The radiologic diagnosis of renal cysts (and their differentiation from renal neoplasms) has come a long way since the 1950s when the approach was surgical exploration, unless clinically contraindicated, for every renal mass detected using urography. Nephrotomography, renal angiography, and cyst puncture have contributed over the ensuing years to the differentiation of cyst from tumor. However, for the most part, sonography and CT (or a combination of these when necessary) have become the main diagnostic techniques for evaluating renal masses, and with their use we have never been more accurate, noninvasive, and relatively economically efficient. The more widespread use of CT has enabled serendipitous discovery of many small renal carcinomas, the removal of which should result in an improvement in the overall cure rate of patients with renal parenchymal neoplasms. On the other hand, we are also discovering many more cysts than we have previously. We must be on guard, therefore, against discovering lesions for which we are unable to establish a radiologic diagnosis of benignity, because this will only increase the need for exploratory surgery once again. It is thus imperative that imaging studies be performed with great care, that diagnoses be based on rigid criteria, and that more experience with difficult lesions be gained so that the proper approach to treatment will be determined. If we are able to accomplish this, then the present radiologic age can be remembered as a time when great advances in the evaluation of renal masses were made, with resultant improved patient management and cure of disease.

With modern computed tomography (CT) and magnetic resonance (MR) imaging equipment, the diagnosis of most renal masses is usually straightforward and accurate. The major question to be answered is whether the mass represents a surgical or nonsurgical lesion or, in some cases, if follow-up studies are necessary. This evaluation usually can be accomplished if a high-quality examination is performed, if the clinical history of the patient is kept in mind, if conditions that mimic a renal neoplasm are considered and excluded, and if there is an awareness of the potential pitfalls and limitations of CT and MR imaging. In this article, the authors present their technique in the performance of CT and MR imaging examinations, summarize their approach to the diagnosis of renal masses, review the imaging findings in these lesions, and stress the limitations in renal mass diagnosis.

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CT and MR imaging findings were similar in the majority of cystic renal masses. In some cases, however, MR images may depict additional septa, thickening of the wall and/or septa, or enhancement, which may lead to an upgraded Bosniak cyst classification and can affect case management.