and a high washout together suggest a lipid-poor adenoma.

According to Korobkin et al., Radiology (1998), an absolute percentage washout above 60 percent, or a relative percentage washout above 40 percent on 15-minute delayed imaging, characterizes an adrenal mass as a benign lipid-poor adenoma.

Contrast handling depends on kidney function, so the washout protocol should be reviewed with the GFR Calculator in any patient whose renal function is borderline before the contrast bolus is given.

Four concepts hold the washout reading together, and they show up on every adrenal mass report.

The thresholds are conservative by design. An adenoma that sits just above the APW threshold may still be called indeterminate, especially in a patient with a known primary cancer. Read the result alongside the unenhanced attenuation, the lesion size, the patient's endocrine labs, and the history of malignancy.

The unenhanced attenuation and the washout are complementary. A lipid-rich adenoma can show a low unenhanced HU and a modest washout, while a lipid-poor adenoma usually has a higher unenhanced HU and a high washout. A non-adenoma pattern has both a higher unenhanced HU and a low washout.

A new adrenal mass on the CT is one trigger for a hypertension workup, and the Blood Pressure Calculator helps frame where the next reading sits on the AHA and ACC/AHA categories before aldosterone or cortisol labs are drawn.

Use the three Hounsfield unit values from the washout CT report. The calculator is designed to read those values, not to estimate them.

1. 1 Open the radiology report: Pull the most recent triple-phase adrenal CT report and find the unenhanced, portal venous, and 15-minute delayed HU values.
2. 2 Enter the unenhanced HU: Type the pre-contrast HU value. Leave it blank if the unenhanced series was not obtained, and the calculator will skip the absolute washout.
3. 3 Enter the enhanced and delayed HU: Type the portal venous and 15-minute delayed HU values, both required for the relative washout.
4. 4 Read the washout numbers: Look at the absolute and relative percentage washout values, then at the interpretation label and the applied threshold.
5. 5 Compare to the adenoma thresholds: Use the 60 percent APW and 40 percent RPW thresholds to label the result as adenoma, indeterminate, or non-adenoma.
6. 6 Bring the summary to the team: Share the washout values and the interpretation with the radiologist, endocrinologist, or oncology team.

A 60-year-old with an incidentally discovered 2.4 cm right adrenal mass has an unenhanced HU of 18, an enhanced HU of 80, and a delayed HU of 30. The APW is 80.6 percent and the RPW is 62.5 percent, and the calculator reads 'Washout pattern consistent with lipid-poor adenoma'. The patient is followed with serial imaging rather than biopsy.

A triple-phase adrenal CT adds up to a non-trivial effective dose, so bring the Radiation Dose Calculator to the protocol review to put the cumulative exposure in the same units used for other imaging studies.

The result helps turn three HU values into the standard washout numbers used in the radiology report. Rerun the adrenal washout calculator whenever a new triple-phase CT is performed.

• • Standardized washout output: Returns both the absolute and relative percentage washout values alongside the interpretation label, so the washout protocol is reported the same way for every adrenal mass.
• • Adenoma threshold built in: Applies the APW above 60 percent and RPW above 40 percent thresholds from the Korobkin protocol, so the result matches the standard radiology report language.
• • Two-phase fallback: Computes the relative washout when the unenhanced phase is missing, and clearly shows 'not applied' for the absolute washout.
• • Lipid-rich flag: Highlights when the unenhanced attenuation is at or below 10 HU, which already supports a lipid-rich adenoma even before the washout is reviewed.
• • Indeterminate and non-adenoma labels: Labels borderline results as indeterminate and clearly distinguishes non-adenoma from adenoma patterns for the next imaging or biopsy step.

Use the adrenal washout calculator to prepare for tumor board, review a discharge summary, or walk a patient through what the washout means. It is also a quick check when an unfamiliar mass appears on a staging CT.

The result is not a diagnosis. The interpretation label organizes the washout values, the threshold, and the lipid-rich flag so the treating team can decide on the next imaging, biopsy, or follow-up scan.

Cortisol excess in the adrenal workup often surfaces first as fasting glucose and insulin changes, so the Insulin Dosage Calculator is a useful companion for turning the endocrine labs into a usable mealtime plan.

Several protocol and patient factors change how the washout numbers should be read, even when the inputs look reasonable.

• • The calculator is a clinical reference. It does not replace the radiology report, does not perform chemical shift MRI, and does not evaluate PET-avid lesions or known adrenal malignancies.
• • The result is only as reliable as the input HU values. Region-of-interest placement, motion artifact, beam hardening, and partial volume effects can each change the reported HU by 5 to 10 units.
• • The adenoma thresholds were derived from adult surgical and imaging cohorts. Pediatric, pregnant, and post-treatment patients may not fit the same thresholds.

An APW below 60 percent in a cancer patient should prompt a search for adrenal metastases, including correlation with FDG-PET, MRI with chemical shift, and the rest of the staging workup.

When the unenhanced and enhanced values are equal, the absolute washout is not defined. The calculator falls back to the relative washout and labels the result as indeterminate, and the team can decide whether to repeat the CT or move to MRI.

According to Blake et al., RadioGraphics, an unenhanced adrenal mass attenuation of 10 Hounsfield units or lower is highly specific for a lipid-rich adenoma, and contrast-enhanced washout CT is reserved for lesions above that threshold.

According to ACR Appropriateness Criteria: Adrenal Mass Evaluation, contrast-enhanced washout CT is the preferred imaging tool for characterizing an incidentally discovered adrenal mass that is not clearly benign on unenhanced CT.

A metabolic workup often follows a borderline washout, and the Cholesterol Ratio Calculator provides the total-to-HDL, LDL-to-HDL, and triglyceride-to-HDL ratios used in cardiovascular risk framing.