Simple Renal Cysts: Are they Really Benign ?

B n i g n r e n a l n e o p l a s m s c o n s t i t u t e a l a r g e a n d heterogeneous group of lesions which include the simple renal cysts (SRCs), selected complex r e n a l c y s t s , c o r t i c a l a n d metanephric angiomyolipoma, oncocytoma, the rarer cyst ic nephroma, mixed epithel ia l / stromal tumor and leiomyoma, as well as other, even more esoteric tumor types. SRCs are discrete lesions within the kidney that are typically cortical, extending outside the parenchyma and distorting the renal contour. They are oval or circular in shape and have a distinct, sharply defined outline. On an average, simple cysts measure about 0.5 to 1.0 cm in diameter, but 3 to 4 cm cysts are not uncommon. The cyst wall is characteristically smooth, transparent, avascular, yel lowish or bluish white in color , and formed by a thin layer of fibrous tissue lined by a single layer of flattened or cuboidal epithelia. They are filled with a homogeneous transudate like, clear or straw-colored fluid of low viscosity. SRCs may be solitary or multiple and bilateral. SRCs remain the most common of benign renal lesions, representing more than 70% of asymptomatic renal masses. Rarely, however, they can be associated with rupture (hemorrhage), hematuria, abdominal or back pain due to bleeding, palpable abdominal mass , evidence of infect ion, and/or hypertension. Clinical symptoms are more common with neoplasms than with simple cysts.1 The Bosniak classification f o r r e n a l c y s t i c l e s i o n s , a s reviewed 2,3 in Table 1 , is the Table 1: The Bosniak classification for renal cystic lesions. (source, Israel and Bosniak, 2005; Warren and Mc Farlane 2005)