Main histological subtypes
- Clear cell carcinoma (Clear cell carcinoma): the most frequent subtype (85%). Characterized by significant contrast enhancement. Associated with aggressive BAP1 syndrome and von Hippel-Lindau disease (often combined with multifocal cysts containing a solid component).
- Papillary tumors (Papillary tumours): 10–15% of cases. Show minimal (mild) contrast enhancement. Homogeneous on unenhanced CT, which can lead to misinterpretation as a hyperdense cyst. Easily missed on single-phase post-contrast CT.
- Chromophobe tumors (Chromophobe tumours): 5% of cases. Show homogeneous contrast enhancement, often featuring a central scar (mimicking oncocytomas on imaging). Hyperechoic on ultrasound.
- Medullary carcinoma: a rare subtype of collecting duct epithelium. Occurs in young patients with sickle cell anemia. Prognosis is extremely poor, with metastases often present at the time of diagnosis.
Diagnostic imaging (ultrasound, CT, MRI)
Ultrasound (US) Small tumors are typically hyperechoic, while up to 86% of large tumors are isoechoic compared to normal parenchyma. Central necrosis may present as a hypoechoic area with posterior acoustic enhancement. Color Doppler ultrasound is excellent for detecting inferior vena cava (IVC) thrombosis.
Computed tomography (CT) The modality of choice. In the unenhanced phase, most solid carcinomas have a density of more than 20 HU. CT is excellent for detecting calcifications associated with malignancy. Evaluation of enhancement:
- Increase of > 10 HU — solid lesion.
- Increase of > 20 HU — sign of malignancy.
Magnetic resonance imaging (MRI) Used in case of iodine allergy and for complex diagnostic tasks. On T1-weighted images, tumors are iso- or hypointense; on T2-weighted images, slightly hyperintense. Gadolinium enhancement is heterogeneous, decreasing on delayed scans. MRI is superior to CT in determining the extent of venous invasion and differentiating tumor thrombus from bland thrombus.
Staging and venous invasion (TNM)
CT accuracy in staging is 72–90%.
- Stage T1/T2: Tumor is confined to the kidney (size threshold between stages is 7 cm). Stranding of the perinephric fat is frequently observed due to edema rather than invasion.
- Stage T3a: Extension into the perinephric fat or renal vein. The presence of a discrete soft-tissue nodule in the fat is a highly specific (98%) but low-sensitivity (46%) sign.
- Stage T3b and T3c: Invasion into the IVC (below and above the diaphragm, respectively).
- Stage T4: Extension beyond Gerota's fascia or direct invasion of the ipsilateral adrenal gland.
Venous invasion: CT detects invasion with 96% accuracy in the corticomedullary phase. Renal vein dilation is an unreliable sign (can result from increased blood flow), whereas a filling defect with contrast enhancement reliably indicates tumor thrombus.
Differential diagnosis of renal tumors
- Transitional cell carcinoma (TCC): a central endophytic mass infiltrating the parenchyma or renal pelvis. Shows mild contrast enhancement. Surgical management requires ureterectomy.
- Lymphoma: usually secondary (in patients with non-Hodgkin lymphoma). On CT, appears as diffuse infiltration ("sheet-like") or hypodense lesions, often accompanied by bilateral hydronephrosis due to enlarged para-aortic lymph nodes.
- Renal metastases: most commonly from lung cancer, breast cancer, or melanoma. Hematogenous, small (<3 cm), multiple, usually hypovascular, and localized in the cortex.
- Wilms tumor (nephroblastoma): extremely rare in adults. Manifests as a large palpable mass and arterial hypertension.
Post-treatment imaging
Radiofrequency ablation and cryotherapy are increasingly used for small tumors. Post-treatment CT typically shows residual soft tissue surrounded by fat stranding that does not enhance with contrast.
The appearance of new, contrast-enhancing tissue at the periphery of the intervention zone indicates a local recurrence. Evaluation of the retroperitoneum for lymph node metastases is also mandatory.
For rapid density assessment on CT, use the "10/20" rule: a density increase of >10 HU after contrast administration proves the presence of a solid component, while >20 HU is the criterion for tumor malignancy.
Class and exam questions
How to distinguish tumor thrombus in a vein from a bland thrombus on CT?
The primary sign of a tumor thrombus is its ability to enhance with contrast medium. MRI outperforms CT in differentiating thrombi.
Why is papillary cancer frequently missed on unenhanced CT?
Papillary tumors often have homogeneous density and minimal contrast enhancement. On unenhanced scans, they can easily be mistaken for a regular hyperdense cyst, necessitating careful density measurement.
How reliable is CT in assessing lymph node metastases (N-stage)?
CT accuracy is 83–89%. Diagnosis is based solely on size (normal threshold 1 cm), so CT may miss micrometastases (false negatives) and yield false-positive results in reactive node hyperplasia.
What else to study on this topic
- Imaging features of hereditary syndromes (VHL, BAP1)
- Renal medullary carcinoma and its association with sickle cell anemia
- Transitional cell carcinoma of the renal pelvis (TCC): differential diagnosis
- Secondary renal involvement in leukemia and lymphoma
- Criteria for evaluating RCC recurrence after cryotherapy and RFA
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