What are the implications of a Bosniak (Bosniak Classification System) category 2 renal cyst?

From the Guidelines

A Bosniak 2 cyst is a minimally complex renal cyst that is benign and requires no treatment or intervention, with a recommended approach of surveillance with follow-up imaging, usually a repeat ultrasound or CT scan in 6-12 months to ensure stability 1. These cysts typically have a few thin septa (internal walls), minimal calcification, or high-density fluid but lack concerning features for malignancy. The risk of malignancy in Bosniak 2 cysts is approximately 0%, as stated in the European Association of Urology guidelines on renal cell carcinoma 1. Some key points to consider include:

• No medications, procedures, or surgeries are needed for Bosniak 2 cysts.
• If the cyst remains stable on follow-up imaging, no further evaluation is typically necessary.
• These cysts are differentiated from more complex Bosniak 2F, 3, or 4 cysts which have progressively higher risk of malignancy and may require more intensive monitoring or intervention.
• Patients with Bosniak 2 cysts can be reassured that these findings are common, especially with aging, and rarely cause symptoms or complications. It's worth noting that the American Urological Association guidelines also discuss the management of renal masses, including the use of active surveillance for certain types of renal masses 1. However, for Bosniak 2 cysts, the focus is on surveillance rather than active treatment, given the low risk of malignancy. Overall, the management of Bosniak 2 cysts prioritizes minimizing unnecessary interventions and monitoring for any changes that may indicate a need for further evaluation or treatment.

From the Research

Implications of a Bosniak Category 2 Renal Cyst

• A Bosniak category 2 renal cyst is considered to have a low risk of malignancy, with a 0-15% chance of cancerous cells 2.
• However, category 2F cysts (F for follow-up) have a 25% chance of malignancy and require surveillance at 6 months and regularly for 5 years 2.
• Active surveillance of small (< 4 cm) Bosniak category 2F lesions has shown that 88% of these lesions remain stable or regress over time 3.
• The management of Bosniak category 2 cysts can include sclerotherapy or laparoscopic deroofing of the cyst if symptomatic 2.
• Cyst biopsy can be a useful tool in preventing unnecessary surgeries for benign lesions, with no risk of malignant cell dissemination 2.

Diagnostic Performance and Interobserver Variation

• The Bosniak classification scheme has been shown to be useful for evaluating renal masses, with a high area under the curve for the pooled ROC analysis 4.
• However, interobserver variation in distinguishing Bosniak II and Bosniak III lesions may present difficulties in recommending surgical versus conservative management 4.
• A study of pediatric cystic renal masses found that the modified Bosniak classification system can allow for reasonable clinical risk stratification, with cystic renal lesions classified as modified Bosniak class 1 or 2 being most often benign 5.

Treatment and Follow-up

• The treatment of Bosniak category 2 cysts can include partial resection of the cyst, which can provide advantages such as the recipient receiving a new donor kidney and the donor being free of surveillance 6.
• Follow-up imaging can be done with CT, MRI, or contrast-enhanced ultrasonography, with the goal of monitoring for any changes in the cyst over time 2.