Renal Masses & the Bosniak System

Here is the plot twist of kidney imaging: the vast majority of "masses" we find are cysts, the kidney's equivalent of a water balloon, and they are utterly harmless. The whole job is sorting the harmless water balloons from the small minority of things that actually want to hurt the patient. And we mostly do it without a knife, just contrast and a CT scanner.

Most renal masses are discovered completely by accident — someone gets a CT for a kidney stone or a sore belly, and there it is, a quiet little lesion minding its own business. So the question is never "did we find a mass," it's "now what." (For the full step-by-step search pattern, see the approach to the renal mass.)

Step one: is it cyst or is it solid?

Everything starts with one question that sounds almost insultingly basic: is this thing fluid, or is it tissue?

A simple cyst is water in a smooth-walled bag. On CT it measures right around water density, it's perfectly round, it has a hair-thin wall, and — the clincher — it does not light up when you give intravenous iodinated contrast. On ultrasound a simple cyst is the most satisfying thing in radiology: jet black inside, sharp back wall, and a bright band deep to it (posterior acoustic enhancement, because sound sails through fluid unimpeded). If it ticks all those boxes, you can stop. It's nothing.

Figure · US

Enhancement: the magic word

The single most important concept in this whole topic is enhancement. We scan once without contrast, inject contrast, then scan again. Solid tumor tissue has its own blood supply, so it grabs the contrast and gets brighter — its attenuation jumps up. Plain old fluid has no vessels, so it just sits there at the same number. That jump, a clear rise between the unenhanced and enhanced images, is the fingerprint of living, vascularized tissue, and enhancement of a mass or of any nodular component is what the ACR calls "concerning for neoplasm."

Findings by modality

CT

CT does most of the work, and the unenhanced attenuation sorts a surprising number of masses on its own. A homogeneous mass measuring −10 to +20 HU with no enhancement is a simple cyst (Bosniak I) and needs no further evaluation, whether the scan was with or without contrast. A homogeneous mass at ≥70 HU on unenhanced CT is almost always a hyperdense (Bosniak II) cyst — old blood or protein — and is also done. A homogeneous mass that falls between, >20 and <70 HU unenhanced (or >20 HU on a contrast-only scan), is the one that needs dedicated pre- and post-contrast characterization, because it could be a cyst with thick contents or it could be a hypovascular tumor. Fat measuring < −10 HU inside a mass means angiomyolipoma (AML).

For the cystic masses, the multiphase study is where Bosniak lives: you're measuring wall and septal thickness in millimeters, counting septa, and hunting for enhancing nodules. For solid masses, you're describing enhancement, heterogeneity, the interface with the kidney, and — for staging — the renal vein, the IVC, and the perinephric fat.

Figure · CT

MRI

MRI is the tie-breaker. The ACR prefers it for masses under 1.5 cm — the "too small to characterize" (TSTC) lesions where a CT region of interest can't be trusted — and for detecting enhancement in suspected hypovascular masses, because subtraction imaging shows subtle enhancement that CT's noisy numbers hide. T2 signal helps too: simple fluid is brilliantly bright, hemorrhagic or proteinaceous cysts are variable, and solid tumors are usually intermediate. Chemical-shift and fat-suppressed sequences confirm the macroscopic fat of an AML.

Ultrasound

Ultrasound is the cheapest way to prove a cyst is simple (anechoic, thin wall, posterior enhancement) and a good first look at an indeterminate low-density CT lesion. Contrast-enhanced ultrasound, where available, can show enhancement of septa and nodules without iodine or gadolinium. Its limitation is the deep or obese patient and the small mass.

The numbers

How good is the test

There is no robust pooled sensitivity or specificity for CT or MRI in separating benign from malignant solid renal masses, so no figure is stated here — the honest summary is that imaging characterizes cysts extremely well (a simple cyst is a diagnosis, not a suspicion) and characterizes solid masses only partly, which is why the size-based benign rates above matter: they are the best pre-test odds you have before a biopsy or a surgeon.

Bosniak: a weather forecast for cysts

Some cysts aren't simple. They grow internal walls (septa), their walls thicken, they pick up calcium, or — worst of all — they sprout enhancing solid bits. The Bosniak classification is a tidy way of saying "how complicated does this cyst look, and therefore how nervous should we be?" Think of it as a weather forecast: from clear skies up to take-cover.

Abbreviated from Bosniak Classification of Cystic Renal Masses, Version 2019 (Silverman et al.). This is the pocket version; the full table with every attenuation band, the MRI criteria, and the worked examples lives on the Bosniak v2019 classification page, and the step-by-step decision flow for a cyst you've just found is the approach to the cystic renal mass. You'll notice the ACR white paper and the Bosniak paper draw the "simple fluid" line a hair apart (−10 versus −9 HU) — same idea, two documents.

The beauty is that the grade climbs in step with the suspicion: the more enhancing tissue and irregular architecture you see, the further up the ladder you go. (If grading lesions by a roman-numeral risk scale feels familiar, it's the same spirit as LI-RADS for liver lesions — radiologists love a good scoring system.)

Solid masses: guilty until proven innocent

If the lesion isn't cystic at all — it's a solid, enhancing lump — the default assumption in an adult is renal cell carcinoma (RCC) until something proves otherwise. RCC is the most common malignant kidney tumor in adults, and a solid enhancing mass is its classic look. Its staging by size is refreshingly simple at the small end: T1a is ≤4 cm and T1b is >4 to ≤7 cm, both confined to the kidney; T2 begins above 7 cm; and the T3 categories are about what the tumor has invaded — the renal vein, the collecting system, the perirenal or sinus fat, the IVC. The subtypes and the rest of the staging are on the RCC subtypes & staging page.

But there's one famous benign impostor worth knowing: the angiomyolipoma. As the mouthful of a name advertises, it contains blood vessels, muscle, and — crucially — fat. That macroscopic fat is a gift, because fat has a very distinctive, very negative attenuation on CT: a region of interest measuring < −10 HU inside the mass points strongly to a benign AML rather than cancer. AMLs are usually left alone, but a big one can bleed, which is why an AML ≥4 cm or one carrying an aneurysm >0.5 cm is referred for prophylactic treatment, and a symptomatic AML is referred at any size. AML and its lookalike oncocytoma get their own page: angiomyolipoma & oncocytoma.

The differential

Why we obsess over getting this right

The stakes are real but the calculus is reassuring. Pure simple cysts are extraordinarily common and need nothing. The middle ground — the IIF cysts and the small indeterminate solid masses — gets follow-up imaging, and a mass that shows no morphologic change and grows ≤3 mm per year for at least 5 years can be declared stable and dismissed. Bosniak III and IV lesions and genuinely worrying solid masses get referred for treatment, often surgery, sometimes a biopsy or ablation first — remembering that the smaller the solid mass, the better the odds it's benign or indolent.

This same "is it fluid, does it enhance" instinct carries straight over to its neighbor upstairs, the adrenal lesion — different organ, same detective work.

Reporting

• Size in three planes and location (pole, anterior or posterior, exophytic or endophytic, relation to the hilum and collecting system).
• Cystic or solid, with the unenhanced attenuation in HU and whether the mass is homogeneous.
• Enhancement: present or absent, and where (wall, septa, nodule, whole mass).
• For a cystic mass: the Bosniak v2019 class, with the feature that drove it (wall or septal thickness in millimeters, number of septa, nodule margins).
• For a solid mass: macroscopic fat present or absent; heterogeneity; the renal vein and IVC; perinephric fat and Gerota fascia; adrenal, nodes, and the other kidney — the T-category ingredients.
• Comparison: growth per year against the ≤3 mm and ≥4 mm definitions, and any change in morphology.
• Recommendation: none, follow-up interval, MRI for a TSTC mass, or urologic referral.

If you remember nothing else: chase the enhancement. Water in a bag is nothing; tissue that lights up is something. Everything else is just deciding how worried to be.

References

• Herts BR, Silverman SG, Hindman NM, et al. Management of the Incidental Renal Mass on CT: A White Paper of the ACR Incidental Findings Committee. J Am Coll Radiol 2018;15(2):264–273. Used for: the −10 to +20 HU simple-cyst rule, the ≥70 HU unenhanced hyperdense-cyst rule, the >20 and <70 HU (and >20 HU contrast-only) indeterminate band and the statement that enhancement of a mass or nodular component is concerning for neoplasm, the < −10 HU fat threshold, the TSTC definition and the MRI preference under 1.5 cm, the benign-by-size figures (≈40%, ≈20%, <10%), the stability (≤3 mm per year for at least 5 years) and growth (≥4 mm per year) definitions, and the AML referral thresholds (≥4 cm, aneurysm >0.5 cm; symptomatic AML at any size) in "Key Points," "Findings by modality," "The numbers," "Solid masses," "Why we obsess," and "Reporting."
• Silverman SG, Pedrosa I, Ellis JH, et al. Bosniak Classification of Cystic Renal Masses, Version 2019: An Update Proposal and Needs Assessment. Radiology 2019;292(2):475–488. Used for: the abbreviated Bosniak v2019 class I–IV definitions (≤2 mm thin, 3 mm minimally thickened, ≥4 mm thick, the −9 to 20 HU and ≥70 HU attenuation criteria, septal counts, nodule margins) and the "roughly half" and "roughly 90%" malignancy summaries in "Bosniak: a weather forecast for cysts" and "Key Points."
• Oesterling JE, Fishman EK, Goldman SM, Marshall FF. The management of renal angiomyolipoma. J Urol 1986;135(6):1121–1124. Used for: the origin of the 4 cm angiomyolipoma treatment cutoff in "Solid masses" and "The numbers."
• Amin MB, Edge SB, Greene FL, et al., eds. AJCC Cancer Staging Manual. 8th ed. Springer; 2017. Kidney chapter (renal cell carcinoma TNM). Used for: the T1a (≤4 cm), T1b (>4 to ≤7 cm), T2 (>7 cm) size cutoffs and the T3 invasion definitions in "The numbers" and "Solid masses."