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Renal cysts and benign masses: Bosniak, AML and oncocytoma

A renal cyst is graded by Bosniak — I and II are benign, IIF needs follow-up, III and IV need treatment or active surveillance; angiomyolipoma is identified by fat on CT and treated when large or bleeding, and oncocytoma cannot be reliably told from RCC on imaging.

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Why it matters in the exam

Simple cysts are very common in older age, and the exam asks when such a finding is benign and when it needs treatment. Bosniak I: thin wall, water density, no septa. Bosniak II: a few thin septa, fine calcification, or a small homogeneous hyperdense cyst — benign and needing no follow-up. Bosniak IIF: minimally and smoothly thickened wall or septa, or many septa — imaging follow-up at 6 months, 12 months and then yearly up to 5 years. Bosniak III: thick or irregular enhancing wall or septa — about half are malignant; excision or active surveillance. Bosniak IV: an enhancing soft-tissue nodule — most are malignant; excision.

Angiomyolipoma (AML) consists of blood vessels, smooth muscle and fat; the fat appears on CT with negative density and allows diagnosis without biopsy. AML is linked to tuberous sclerosis — where it is multiple and bilateral. The main risk is spontaneous retroperitoneal bleeding (Wunderlich), and treatment is considered for a tumour over 4 cm, symptoms, a woman of childbearing age or when follow-up is difficult. Treatment: selective embolisation (also in acute bleeding), partial nephrectomy, and in tuberous sclerosis — everolimus (an mTOR inhibitor). Fat-poor AML looks like RCC.

Oncocytoma is a benign tumour; a "central scar" on imaging is not a reliable sign, and even at biopsy it is hard to tell from chromophobe RCC, so it is usually treated like a solid mass. About one in five small solid masses (up to 4 cm) are benign. Renal mass biopsy helps when the result will change management: before active surveillance, before ablation, when lymphoma or metastasis is suspected, and in a patient unfit for surgery.

Key points

I and II — benign.
IIF — follow-up up to 5 years.
III — about half malignant; IV — most malignant.
AML — fat on CT; treat over 4 cm.
Oncocytoma — no reliable diagnosis on imaging.

Summary table

Finding	Feature	Treatment or follow-up
Bosniak I and II	Thin wall, a few thin septa	No follow-up needed
Bosniak IIF	Minimal smooth thickening, many septa	Follow-up: 6 months, 12 months, then yearly up to 5 years
Bosniak III	Thick enhancing wall or septa	Excision or active surveillance; about half malignant
Bosniak IV	Enhancing soft-tissue nodule	Excision; most malignant
Angiomyolipoma	Fat on CT, tuberous sclerosis	Treat over 4 cm or if bleeding: embolisation, partial nephrectomy, everolimus
Oncocytoma	Unreliable central scar	Treated as a solid mass

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Sources

Bosniak Classification of Cystic Renal Masses, Version 2019: An Update Proposal and Needs Assessment, 2019 — Classification criteria
EAU Guidelines on Renal Cell Carcinoma, 2025 edition — Cystic renal lesions and renal tumour biopsy
AUA Guideline: Renal Mass and Localized Renal Cancer, 2021 — Evaluation and management

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.