Approach to the Cystic Renal Mass
- Most cystic renal masses close in one step: a homogeneous mass measuring −10 to +20 HU on CT is a simple cyst, and a homogeneous mass ≥70 HU on unenhanced CT is a hyperdense cyst. Both are benign and need nothing more.
- A homogeneous mass between those bands (>20 and <70 HU unenhanced, or >20 HU on a contrast-only scan) is the one that needs a proper pre- and post-contrast study.
- Once you have enhancement to judge, Bosniak v2019 takes over: wall or septum ≤2 mm is thin, 3 mm is IIF, ≥4 mm is III, and an enhancing nodule (≥4 mm with obtuse margins, or any size with acute margins) is IV.
- Fat attenuation (< −10 HU) inside the mass changes the conversation entirely: think angiomyolipoma, not cystic cancer.
- Stability matters. No change in features and growth ≤3 mm per year for at least 5 years means you can stop.
You will find cystic renal masses on a startling fraction of the abdominal scans you read, and almost all of them are water balloons that deserve exactly one sentence in the report. The work is not in the great majority that are obviously simple. It is in building a routine so reliable that the occasional cystic cancer cannot slide past you dressed as a balloon. This page is that routine, and it runs on two rulebooks: the ACR white paper on incidental renal masses for the first look, and Bosniak version 2019 for the grading.
What you're looking at
A cystic renal mass is any well-defined rounded lesion in the kidney whose bulk is fluid. On CT that fluid measures near water on the Hounsfield scale; on ultrasound it is anechoic with a bright back wall and through-transmission; on MRI it is dark on T1 and bright on T2. Around and inside the fluid there may be a wall, septa (internal partitions), calcification, and, in the ones that matter, enhancing soft tissue.
The thing you are actually hunting is enhancing tissue in the wall or septa, because that is where a cystic renal cell carcinoma lives. Everything else on this page is a way of getting to that judgment quickly, or of proving you don't need to make it at all.
Three descriptors carry the weight:
- Homogeneity. Is the interior uniform, or are there mixed attenuations, layering, or lumps? Homogeneous is the password for every shortcut below; a heterogeneous mass never qualifies for them.
- Attenuation (or signal). The number inside the mass, measured with a generous region of interest away from the wall, on the phase you actually have.
- Wall and septal morphology. How thick, how many, how smooth, and whether anything enhances.
The decision flow
Run every cystic renal mass through the same gates in the same order. The thresholds are the ACR white paper's and Bosniak v2019's, and the actions come straight from them.
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Is there macroscopic fat? Drop a region of interest on any dark focus. Fat attenuation is < −10 HU. If you find it, this is not a cystic mass problem any more; it is an angiomyolipoma problem. Move to that page's flow (referral thresholds live there).
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Is the mass homogeneous? If so, measure it and try to close the case. On unenhanced CT, a homogeneous mass at −10 to +20 HU without enhancement is a simple cyst (Bosniak I); no further evaluation. A homogeneous mass at ≥70 HU on unenhanced CT is almost always a hyperdense cyst (Bosniak II); no further evaluation. On a contrast-enhanced-only CT, a homogeneous mass at −10 to +20 HU is still a simple cyst, and a homogeneous mass at 21–30 HU on the portal venous phase is Bosniak II. Stop here for all of these.
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Is it too small to measure? A well-circumscribed, homogeneous mass that is too small to characterize (TSTC) and that looks visually much lower in attenuation than the parenchyma (or, unenhanced, much higher) is probably a benign cyst. If the mass genuinely needs an answer, MRI is preferred for masses <1.5 cm and for detecting enhancement in suspected hypovascular masses.
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Does the number land in the gray zone? A homogeneous mass measuring >20 and <70 HU on unenhanced CT, or >20 HU on a contrast-enhanced-only CT (outside the 21–30 HU portal venous exception), needs pre- and post-contrast characterization. Enhancement of the mass or of any nodular component is concerning for neoplasm. Order the renal-mass-protocol CT or MRI.
