Outcomes of Bosniak Classification Version 2019 Class IIF Cystic Renal Masses at Imaging Surveillance.

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Abstract

Background: Bosniak classification system version 2019 (v2019) recommends that class IIF masses undergo follow-up imaging at 6 months, 12 months, and then annually for 5 years. The frequency and timing of upgrade on follow-up imaging are incompletely understood. Purpose: To describe the temporal evolution of Bosniak v2019 class IIF cystic renal masses, with attention to outcomes at 6-month follow-up, time to class upgrade, and malignant histologic diagnoses. Methods: This retrospective study included 219 patients (91 women, 128 men; median age, 72 years) with 246 localized class IIF masses from January 2005 to June 2022. Patients underwent both a baseline and at least one follow-up renal-mass protocol contrast-enhanced CT or MRI. Two radiologists evaluated masses at all follow-up time points to categorize masses as downgraded (class I or II), stable (localized class IIF), or upgraded (class III or IV, solid, or =T3a, N1, or M1 disease); a third radiologist resolved discrepancies. Incidence rate of upgrade was determined. Histopathologic outcomes were assessed for resected masses. Results: Median follow-up was 28.4 months (IQR, 13.7-59.4 months). At 6-month follow-up, 5 (2%) masses were downgraded, 241 (98%) were stable, and none were upgraded. Based on final follow-up, 14 (6%) masses were downgraded, 223 (91%) were stable, and 9 (4%) were upgraded. All upgrade events entailed a class increase to III (n=7) or IV (n=2); no mass became solid or developed T3, N1, or M1 disease. Among the nine upgraded masses, median time to upgrade was 53.5 months (IQR, 23.2-63.7 months). Incidence rate of upgrade was 3.006 per 100,000 person-days (95% CI, 1.466-5.516). Ten masses were resected; histopathology was benign in six, and malignant in four. Of the four malignant masses, one was upgraded to class III after 15 months of preoperative follow-up imaging, and three remained class IIF on preoperative follow-up imaging. No resected malignant mass developed postoperative recurrence. Conclusion: Bosniak v2019 class IIF masses are unlikely to represent aggressive malignancy; only 4% were upgraded over time, and never on initial 6-month follow-up. Clinical Impact: The currently recommended initial 6-month follow-up imaging examination for class IIF masses is of questionable clinical utility.

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