Introduction:

Short- and intermediate-term outcomes for Bosniak cysts have been characterized, however there is a paucity of long-term data. When patients are counseled to discontinue surveillance, many are left wondering whether the cysts may continue to evolve and what the potential risk is.  We aimed to evaluate the natural history, intervention rates, and survival outcomes of patients diagnosed with Bosniak II, IIF, III, or IV cysts with 15 years of follow-up.

Methods:

Consecutive patients enrolled in a renal cyst observational cohort (Bosniak 2 or greater) between 2003 and 2012 at a single institution were included in the study. Patients were categorized into four groups based on management strategy: active surveillance, discharge without further imaging, or definitive surgical treatment, or lost to follow-up. Active surveillance consisted of yearly abdominal imaging to 5 years then every 2-3 years thereafter. Patients with minimally complex cysts were discharged after 2-3 years of imaging. Longitudinal imaging data including Bosniak class and cyst size were captured through 2025 using a Provincial-wide central imaging bank. Renal function, recurrence/metastasis, and survival were captured using the electronic medial record.

Results:

A total of 131 patients were included, which includes 161 cysts comprising of 34 Bosniak 2, 39 IIF, 43 Bosniak III, and 29 Bosniak IV cysts. Median cyst size was 3.2cm (IQR 1.8, 5.2). The median follow-up duration was 15.2 years (IQR 11.9,17.1). Thirty patients underwent surgery, the median time to surgery was 352 days (IQR 224,784); 17/30 were malignant. One patient who had a nephrectomy and node positive disease developed local recurrence 14.7 years after surgery. One treated patient developed pulmonary nodules at 3.4 years after nephrectomy, who had pT3a papillary RCC N1 disease; this patient had very atypical imaging findings (concern for urothelial carcinoma) and had surgery within 1 year of presentation.

Of the 57 patients who remained on active surveillance: 8 had Bosniak 4 lesions, 10 were Bosniak 3, and 14 had Bosniak 2F lesions after a median follow-up of 14.5 (IQR 12,16.5) years. None of these patients developed metastatic disease. Thirty-nine out of 131 patients died with no deaths from renal cancer; median time to death 11 years (IQR 6.7, 13.5). Notably, of those who were initially discharged but had more recent incidental imaging (27 patients), most cysts were noted to be Bosniak 2F or less and some cysts had completed involuted (4/27) including one Bosniak 4 cyst. Three cysts progressed from 2 to 2F; one cyst progressed from 2F to 3. No patient ended up on dialysis.  

Conclusion:

With more than 15 years of longitudinal follow-up of from a renal cyst registry we demonstrate that active surveillance is a safe strategy. Even among those patients discharged, ten years later had little variation in their cyst size and classification. The frequency of surveillance imaging was de-escalated safely in the actively followed active surveillance group. There was excellent long-term oncologic and overall survival outcomes among patients with Bosniak III and IV cysts. Of those that required surgery the decision was made within 1 year. These findings suggest that candidates selected for surveillance can do so safely and may choose a period of active surveillance prior to decisions made for surgery. Accurate characterization of the lesion is of pre-eminent importance and may take 1-2 years.

Funding: Henry B. and Edna Smith Dielmann Memorial Chair in Urologic Science