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Now grade the walls and septa (Bosniak v2019). With a study that can show enhancement, measure the thickest enhancing wall or septum and count the septa:
- Thin smooth wall (≤2 mm), simple fluid (−9 to 20 HU), no septa: class I. The wall may enhance.
- Thin wall with few (1–3) thin (≤2 mm) septa (may enhance, any calcification): class II. No follow-up.
- Smooth minimally thickened (3 mm) enhancing wall or septa, or many (≥4) smooth thin enhancing septa: class IIF. Follow-up imaging.
- Enhancing thick (≥4 mm) or irregular (≤3 mm obtusely margined convex protrusions) wall or septa: class III. Roughly half malignant; urologic referral.
- Enhancing nodule (a ≥4 mm convex protrusion with obtuse margins, or any size with acute margins): class IV. Roughly 90% malignant; treatment for appropriate candidates.
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Check the priors before you write anything. A mass with no change in imaging features and an average growth of ≤3 mm per year for at least 5 years is stable and needs no further workup. Growth is defined as ≥4 mm per year on average, and morphologic change is any change in heterogeneity: contour, attenuation, or number of septa. A stable-for-years cystic mass with new septa is not stable.
Step 2's ≥70 HU rule is an unenhanced rule. On contrast-enhanced CT, both a hyperdense cyst and a renal cell carcinoma can measure ≥70 HU, so a bright homogeneous mass on a portal venous scan is not a cyst until an unenhanced or MRI comparison says so.
Skipping "homogeneous." Every shortcut in step 2 begins with that word. A mass that averages 15 HU but has a 40 HU lump on one edge is heterogeneous, and heterogeneous masses go to step 4 no matter what the mean says. Renal cell carcinoma is almost always heterogeneous on unenhanced CT; a homogeneous mass under 20 HU is rarely RCC.
Discriminators
The mimics of a cystic renal cancer are mostly other cystic things, plus two solid impostors that can look cystic on the wrong phase.
| Mimic | Looks similar because | Tell them apart by |
|---|---|---|
| Simple cyst (Bosniak I) | Round, fluid-filled, sharply marginated | Homogeneous −10 to +20 HU (−9 to 20 HU in Bosniak's wording), wall ≤2 mm, no septa; no enhancing thickening anywhere |
| Hemorrhagic or proteinaceous (hyperdense) cyst | Bright on a single phase, easily mistaken for solid | Homogeneous ≥70 HU on unenhanced CT is a Bosniak II cyst; on a pre/post pair it does not enhance |
| Minimally complex cyst (Bosniak II or IIF) | Has septa or a slightly thick wall | Count and measure: 1–3 thin (≤2 mm) septa is II; a 3 mm smooth enhancing wall or septum, or ≥4 thin septa, is IIF; thick (≥4 mm) or irregular tissue is III |
| Cystic renal cell carcinoma (Bosniak III or IV) | Same fluid core as a benign cyst | Enhancing thick (≥4 mm) or irregular walls or septa (III), or an enhancing nodule ≥4 mm with obtuse margins or any size with acute margins (IV) |
| Necrotic solid RCC | A central fluid-attenuation cavity | The bulk is enhancing solid tissue with a fluid center, not a fluid mass with a thin enhancing rim; heterogeneous on unenhanced CT |
| Angiomyolipoma with a low-attenuation region | A dark interior region on soft-tissue windows | Fat attenuation < −10 HU on a region of interest, versus water attenuation for fluid |
| Papillary RCC (hypovascular solid mass) | Enhances weakly; on one phase it can pass for a cyst above 20 HU | Homogeneous >20 HU on a contrast-only scan (outside the 21–30 HU portal venous exception) needs a pre/post pair; MRI is preferred for detecting enhancement in suspected hypovascular masses |
| Pseudoenhancement of a small cyst | A small intraparenchymal cyst appears to gain attenuation after contrast | Recognise the setting (small cyst inside bright cortex); there is no accepted HU cutoff, so use MRI or ultrasound to arbitrate rather than a number |
The numbers
| What | Threshold / value | Why it matters |
|---|---|---|
| Macroscopic fat | < −10 HU | Diverts the mass to the angiomyolipoma pathway |
