On contrast-enhanced CT of the abdomen-pelvis in a patient with renal cell carcinoma (RCC), the tumor enhancement is measured as +40 HU on pre-contrast and +100 HU on post-contrast study. CT enhancement of ≥15 HU in a renal mass indicates:

• A Pseudoenhancement artifact that should be dismissed
• B True enhancement, indicative of a vascular (potentially malignant) lesion ✓
• C Calcification within the lesion causing high attenuation
• D Hemorrhagic cyst with high intrinsic pre-contrast density

Explanation

The accepted threshold for true enhancement of a renal mass on CT is ≥15 HU increase from pre-contrast to post-contrast images. Enhancement indicates vascularity, which is a feature of solid tumors (RCC, oncocytoma) as opposed to simple cysts. In this case, the mass enhanced by 60 HU (100-40), clearly above the threshold and confirming a hypervascular solid lesion. Pseudoenhancement can occur in small intrarenal cysts due to beam-hardening artifact but is typically <10-15 HU. Hemorrhagic cysts are hyperdense on pre-contrast but show no true enhancement post-contrast (Bosniak IIF or III if complex).

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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