The detection of small renal masses has increased due to the increased use of imaging tests, such as ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI), performed for unrelated indications . The majority of renal lesions can be characterized as benign simple cysts, which would require no further work-up. However, completely solid or mixed solid and cystic renal lesions that enhance with contrast are likely to be malignant and warrant further evaluation and management .
The initial evaluation, differential diagnosis, diagnostic approach, and treatment of small renal masses will be reviewed here. The evaluation of renal cell carcinoma and renal cysts is discussed separately. (See "Clinical manifestations, evaluation, and staging of renal cell carcinoma" and "Simple and complex renal cysts in adults".)
A renal lesion measuring <4 cm in largest dimension that demonstrates contrast enhancement on abdominal imaging meets criteria for a small renal mass . These lesions are classified as either solid (image 1) or complex cystic (Bosniak class III or class IV ) (image 2) based on their imaging appearance. (See "Simple and complex renal cysts in adults".)
Small renal masses are concerning because of the risk of renal cell carcinoma (RCC); this risk increases as the mass gets larger (table 1). Over three-quarters of small renal masses are malignant , and among malignant masses, larger size correlates with a higher pathologic grade . However, almost 70 percent of masses <2 cm are malignant .
There are no clinical or radiographic features of solid or complex cystic masses that accurately predict which lesions will grow; in addition, there are no accurate features that predict histologic diagnosis, risk of malignancy, or prognosis [7-11]. Growth rates also do not reliably predict the biologic behavior of a mass . Representative studies that illustrate this point include the following: