higher than mild and moderate exophytic renal cyst.
INTRODUCTIONRenal cysts are acquired lesions that most commonly affect the elderly population [1] . Autopsy studies have reported a 50% incidence of renal cysts after the age of 50 years [2] . The prevalence of renal cysts is approximately 10%, ranging from 4% in the third decade to 19% in the sixth decade [3] . There is an increase in the size and number of cysts over time in cyst-bearing individuals. Previous longterm follow-up data have shown that, simple renal cysts continue to increase in size with age > 10 years, and sometimes increase rapidly, particularly in younger patients [4] . However, this study also showed decreased growth rate of cysts with increased age. Renal cysts originate from weakened of the tubular basement membrane of the distal convoluted or collecting duct cells [2] . As results, a diverticulum is formed, which can subsequently develop into a simple renal cyst [5,6] . The reported risk factors for renal cysts include old age, male gender, renal dysfunction, and hypertension [7] . However, given the retrospective nature of these studies, the associations could be coincidental. The only persistent confounder among all reported associations was increasing age [4,7,8] . Therefore, we analyzed the discrepancy of an increasing rate of simple renal cysts according to initial size, location, and exophytic index stratified using the RENAL nephrometry scoring system in this study.
MATERIALS AND METHODS
Study populationThis study was conducted after receiving approval from our ABSTRACT AIM: To investigate the growth patterns of renal cysts according to location, exophytic degree and Bosniak classification. METHODS: From December 2000 to September 2012, a total of 252 patients (340 renal cysts) were included in this study. All patients had undergone computed tomography (CT) scans of the kidney level at least twice, and six months apart for any reason. Renal cyst data were assessed according to cyst side (right or left), location (upper, mid or lower), characteristics (Bosniak classification), and exophytic degree (1,2, and 3) which were stratified using the RENAL nephrometry score. The growth rate of renal cyst was analyzed according to each parameter using linear correlation analysis.
RESULTS:The mean size of renal cyst increased from 3.81 cm to 3.85 cm during a mean follow-up of 20.1 months, not significantly (p = 0.121). Among 340 renal cyst units, 180 renal cysts had an exophytic degree index of 1 (52.9%), 90 had an index 2 (26.5%), and 70 had an index of 3 (20.6%). After a mean of 20.1 months of follow-up, only exophytic degree 3 renal cysts increased in size significantly from 4.54 to 5.76 cm (p = 0.018). The annual growth rate in exophytic degree 3 renal cysts was 2.4 mm (r 2 = 0.263), which was higher than that for renal cysts at other site locations. CONCLUSION: The growth rate of renal cysts varied according to exophytic location. The growth rate of exophytic renal cysts was