The mass on the kidney
Most are found by accident on a scan ordered for something else. The Bosniak score tells you how worried to be about a cyst; the size tells you whether to watch, ablate, or operate — and 'spare the nephron' is the quiet rule behind all of it.
The one-liner
Bosniak classifies cystic masses (I–II benign, III–IV surgical). For solid masses, size drives the plan: small renal masses <4 cm allow surveillance, ablation, or partial nephrectomy; partial is preferred wherever technically feasible to preserve renal function. A fat-containing mass is an angiomyolipoma, not a cancer.
The modern renal mass is usually a surprise — an incidental finding on a CT ordered for abdominal pain or a fall. The old triad of flank pain, a mass, and hematuria now signals advanced disease and is rarely how cancers present. That shift toward small, asymptomatic tumors is exactly why the management has become so nuanced: many of these masses do not need to come out, and the kidney they sit in is worth saving.
First question: is it even solid?
For a cystic mass, the Bosniak classification (v2019) grades malignant potential on contrast CT or MRI:
- I — simple cyst, benign. Ignore it.
- II — minimally complex, benign. No follow-up.
- IIF— “follow”; small malignant risk, surveil with imaging.
- III — indeterminate, ~50% malignant; treat as surgical.
- IV — clearly malignant features (enhancing soft tissue); surgical.
What everyone gets wrong
Forgetting the two benign masses that masquerade as cancer. Macroscopic fat on imaging means angiomyolipoma— a benign lesion, not RCC (the one caveat: large ones > 4 cm can bleed and may warrant embolization). And an oncocytoma can look identical to RCC on imaging and is only confidently called on pathology — which is part of why a biopsy has a role before committing some patients to surgery.
Solid masses: size runs the algorithm
For an enhancing solid mass, the diameter sets the options:
- Small renal mass, < 4 cm (cT1a) — the flexible zone. Reasonable choices are active surveillance (especially in elderly or comorbid patients, where the competing risk of dying from something else is high), thermal ablation (cryo or radiofrequency), or partial nephrectomy.
- cT1b, 4–7 cm — partial nephrectomy wherever technically feasible; otherwise radical.
- cT2 and beyond, > 7 cm — radical nephrectomy.
Spare the nephron whenever you can. Every patient has a renal future, and a partial nephrectomy that preserves function protects against the cardiovascular cost of chronic kidney disease down the line.
When it has spread — and the surgical heroics
RCC has a signature trick: it grows as a tumor thrombus up the renal vein into the IVC, sometimes all the way to the right atrium. The Mayo level (0–IV) grades how high it reaches and dictates the surgical approach — a level IV thrombus may need cardiopulmonary bypass. The thrombus itself is not a contraindication to cure; it is a planning problem.
Metastatic disease
Clear-cell RCC is famously chemo- and radio-resistant, so systemic therapy is built on different tools: immune checkpoint inhibitors combined with tyrosine kinase inhibitors (IO + TKI) are now first-line for most metastatic clear-cell disease. Cytoreductive nephrectomy retains a role in selected patients, and a solitary metastasis can be resected (metastasectomy).
If you remember one thing
Two scores and one principle. Bosniak tells you how worried to be about a cyst; size tells you whether to watch, ablate, or cut a solid mass; and nephron-sparing is the default whenever the anatomy allows. And never call a fatty mass a cancer.