Differential Diagnosis of Complex Renal Cysts Based on Lesion Size along with the Bosniak Renal Cyst Classification

PURPOSE: To identify size criteria for complex cystic renal masses that can distinguish renal cell carcinoma from benign cysts supplementing the Bosniak classification. MATERIALS AND METHODS: We reviewed the records of 97 patients who underwent surgery for complex cystic renal masses from January 2001 to April 2010. The pathological results were compared with the lesion sizes measured by preoperative computed tomography and other radiological features (contrast enhancement, irregularities of cyst walls and septa, and calcification) were also obtained for categorization according to the Bosniak renal cyst classification. RESULTS: Malignancy was significantly associated with cyst size (>2 cm), male gender, and younger patient age (<50 years). According to the Bosniak classification, there was no category I cyst, and all 8 category II cysts were benign. However, 3 of 18 (17%) category IIF cysts, 21 of 39 (54%) category III cysts, and 29 of 32 (90%) category IV cysts were malignant. All category IIF cysts were benign in patients older than 50 years of age. CONCLUSION: Many complex cystic renal masses smaller than 2 cm were benign. We suggest that lesion size should be taken into account when formulating treatment plans for complex cystic renal masses.