Long-term follow-up study of the malignant transformation potential of the simple renal cysts

Background: We conducted a multi-center study to investigate the prevalence, the malignant transformation potential of the simple renal cysts and the factors that might predict malignancy. Methods: We defined the simple renal cysts as Bosniak class I & II (including IIF) lesions. In the prevalence study, data from 115,132 ultrasonographies was collected from individuals who participated in a multiphasic health wellness screen. In the natural history and progression study, we retrospectively reviewed 333 participants with simple renal cysts and were followed for at the least 3 years with a mean of 6.3±2.9 years (ranged from 3 to 13 years). Results: About 7.2% (8,303) of the individuals who participated in the study were found to have at the least one simple renal cyst. The incidence increased with age from 0.6% in the first decade to 28.0% in the eighth or later decade of life. The Bosniak class I lesion accounted for 7,559 or 91.0% of the cysts whereas 744 or 9.0% were class II. A slower growth rate was observed in the older age group. Twenty-four patients (7.2%) had their renal cysts upgraded according to the Bosniak classification. Gender, age, initial cyst size, number of cysts and bi-laterality failed to predict the malignancy tendency. Conclusions: Most of the cysts discovered were the Bosniak class I and II lesions and they rarely progress further. Treatment for the asymptomatic simple renal cyst is not warranted. Treatment decision-making based on older age or larger initial cyst size should not be recommended.