Indeterminate renal mass or suspected renal cell carcinoma

ACR AC Indeterminate Renal Mass; Bosniak v2019; ACR AC RCC Staging 2022; ACR Contrast Manual (GBCA groups)

The question is whether the lesion enhances, and enhancement is a subtraction. Any protocol without an unenhanced acquisition in matched geometry cannot answer it, which is why a routine single-phase portal-venous CT — the study most often requested — is non-diagnostic here.

Renal lesion found incidentally or during investigation of haematuria or flank pain, which cannot be confidently called a simple cyst on the study that found it.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

MRI Renal — mass characterisation

MRI Renal

What we'd amend, and why

MRI measures enhancement by subtracting an unenhanced acquisition of identical geometry from each dynamic phase, which is the only reliable way to assess a lesion that is intrinsically bright on T1 — a haemorrhagic or proteinaceous cyst looks enhancing on visual comparison and is unmasked by subtraction. It also avoids the pseudoenhancement that inflates small intrarenal lesion attenuation on CT, adds chemical-shift imaging for microscopic fat, and images the renal vein and inferior vena cava for tumour thrombus without a second radiation exposure. The corticomedullary acquisition is the phase on which clear cell likelihood is judged, so mistiming it costs the specific information the study was requested for.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwaystandard — Adults — renal mass characterisation
rulerule-mr-device-screening — MR safety screening for implants and foreign bodies; checked by Radiographer at the scanner
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-gadolinium-renal — Kidney function and gadolinium-based contrast; checked by Radiographer at the scanner
rulerule-pregnancy-gadolinium — Gadolinium in known or possible pregnancy; checked by Radiographer at the scanner
rulerule-iv-access — Intravenous access adequate for the planned injection; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(2)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

MR safety screening for implants and foreign bodies
Complete the MR safety questionnaire, verify implant labelling and its stated conditions of use against this scanner and this protocol, and ensure no ferromagnetic object enters Zone IV.
Radiographer at the scannerBefore the scan
Flags back if: An implant or retained foreign body that is MR Unsafe, unlabelled, or cannot be identified; or an MR Conditional device whose stated conditions this scanner or the requested protocol cannot satisfy; or a credible unexcluded intraocular metallic foreign body history.
Intravenous access adequate for the planned injection
Site and test a cannula that supports the protocol flow rate, preferring an antecubital or large forearm vein, and observe the injection for extravasation. A 20-gauge or larger cannula is preferred for flow rates of 3 mL/s or more.
Radiographer at the scannerAt the scanner
Flags back if: No cannula can be sited that supports the protocol flow rate — for example only a 22-gauge hand or foot cannula for a CT angiogram needing 4–5 mL/s; or the only available access is a central line or port that is not labelled power-injectable; or an extravasation occurs.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Adults — renal mass characterisation

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	MRI Renal MRI Renal — mass characterisation usually appropriate	MRI measures enhancement by subtracting an unenhanced acquisition of identical geometry from each dynamic phase, which is the only reliable way to assess a lesion that is intrinsically bright on T1 — a haemorrhagic or proteinaceous cyst looks enhancing on visual comparison and is unmasked by subtraction. It also avoids the pseudoenhancement that inflates small intrarenal lesion attenuation on CT, adds chemical-shift imaging for microscopic fat, and images the renal vein and inferior vena cava for tumour thrombus without a second radiation exposure. The corticomedullary acquisition is the phase on which clear cell likelihood is judged, so mistiming it costs the specific information the study was requested for.
Reasonable alternative	CT Urogram CT Urogram — Conventional Three Phase	The CT route where MRI is unavailable, contraindicated or not tolerated. What makes it diagnostic is the true unenhanced series and a nephrographic acquisition at roughly 90-100 seconds, from which the attenuation change is calculated using identically placed regions of interest; the excretory series simultaneously interrogates the urothelium, which is frequently the other half of the clinical question. A dedicated multiphase renal CT protocol without urographic phases is the more targeted equivalent where local practice offers it.
Problem solving	Ultrasound renal tract Ultrasound renal tract	Confirms a simple cyst outright when the appearance is anechoic, thin-walled and through-transmitting, ending the pathway with no radiation and no contrast. Where a microbubble contrast agent is available, contrast-enhanced ultrasound characterises septal and nodular enhancement in real time and is the recommended alternative when neither iodinated nor gadolinium-based contrast can be given.
Second line	CT Chest CT Chest — Contrast-Enhanced (Venous Phase)	Added once a renal malignancy is established rather than before, and then only where the stage warrants it: the lung is the commonest site of distant spread, and thoracic staging is what completes the assessment for a large or locally advanced primary. It is not part of characterising an indeterminate lesion. A small renal mass — clinical T1a, up to 4 cm, with no adverse imaging features — has a very low rate of synchronous thoracic metastasis, and adding a chest CT before the diagnosis exists is a second exposure bought on speculation. Where the mass is confirmed and small, a chest radiograph is a defensible baseline; CT belongs to the larger or locally advanced tumour.

Pitfalls

Accepting a routine portal-venous CT abdomen for a renal mass. With no unenhanced baseline, enhancement cannot be measured and the report can only say the lesion is indeterminate.
Pseudoenhancement: attenuation of a small intrarenal cyst rises by 10-20 HU purely from beam-hardening within the enhanced kidney, and is repeatedly reported as a solid mass.
Measuring on a corticomedullary or portal-venous acquisition and calling a slowly enhancing papillary tumour non-enhancing.
Misregistration between MRI phases from inconsistent breath-holds, which produces artefactual rim enhancement on subtraction images.
Classifying a necrotic solid tumour as a complex cyst. The cystic categories assume the enhancing component is a minority of the mass.
Forgetting to look at the renal vein and inferior vena cava — tumour thrombus changes the operation and is easy to overlook when attention is on the mass.
Adding a staging chest CT to the characterisation of a small renal mass. The lesion has to be malignant, and large or locally advanced enough to plausibly have spread, before the thorax is worth a second acquisition.
Surveying a small renal mass on active surveillance with repeated multiphase CT. The question there is growth rate, not diagnosis: a single matched acquisition, or ultrasound where the lesion is well seen, answers it at a fraction of the cumulative dose.
Withholding gadolinium for an eGFR in the 30s. Group II agents need no eGFR threshold, and an unenhanced renal MRI does not characterise anything — the contrast was almost never the risk in this request.

Priors — what to pull first

Search hard for older cross-sectional imaging. A lesion unchanged over several years is effectively characterised, and that comparison prevents a great deal of investigation.
Where a prior study exists, match its technique for the follow-up — apparent growth is otherwise attributable to phase and slice thickness rather than to the tumour.

What makes a good request

Enhancement on CT is defined by comparison with an unenhanced baseline. An attenuation increase of more than about 20 HU is accepted as enhancement, 10 HU or less as absent, and the range between as indeterminate and needing another modality. None of those statements is available without the unenhanced series.
Nephrographic timing matters as much as the baseline: some papillary tumours enhance slowly and are called non-enhancing if measured only on a corticomedullary or portal-venous acquisition.
Bosniak version 2019 applies to cystic masses on both CT and MRI and defines a cystic mass as one in which less than a quarter is enhancing tissue — which stops a necrotic solid tumour being classified as a cyst.
Small renal masses in the elderly or comorbid are increasingly managed by active surveillance, so the imaging question is often growth rate rather than diagnosis, and technique consistency between studies matters more than any single scan.

Scoring this once it is done

The classification and risk tools this question ends in.

Bosniak Classification v2019 (Cystic Renal Mass, CT/MRI)RENAL Nephrometry Score eGFR (CKD-EPI 2021 Creatinine)Clear Cell Likelihood Score (ccLS) v2.0 — Renal Mass mpMRI RECIST 1.1 Tumor Response

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

MRI Renal — mass characterisation CT Urogram — Conventional Three Phase Ultrasound renal tract CT Chest — Contrast-Enhanced (Venous Phase)

Confirm locally

MRI Renal — mass characterisation: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.