Renal mass-like lesions are ubiquitous. Fortunately, most are benign. The most common renal mass is a benign cyst. In fact, pathologic investigations have shown that approximately one-half of the population over the age of 50 has one or more renal cysts [1]. Computed tomography (CT) investigations show that one-quarter to one-third of patients over 50 years of age has at least one cyst [2]. Renal cell carcinoma (RCC) is most commonly identified because of the widespread use of cross-sectional imaging in the abdomen, either by CT or ultrasound (US), usually performed for a non-renal complaint [3]. With advances in CT and US technology, radiologists are detecting more masses and characterizing cysts as small as 5 mm [4]. Similarly, small solid masses may be also characterized with confidence.

To determine whether a solid or cystic renal mass is benign or malignant, the mass is initially examined by US, CT, magnetic resonance imaging (MRI), or a combination of these techniques [5]. Occasionally, percutaneous biopsy is needed [6]. For lesions considered indeterminate after an imaging evaluation, there are many factors that contribute to the management of an individual case, including patient needs, co-morbidity, equipment availability, and the experience of the radiologist and urologist [4].

The differential diagnosis of mass-like lesions of the kidney includes pseudotumors, cysts, inflammatory lesions, and neoplasia. Pseudotumors refers to normal variants that mimic renal masses, and include persistent fetal lobation, column of Bertin, and hypertrophied parenchyma. These entities might cause a renal contour abnormality that mimics a renal mass. However, these entities are generally identified at CT by noting that they are isodense compared to normal renal parenchyma at all phases of the examination. Inflammatory lesions are caused by infection, vascular infarction, and trauma. These lesions are often diagnosed in the appropriate clinical setting but may also be identified on the basis of their radi-ologic appearance. Thickening of Gerota's fascia and per-inephric stranding may be indicative of an inflammatory etiology. Pseudotumors and inflammatory lesions should always be considered first when a mass-like lesion is en countered in the kidney. After their exclusion, solid and cystic renal masses may be considered. This article focuses on solid and cystic renal masses and summarizes an approach that is useful in clinical practice.

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