Bone metaplasia in the urinary tract: a new radiological sign.

We report 7 cases of bone tissue in the upper urinary tract. Routine excretory urography was suspicious for renal lithiasis. However, pathological evaluation of the kidney tissue revealed bone metaplasia in all 7 cases. Retrospective examination of the first 6 cases showed a typical radiological sign, consisting of a radiopaque eccentric halo surrounding an area of lesser radiological density that, in turn, connected with the urothelium. These findings permitted a correct preoperative diagnosis of bone metaplasia in case 7. Accurate preoperative differentiation of bone metaplasia (false calculi) and true renal lithiasis is important owing to the different treatments required.