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Renal masses: Bosniak 2019, angiomyolipoma and RCC

A renal cyst is classified by Bosniak 2019 by its wall, septa and enhancing nodules — I–II benign, IIF for follow-up, III–IV for treatment — and macroscopic fat in a solid mass diagnoses angiomyolipoma.

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Why it matters in the exam

Bosniak 2019: I — a simple cyst with a thin wall (2 mm or less), homogeneous fluid of -9 to 20 HU, no septa. II — few thin septa (1–3), calcification, a homogeneous hyperdense cyst (70 HU or more without contrast), or a homogeneous non-enhancing cyst. IIF — a minimally thickened (3 mm) enhancing wall or septa, or many thin septa (4 or more). III — a thick (4 mm or more) or irregular enhancing wall or septum. IV — an enhancing nodule. I and II — benign without follow-up; IIF — follow-up (6 and 12 months, then yearly to 5 years); III — about half malignant, surgery or active surveillance; IV — mostly malignant, treatment. Enhancement on CT is defined as a rise of 20 HU or more.

Angiomyolipoma (AML): macroscopic fat (-10 HU or less) in a solid mass — diagnostic; multiple and bilateral in tuberous sclerosis. Above 4 cm there is a bleeding risk and preventive embolisation is considered. "Fat-poor" AML mimics RCC. RCC by subtype: clear cell — the commonest, hypervascular and heterogeneous, bright on T2, and multiple in von Hippel-Lindau disease; papillary — hypovascular, homogeneous and dark on T2; chromophobe — intermediate, sometimes with a central scar. Oncocytoma with a central scar cannot be reliably distinguished from RCC. Renal lymphoma — multiple bilateral weakly enhancing masses, or tissue encasing the kidney.

Staging (AJCC 8): T1a up to 4 cm, T1b 4–7 cm, T2 above 7 cm and limited to the kidney, T3a invasion of perinephric or sinus fat or the renal vein, T3b thrombus in the vena cava below the diaphragm, T3c above the diaphragm or invasion of the caval wall, T4 beyond Gerota's fascia or into the ipsilateral adrenal. A 5 cm mass involving the perinephric fat is T3a. Cystic diseases with malignancy risk: von Hippel-Lindau disease, tuberous sclerosis and acquired cystic disease in dialysis; ADPKD — large kidneys with many cysts, liver cysts and cerebral aneurysms, without increased RCC risk. Adrenal adenoma: 10 HU or less without contrast, or absolute washout of 60% or more, or signal drop on chemical shift.

Key points

• I–II benign; IIF follow-up; III–IV treatment.
• Enhancement = a rise of 20 HU or more.
• Macroscopic fat — AML; above 4 cm bleeding risk.
• Clear cell — hypervascular; papillary — hypovascular and dark on T2.
• T1a up to 4 cm; T3a fat or renal vein.

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Sources

• Bosniak Classification of Cystic Renal Masses, Version 2019 (Silverman et al.), 2019 — Definitions and management
• AJCC Cancer Staging Manual, 8th edition (2017) — Kidney
• ACR Appropriateness Criteria: Indeterminate Renal Mass, 2020 — CT and MRI renal mass protocol

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.

See also: Renal cysts and benign masses: Bosniak, AML and oncocytoma