Introduction
Advancement and clinical application of cross-sectional imaging techniques such as dual-energy CT (computed tomography) have increased the visualization of renal lesions. The renal mass can be classified as benign or malignant depending on the attenuation (radiodensity) and imaging characteristics and requires further evaluation with contrast-enhanced CT. If the renal mass is suggested to be intermediate (it cannot be diagnosed as benign or malignant renal mass), then additional diagnostic measures are necessary. The presence of attenuation greater than 20 HU (Hounsfield scale) in CT accurately detects solid renal mass. However, the presence of small endophytic benign cysts as pseudo attenuation limits renal lesion imaging.
What Are Renal Masses?
A renal mass is an abnormal growth in the kidney. Most renal masses are benign (noncancerous growth) and may require no intervention. The renal masses can be solid or cystic. The solid renal mass has a higher risk of developing cancer. Therefore, diagnosing new renal masses early is vital to improve a patient's quality of life and prognosis. Imaging techniques such as CT, MRI (magnetic resonance imaging), and ultrasound are helpful in the identification of renal masses.
What Are the Causes of Developing Renal Masses?
The cause of developing renal mass is dependent on multiple factors. Smoking can cause an increased risk of developing renal cell carcinoma. Other agents such as benzene, herbicides, and vinyl can also increase cancer development incidence. Specific medications like NSAIDs (nonsteroidal anti-inflammatory drugs) can also be attributed to increased cancer risk. Few patients may develop renal masses due to genetic factors. For example, in patients with polycystic kidney disease (a disease where a cluster of cysts forms in the kidney), the genetic factors may be inherited in autosomal recessive (inheritance of two copies of the abnormal gene) or dominant (inheritance of abnormal copy of the single gene) forms. In addition, a few medical disorders like hepatitis C and tumors are associated with renal cell carcinoma (kidney cancer) formation.
How Can CT Scans Be Utilized in the Identification of Renal Masses?
CT scans often detect renal masses with a slice thickness of two to three millimeters. Following contrast administration, the liver parenchyma appears homogeneously enhanced, making the small renal mass identification easier. The corticomedullary and excretory phase visualization may be required to identify the type of renal cell cancer and differentiate it from other tumors.
What Is the Standard Protocol for Renal Mass Visualization With CT?
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Non-enhanced CT- This phase of CT imaging provides a baseline of attenuation before iodinated contrast agent administration. In addition, the imaging can also detect bulk fat and calcification. If there is the presence of bulk fat in imaging studies, it indicates angiomyolipoma (benign tumor of fat and muscle tissue), which does not require intervention. The images must be taken with a slice thickness of 1.5 to 3 mm for accurate diagnosis. In renal cell carcinoma, calcification is more prominent. The corticomedullary phase begins when the contrast media reaches proximal renal tubules (region of kidney cell responsible for reabsorption and secretion of solute) in the kidney. The timing is 30 to 40 seconds after contrast injection. The acquisition of this imaging phase helps identify subtypes of renal cell cancer. Thrombosis in renal arterial or vein vasculature can be detected as they appear enhanced. In addition, this phase can also differentiate between renal mass mimics such as arterio-venous fistula (an abnormal connection between artery and vein).
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Nephrographic Phase Contrast-Enhanced CT - It begins when the contrast agent reaches the collecting duct (responsible for the concentration and transport of urine). The timing is determined as 90 to 120 seconds after contrast injection. During this phase, the renal cortex (outer part of the kidney) and medulla (inner part of the kidney) are enhanced homogeneously. This is the main phase for characterizing renal mass as it is more sensitive in detecting renal cystic mass and internal renal enhancement of the mass. In addition, the extent of thrombus (clot) formation in the renal vein and inferior vena cava can be measured.
What Is the Role of Dual-Energy CT in the Detection of Renal Masses?
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Studies are ongoing on using dual-energy CT effectively in identifying renal masses. The imaging for renal masses can be obtained in a single phase, double layer phase, or multiphase manner. The imaging showed sensitivity and specificity greater than 95 % for renal mass identification. The imaging is better than conventional CT in renal mass identification. The dual-energy CT can eliminate cysts' pseudo attenuation by correcting iodine's beam hardening effect. In addition, the imaging can also characterize intermediate cysts that are accurately identified without the need for further imaging. Therefore, dual-energy CT can replace multiphase CT for detecting renal masses and is also fast and accurate in identifying renal masses in single-phase acquisition.
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Characteristics of Renal Mass in Dual-Energy CT: The non-enhanced phase of the CT scan for baseline determination is obtained virtually from contrast-enhanced CT. Contrast-enhanced dual-energy CT with iodine and water density images can differentiate hyper-attenuating cysts from enhancing masses. In iodine density imaging, the renal masses appear brighter than the non-enhancing cyst, even when the conventional CT images are similar. The reconstruction of data obtained from contrast-enhanced CT can eliminate the pseudo-enhancement defect. Color-coded images show the presence or absence of enhancement. Water density images are used as unenhanced images to identify calcification in renal masses.
- Advantages: Dual-energy CT facilitates the reconstruction of non-contrast images from the single-phased contrast-enhanced examination. It may lead to the elimination of multiphase CT imaging for determining intermediate cysts. The imaging can also reduce a patient's radiation exposure by 50 %. In addition, spatial misregistration can also be eliminated.
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Limitations: Variations in imaging among the dual-energy CT can increase the risk of developing defects and reduce reliability. Even a 10 HU error in evaluating baseline attenuation can lead to an incorrect diagnosis of the renal lesion.
What Are the Other Imaging Techniques Used for Renal Mass Detection?
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MRI Scan - It is a preferred technique for detecting small renal masses. Kidney cancer growth into major blood vessels is visualized with this imaging. This technique does not subject the patient to radiation. The imaging is also helpful in the diagnosis of patients that are allergic to contrast dye.
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Ultrasound - The imaging technique can identify whether the cyst is solid, fluid-filled, or containing gas. The imaging technique can also differentiate between benign and malignant tumors.
Conclusion
Dual-energy CT is essential for determining renal lesions. It is a fast and accurate imaging technique for identifying renal masses. In addition, imaging is also helpful for the differentiation between benign and malignancy. Studies are ongoing on utilizing dual-energy CT for determining renal mass to yield better outcomes.
