Purpose: To assess the clinical reliability of the Bosniak IIF category and to determine the proper radiologic follow-up duration and intervals for category IIF complex renal cysts. Materials and Methods: We studied 201 patients with category IIF renal cysts from January 1996 to January 2011. Renal cyst progression to category III was defined as an increase in complexity of the cyst in follow-up radiologic studies. We monitored radiologic changes and progression of renal cysts during the follow-up period and analyzed the pathologic results of those patients who were treated surgically. Results: At a mean follow-up of 20 months, only 14 cases (7%) showed evidence of progression to stage III, with a mean time to progression of 11 months (range, 3 to 65 months). There were no significant differences in age, gender, cyst size, or change in cyst size between the progressive and non-progressive groups. Of 12 cases treated surgically, 10 cases (83.3%) showed renal cell carcinoma with pT1 stage, and there was no recurrence during postoperative follow-up of 23 months. Of the 187 patients without radiologic progression, 23 cases were treated surgically, and all of them showed benign cysts. Conclusions: The IIF category showed significant clinical reliability by a low rate of radiologic progression and a high rate of malignancy in the radiologic progressive group but a low rate of malignancy in the non-progressive group. Although it is hard to decide on a proper follow-up duration because of the variable time to progression, too frequent follow-up study seems to be unnecessary considering that most malignant cases were of a low stage.
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