Milk of calcium (MOC) cysts masquerading as renal calculi - a trap for the unwary.

Milk of calcium (MOC) is a colloidal suspension of calcium salts occurring in calyceal cysts and diverticula. Although reported to be rare, in fact it seems to be more common than previously thought. It has characteristic appearances on plain x-rays, ultrasound, and CT imaging particularly in the prone and supine positions. Often entirely asymptomatic, its appearance may be mistaken for renal stones on radiography or angiomyolipomas on ultrasonography. In this paper, a series of cases is presented outlining its characteristic features and reviewing the relevant literature.