Which CT findings are characteristic of renal cell carcinoma (RCC)?

Updated: Dec 13, 2018
  • Author: Deborah A Baumgarten, MD, MPH; Chief Editor: Eugene C Lin, MD  more...
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Answer

A dedicated renal CT examination consists of thin-section (2.5-5 mm) helical imaging of the kidneys before the intravenous administration of contrast agent, followed by imaging 60-70 seconds and 3-5 minutes after administration of the contrast agent.

The imaging parameters (kilovoltage, microamperage, field of view, section thickness) should be kept constant for all phases of imaging to enable comparison of the attenuation measurements. The addition of an arterial phase CT (either with bolus tracking or after a 20-25 second delay) with thin slices (1-2 mm) may be helpful to evaluate arterial anatomy, especially if partial resection is contemplated or if renal parenchymal or vascular anomalies are suspected. [1, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 28, 33]

(For CT scans of RCC, see the images below.)

Case 4. Renal cell carcinoma. Dedicated renal CT s Case 4. Renal cell carcinoma. Dedicated renal CT scan. Before contrast enhancement, right kidney.
Case 4. Renal cell carcinoma. Dedicated renal CT s Case 4. Renal cell carcinoma. Dedicated renal CT scan obtained before contrast enhancement. Right kidney has an attenuation measurement of 45.7 HU.
Case 4. Renal cell carcinoma. Contrast-enhanced de Case 4. Renal cell carcinoma. Contrast-enhanced dedicated renal CT scan. Right kidney.
Case 4. Renal cell carcinoma. Contrast-enhanced de Case 4. Renal cell carcinoma. Contrast-enhanced dedicated renal CT scan with an attenuation measurement of 101.7 HU.
Case 5. Typical renal cell carcinoma. CT scan obta Case 5. Typical renal cell carcinoma. CT scan obtained before contrast enhancement has an attenuation measurement of 33.9 HU.
Case 5. Typical renal cell carcinoma. Contrast-enh Case 5. Typical renal cell carcinoma. Contrast-enhanced CT scan has an attenuation measurement of 75.8 HU.
Case 6. Multifocal renal cell carcinoma in a patie Case 6. Multifocal renal cell carcinoma in a patient with Von Hippel-Lindau disease. Contrast-enhanced CT scan.
Case 6. Multifocal renal cell carcinoma in a patie Case 6. Multifocal renal cell carcinoma in a patient with Von Hippel Lindau disease. Patient had already undergone a right nephrectomy. Contrast-enhanced CT scan.
Case 7. Multifocal renal cell carcinoma in patient Case 7. Multifocal renal cell carcinoma in patient presenting with palpable mass. Nonenhanced CT scan.
Case 7. Multifocal renal cell carcinoma. Nonenhanc Case 7. Multifocal renal cell carcinoma. Nonenhanced CT scan.
Case 7. Multifocal renal cell carcinoma. Contrast- Case 7. Multifocal renal cell carcinoma. Contrast-enhanced CT scan.
Case 7. Multifocal renal cell carcinoma. Contrast- Case 7. Multifocal renal cell carcinoma. Contrast-enhanced CT scan.
Case 8. Cystic renal cell carcinoma. Nonenhanced C Case 8. Cystic renal cell carcinoma. Nonenhanced CT scan with an attenuation measurement of 25.8 HU.
Case 8. Cystic renal cell carcinoma. Contrast-enha Case 8. Cystic renal cell carcinoma. Contrast-enhanced CT scan with an attenuation measurement of 47.1 HU.

Historically, enhancement was considered present if the attenuation of the lesion increased by more than 10 HU from baseline. However, with recent advances in CT hardware, this definition may need to be changed to 15-20 HU.

On initial nonenhanced CT scans, RCCs may appear as isoattenuating, hypoattenuating, or hyperattenuating relative to the remainder of the kidney. Calcifications may be present and are usually amorphous and internal, although rimlike calcifications can also be present.

On contrast-enhanced CT scans, RCC is usually solid, and decreased attenuation suggestive of necrosis is often present. Sometimes, RCC is a predominantly cystic mass, with thick septa and wall nodularity.

RCC may also appear as a completely solid and highly enhancing mass.

Staging of RCC, which can be performed by using CT or MRI, includes the assessment of ipsilateral or contralateral adrenal involvement, direct extension into adjacent organs, enlargement of retroperitoneal lymph nodes, invasion of the ipsilateral renal vein (with or without extension into the inferior vena cava), and distant metastatic disease (liver, bone, lungs). Retrocrural, subcarinal, or mediastinal lymph nodes can also be enlarged.

If a solitary mass is enhancing, the degree of confidence in diagnosing RCC is high. When masses are multiple, metastatic disease and lymphoma must be considered, especially if the patient has a history of a primary malignancy. When a mass is predominantly cystic, the confidence level decreases. In these patients, US may be useful.

A false-positive diagnosis of enhancement may occur in small masses. This so-called pseudoenhancement may be the result of reconstruction algorithms, beam-hardening artifact, or cupping artifact. It is more pronounced on multi-detector spiral CT scans than on single-detector spiral CT scans. In patients with these findings, MRI or US is helpful in proving that the lesions are cysts. In patients in whom nonenhanced imaging was not obtained, differentiation of solid masses from hyperattenuating cysts (Bosniak class 2 lesions) may be difficult. In one series, RCCs were significantly larger, had greater mean attenuation (>70 HU), and had increased central heterogeneity compared with hyperattenuating cysts.

Occasionally, masses are predominantly cystic but indeterminate because of septa or nodularity (Bosniak class 3 lesions). The lesions are often complex cysts, but they may be removed because of their suggestive appearance.

Briefly, the Bosniak classification of renal masses is as follows: class I includes simple cysts; class II, minimally complicated but overwhelmingly benign masses with thin septa, hyperattenuation, or small amounts of mural or septal calcification; class III includes moderately complicated masses with mural nodularity, thick septa, or irregular or thick calcifications that often require surgical exploration; and class IV includes significantly complicated and generally malignant masses with thick and irregular enhancing regions and definite solid components.

Oncocytomas cannot be reliably differentiated from RCCs without pathologic analysis. Macroscopic areas of fat in the tumor mass are reported in RCC, but they are extremely rare. Almost all renal tumors with measurable areas of fat are angiomyolipomas (AMLs); however, some AMLs do not contain visible fat and may be mistaken for RCCs. In one series, homogeneous and prolonged enhancement were valuable predictors for differentiation of AML with minimal fat from RCC. High attenuation on nonenhanced CT scans and the degree of enhancement were helpful but less valuable.

A false-negative diagnosis can occur if the attenuation is not carefully measured before and after the administration of contrast material, if cystic masses are not carefully examined for septa or nodularity, if the enhancement of a lesion is missed (as a result of lack of enhancement at the time of scanning), or if the masses are too small for adequate characterization at the time of their discovery.

In patients with renal failure and long-standing dialysis dependence, detection of an RCC (especially the papillary type) is increased when imaging is performed soon after the contrast bolus passes (in the arterial phase). In patients with normal renal function, imaging during the arterial or cortical phases may make the lesions less visible because the typical hypoenhancement of the tumors cannot be distinguished from the nonenhancement of the adjacent medulla.


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