What does a renal cyst measuring about 70 Hounsfield units on a non‑contrast CT scan indicate?

What is a 70 HU Renal Cyst?

A renal cyst measuring 70 HU on non-contrast CT is classified as benign and requires no further evaluation or urologic referral. 1

Definition and Clinical Significance

A 70 HU renal cyst is a hyperdense (high-attenuation) renal cyst that appears brighter than normal kidney tissue on non-contrast CT imaging. 2 According to the American College of Radiology Appropriateness Criteria, homogeneous renal masses measuring >70 HU on unenhanced CT are characterized as benign lesions and do not require additional imaging workup or specialist consultation. 1, 3

Underlying Pathophysiology

These hyperdense cysts are typically simple cysts that have undergone hemorrhage or contain proteinaceous fluid:

• Hemorrhagic cysts are the most common cause, where bleeding into a simple cyst increases the attenuation value to 30-60 HU above adjacent parenchyma (typically 70-90 HU range). 2
• High protein content within the cyst fluid can also cause elevated attenuation values, sometimes reaching 35 HU or higher. 4
• The cyst remains benign despite the elevated density. 5

Diagnostic Criteria

Key imaging features that confirm benignity at 70 HU: 1

• Homogeneous appearance without nodules, septations, or calcifications
• No enhancement after IV contrast administration (enhancement <10 HU between phases)
• Well-defined margins with smooth borders
• Attenuation value >70 HU on non-contrast CT

Important Caveats and Pitfalls

The 70 HU threshold applies ONLY to non-contrast CT. 1 Several critical considerations exist:

• Masses between 20-70 HU are indeterminate and require multiphase contrast-enhanced CT or MRI for further characterization. 3, 6
• Papillary renal cell carcinomas can occasionally demonstrate low enhancement (13.6-59.1% may not reach 15-20 HU enhancement thresholds) and could theoretically be mistaken for hyperdense cysts. 7 However, these would typically show some enhancement pattern change between phases.
• Ultrasound findings may be misleading: hyperdense cysts often demonstrate internal echoes and lack of posterior acoustic enhancement, mimicking solid masses. 2
• If any enhancement is detected (>10-15 HU change between corticomedullary and nephrographic phases), the lesion should be considered a solid renal neoplasm requiring urologic referral. 8

When Further Evaluation IS Required

Despite the >70 HU benign classification, obtain contrast-enhanced imaging if: 1

• The mass is heterogeneous rather than homogeneous
• Nodules, thick septations, or calcifications are visible
• The mass demonstrates any enhancement on available contrast phases
• Clinical suspicion exists based on patient symptoms or risk factors

Management Algorithm

For a truly homogeneous 70 HU renal mass on non-contrast CT: 3, 9

1. No further imaging is required
2. No urologic referral is necessary
3. No surveillance imaging is indicated
4. Document as benign hyperdense cyst (likely hemorrhagic or proteinaceous)

If uncertainty exists about homogeneity or enhancement: 3, 6

1. Obtain multiphase contrast-enhanced CT (unenhanced, corticomedullary at 25-70 seconds, nephrographic at 80-180 seconds)
2. Measure attenuation change between phases
3. If enhancement >10 HU occurs, refer to urology for solid renal mass evaluation