What are the guidelines for following Bosniak (Bosniak Classification System) I and II renal (kidney) cysts?

From the Guidelines

Bosniak I and II kidney cysts do not require follow-up imaging or intervention as they are considered benign with a negligible risk of malignancy. According to the European Association of Urology guidelines on renal cell carcinoma, updated in 2022 1, the Bosniak classification is used to predict the risk of malignancy and provide guidance for management of renal cystic masses. Bosniak I and II cysts are malignant in approximately 0% of surgically treated cases, and therefore, cautious surveillance or intervention is not typically recommended unless the patient develops symptoms related to the cysts.

Some key characteristics of Bosniak I and II cysts include:

• Bosniak I cysts: simple cysts with thin walls, no internal echoes, and no enhancement on contrast imaging
• Bosniak II cysts: may have a few thin septa, minimal calcification, or high-density fluid but still have a negligible risk of malignancy (less than 1%) The rationale behind this approach is that these cysts have been extensively studied and shown to have virtually no malignant potential, making routine surveillance unnecessary and potentially leading to patient anxiety and healthcare resource overutilization, as noted in the guidelines 1. However, proper initial classification is crucial, as misclassification of higher-risk cysts (Bosniak IIF, III, or IV) as category I or II could lead to missed malignancies.

In terms of management, the guidelines suggest that no further imaging or intervention is typically recommended for Bosniak I and II cysts unless the patient develops symptoms such as pain, hematuria, or infection, in which case follow-up with a urologist is warranted 1. This approach prioritizes the patient's quality of life and minimizes unnecessary medical interventions, while also ensuring that any potential symptoms or complications are promptly addressed.

From the Research

Guidelines for Following Bosniak I and II Kidney Cysts

• The Bosniak classification system is used to categorize cystic renal masses, with Bosniak I and II being the most common categories 2, 3, 4, 5, 6.
• Bosniak I cysts are simple cysts with a low risk of malignancy, while Bosniak II cysts are more complex with a higher risk of malignancy 2, 3, 4, 5, 6.
• The Bosniak classification system version 2019 recommends that class IIF masses undergo follow-up imaging at 6 months, 12 months, and then annually for 5 years 2.
• A study found that only 4% of Bosniak IIF masses were upgraded over time, and none were upgraded on initial 6-month follow-up, suggesting that the currently recommended initial 6-month follow-up imaging examination for class IIF masses may be of questionable clinical utility 2.
• Another study found that the CEUS-Bosniak classification is an essential tool in clinical practice to differentiate and monitor renal cystic lesions, and that it empowers diagnostic work-up and patient care 3.
• A comparison of the Bosniak classification of cystic renal masses version 2019 assessed by CT and MRI found that there was 70.8% agreement between CT and MRI for Bosniak.original and 72.3% agreement for Bosniak.v2019, and that Bosniak.v2019 improved specificity compared to Bosniak.original 4.
• A study investigating the Bosniak category IIF designation and surgery for complex renal cysts found that adding the IIF categorization improved the accuracy of the Bosniak renal cyst classification, as evidenced by a low rate of progression in IIF lesions and a high rate of malignancy in category III lesions 5.
• A pictorial guide to the clinical use of the Bosniak Classification of Cystic Renal Masses, Version 2019, provides practical tips and decision tree rules to help radiologists and other physicians apply the classification and better manage cystic renal masses 6.

Key Points for Bosniak I and II Cysts

• Bosniak I cysts have a low risk of malignancy and do not require follow-up imaging 2, 3, 4, 5, 6.
• Bosniak II cysts have a higher risk of malignancy and require follow-up imaging, with the frequency and timing of follow-up depending on the specific characteristics of the cyst 2, 3, 4, 5, 6.
• The Bosniak classification system version 2019 provides a framework for categorizing and managing cystic renal masses, and can help guide clinical decision-making 2, 3, 4, 5, 6.

Imaging Modalities for Bosniak I and II Cysts

• CT and MRI are commonly used imaging modalities for evaluating cystic renal masses, with CT being more sensitive for detecting small lesions and MRI being more sensitive for detecting septa and wall thickening 2, 3, 4, 5, 6.
• Ultrasound can also be used to evaluate cystic renal masses, particularly for follow-up imaging, but may not provide as much detail as CT or MRI 3, 6.
• The choice of imaging modality will depend on the specific characteristics of the cyst and the clinical context, as well as patient factors such as renal function and allergy to contrast agents 2, 3, 4, 5, 6.