| Simple cyst on CT | Homogeneous −10 to +20 HU, no enhancement | Bosniak I; no further evaluation |
| Hyperdense cyst | Homogeneous ≥70 HU on unenhanced CT | Bosniak II; no further evaluation; unenhanced rule only |
| Indeterminate homogeneous mass | >20 and <70 HU unenhanced, or >20 HU on contrast-only CT | Needs pre- and post-contrast characterization |
| Portal venous exception | Homogeneous 21–30 HU on portal venous phase | Bosniak II; no follow-up |
| MRI preferred | Masses <1.5 cm; suspected hypovascular masses | Better at detecting subtle enhancement |
| Thin wall or septum | ≤2 mm | Bosniak I or II territory |
| Minimally thickened | 3 mm (smooth, enhancing) | Bosniak IIF; follow-up imaging |
| Many septa | ≥4 smooth thin enhancing septa | Bosniak IIF |
| Thick wall or septum | ≥4 mm, enhancing | Bosniak III; urologic referral; roughly half malignant |
| Irregular wall or septum | ≤3 mm obtusely margined convex protrusions, enhancing | Bosniak III |
| Nodule | ≥4 mm convex protrusion with obtuse margins, or any size with acute margins, enhancing | Bosniak IV; roughly 90% malignant |
| Stable mass | No feature change and growth ≤3 mm per year for at least 5 years | No further workup |
| Growth | ≥4 mm per year on average | Counts as change |
What to say in the report
The referrer needs four things: where it is, how big it is, what class it is, and what to do. Something like:
Left kidney, interpolar, exophytic cystic mass measuring 3.4 × 3.0 × 2.9 cm. Homogeneous fluid attenuation on the unenhanced phase. Smooth wall measuring 3 mm that enhances on the nephrographic phase; no septa; no nodule. Bosniak IIF (version 2019). Recommend follow-up imaging.
The rules for a clean cystic-mass report:
- Name the version. "Bosniak IIF (v2019)" tells the next reader which ruler you used.
- Give the measurement that decided the class. "Wall 3 mm" or "septum 5 mm" makes your reasoning checkable and comparable next time.
- State the phase you measured on. "78 HU on unenhanced CT" is a closed case; "78 HU on portal venous CT" is an open one.
- If you cannot grade it, say why and what would. "Enhancement cannot be assessed on this single-phase study; a renal-mass-protocol CT or MRI would allow Bosniak classification."
- Use the ACR language for the benign ones, and mean it: "simple cyst, no further evaluation needed." A benign cyst that gets recommended for follow-up is a small harm delivered with good intentions.
The whole page compresses to one habit: homogeneous and in a benign attenuation band, stop; otherwise get the pre/post pair, measure the thickest enhancing tissue, and read the class off the table. Do that on every kidney and the balloon with a passenger inside will not get past you.
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 HU fat threshold; the −10 to +20 HU simple-cyst band; the ≥70 HU unenhanced hyperdense-cyst rule and its restriction to unenhanced CT; the >20 and <70 HU (unenhanced) and >20 HU (contrast-only) indeterminate bands; the too-small-to-characterize guidance and the <1.5 cm and hypovascular-mass MRI preference; the enhancement-is-concerning-for-neoplasm statement; the ≤3 mm per year for at least 5 years stability rule and the ≥4 mm per year growth definition; and the statement that RCC is almost always heterogeneous unenhanced while a homogeneous mass under 20 HU is rarely RCC, all in "The decision flow," "Discriminators," and "The numbers."
- 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 Bosniak v2019 class definitions in step 5 of "The decision flow," "Discriminators," and "The numbers" (≤2 mm thin, 3 mm minimally thickened, ≥4 mm thick; 1–3 few and ≥4 many septa; ≤3 mm irregular protrusions; ≥4 mm obtuse-margined or any-size acute-margined nodule; −9 to 20 HU simple fluid; the 21–30 HU portal venous class II subtype), the "roughly half" and "roughly 90%" malignancy summaries for classes III and IV, and the per-class actions; also the example report's IIF assignment in "What to say in the report."