A 45 mm multilocular renal cyst: IIF or III?
A 58-year-old woman has an incidental 45 mm cystic lesion in the left kidney found on ultrasound performed for urinary frequency. She is otherwise well, with a blood pressure of 132/80 mmHg and an eGFR of 72. A contrast-enhanced CT is performed in the corticomedullary and nephrographic phases. The lesion is well circumscribed, predominantly water density, and contains at least four smooth septa measuring 1–2 mm. The septa show perceived enhancement on the nephrographic phase but no measurable enhancement, no nodularity, no calcification and no solid component. The contralateral kidney and the rest of the abdomen are normal.
1. Assign a Bosniak category using the 2019 classification and explain which features drove your decision.
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Bosniak IIF.
The discriminators that matter, in order:
| Feature | This lesion | Consequence |
|---|---|---|
| Wall/septal thickness | 1–2 mm, smooth | Thin - keeps it out of III |
| Number of septa | ≥4 | Moves it above II, which allows only a few thin septa |
| Enhancement | Perceived only, not measurable | Perceived enhancement alone does not force III |
| Nodularity | None | Excludes IV |
| Calcification | None | - |
| Attenuation | Water density (not homogeneous high attenuation) | - |
In the 2019 system, category III requires measurable enhancement of a thick (≥4 mm) or irregular wall or septum; category IV requires an enhancing nodule of ≥4 mm. Category IIF is where multiple thin septa, minimally thick (about 3 mm) smooth walls/septa with perceived enhancement, or homogeneous high-attenuation lesions ≥3 cm belong. This lesion is a textbook IIF - but the honest caveat is that 'perceived versus measurable' enhancement is a subjective call, and if the septa measured 3–4 mm the category would shift.
2. What is the risk of malignancy and what surveillance would you recommend?
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Risk of malignancy in IIF is roughly 5% (reported ranges about 0–9%), compared with around 50% for III and over 90% for IV. So the overwhelming likelihood is that this is benign - a complicated or septated cyst - and the purpose of surveillance is to catch the minority that declare themselves.
Plan:
- Follow-up imaging at 6 months, then annually if stable, for about 5 years.
- MRI is preferable to CT for the follow-up: better contrast resolution for septa and nodules, and it avoids stacking several CT doses on a 58-year-old woman over 5 years. Non-contrast T2 plus a post-gadolinium series is adequate.
- Review the prior ultrasound and ask specifically whether this could be a haemorrhagic cyst with retracted clot mimicking septa - a common cause of over-grading.
- Stop surveillance if the patient develops significant comorbidity or frailty, or after sustained stability, since the yield of further imaging falls and the clinical relevance of a slow-growing IIF lesion in an elderly patient is low.
I would explain the numbers explicitly: the scan is far more likely to reassure than to find cancer, and the plan is to watch, not to treat.
3. The patient is anxious and asks for the lesion to be removed. How do you respond, and what would make you change to intervention or biopsy?
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Removal would be overtreatment for a IIF lesion, and I would say so plainly.
- Roughly 19 in 20 IIF lesions are benign. A partial nephrectomy carries real risks - bleeding, urine leak, loss of renal function, anaesthetic risk - and would expose her to those risks for a lesion that most likely needs nothing.
- Active surveillance is the guideline-endorsed management for IIF. The anxiety is best treated with a clear explanation of the numbers and a defined, dated follow-up plan rather than with an operation.
I would change to intervention if any of the following appear:
- Measurable enhancement of wall or septa, or thickening to ≥4 mm or irregularity (upgrades to III).
- Any enhancing nodule ≥4 mm (upgrades to IV).
- Sustained growth, or new symptoms (pain, haematuria, hypertension).
On biopsy: percutaneous sampling of a cystic lesion has a high non-diagnostic rate and cannot reliably exclude malignancy; it is not routine for IIF and a benign result would not remove the need for follow-up. I would reserve it for a lesion where the result would genuinely change the decision - for example a Bosniak III lesion in a patient in whom surgery is high risk and a benign diagnosis would allow surveillance instead.