Introduction

Cystic renal masses are usually classified according to the Bosniak classification. It was introduced in 1986, later modified and is now accepted by urologists and radiologists worldwide [1]. Five groups have been delineated including I, II, IIF, III, and IV. The Bosniak classification is based on findings of contrast-enhanced computed tomography (CT), but can be also applied for MRI [1]. MRI in most circumstances offers no advantage over CT. However, in some cases, MRI can better demonstrate the septa and wall thickening when compared with CT [2]. In contrast, ultrasound plays a limited role in classifying cystic renal masses [1]. New technical improvements, such as contrast enhanced US may play a limited role in patients who are at risk for injection of iodinated or MR contrast media.

Details of the current classification are shown in (Table 1). Generally, management of renal cysts is largely dependent on the assigned group; however, there are still controversies in diagnosis and management of these lesions.

Table 1 Bosniak classification of renal cysts [1]
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The basic morphological features of complex renal cysts are the presence of: (1) septa; (2) calcifications; (3) nodular or solid structures; and (4) enhancement. In the past, the presence of thick, nodular or irregular calcifications has placed the lesion into surgical Bosniak III category. Israel et al. have proved that according to the presence of calcifications alone, a lesion should not be classified as surgical. In their study, the value of calcification score was similar between surgical and non-surgical lesions [3] (Table 2). Therefore, the presence of thick, irregular calcifications may upgrade the Bosniak II lesion into Bosniak IIF category. The presence of enhancement is considered as the critical parameter to separate potentially benign from malignant lesions. It also seems that enhancement is one of the major determinants of progression for depicting malignant lesions during observation [4–7].

Table 2 The mean values of calcification scores in each Bosniak category [3]
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Diagnosis and management of Bosniak I and IV lesions is straightforward and usually leads to expectant or surgical management, respectively. Bosniak IIF masses, however, harbor a significant risk of renal cell carcinoma (RCC) that may be as high as 24% [4, 5, 7, 8] (Table 3). At most institutions, these cysts are only explored when they progress over time or become symptomatic. In Bosniak III category, a significant proportion of masses are malignant (0–100%, generally up to 50% are benign) [8–12] (Table 3). Differentiation of the malignant Bosniak III from benign masses on imaging is crucial in order to avoid unnecessary surgeries. The main problem from 1986 till 1993 was to differentiate some complicated Bosniak II from Bosniak III lesions. Bosniak II lesions that have some worrisome features (but not enough to categorize them to Bosniak III group) were suggested and designated as Bosniak IIF lesions [13] to establish their character during regular observation with CT or MRI. Certainly, this new category was beneficial and increased the incidence of malignancy in Bosniak III category [6], but could potentially increase the interobserver variability. Regular follow-up of CT, apart from the added expense, additional radiation exposure has potential, albeit low a risk of developing secondary malignancies.

Table 3 Incidence of malignancy in each Bosniak category, when considering that the observed lesions in Bosniak III and IV class are malignant
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[14]. Although the Bosniak classification system is the only preoperative diagnostic tool that has proven its efficiency in the management of complex renal cystic masses (CRCM), it is highly reader dependent despite clear definition of each category.

In this review, the authors point out diagnostic dilemmas and current controversies in the management of CRCM.

Bosniak II and IIF controversies

Follow-up of Bosniak IIF cystic masses has been proven as a safe management. Minimum of 5-year follow-up is important to determine the stability and benign nature of the mass. When the lesion progresses (in terms of enhancement, change in internal architecture by developing irregular, thick enhancing septa, solid component or multilocular character) on control CT scan or MRI, the lesion is upgraded and indicated for surgical revision [4]. Three patients from our group progressed, and the detection and presence of CT enhancement was the major indicator for surgery. Final histopathology confirmed RCC in all cases [23]. Similar results were recently observed by Gabr. et al., where 7 pts with Bosniak II and IIF progressed in terms of size, complexity, or enhancement. In 3 cases (1 pt with Bosniak II and 2 pts with Bosniak IIF), enhancement was detected as the parameter of progression, final histology confirmed malignancy [5].

Bosniak II and IIF cysts harbor more than 10% risk of having carcinoma (Table 3). The overall incidence of malignancy in both groups can be presumed to be much lower, because most of the masses are generally followed and only few of them are surgically resected.

Recently published study by O’Malley et al., with the largest cohort of Bosniak IIF lesions, has demonstrated 14.8% rate of progression. Three patients were lost on follow-up, 4 patients are still observed, while the progression was considered marginal or pts are in poor medical condition and in 5 surgically managed patients, RCC was confirmed [6] (Table 4).

Table 4 Incidence of malignancy in Bosniak IIF based on number of surgically resected lesions and benignity persumed on radiographic stability of the remaining lesions
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The overall incidence of malignancy in Bosniak IIF can be influenced by interobserver variability, number of surgically resected lesions, presumed benign character based on radiographic stability, length of follow-up and the character of progression as well. Generally, increase in size does not result in surgical procedure. For that reason, it seems important to define the most accurate parameters of progression as the indicator for intervention in Bosniak II and IIF group (Table 4); furthermore, some radiologists tend to group these lesions in one category [5].

Major and minor criteria of Bosniak III lesions

Bosniak III lesion is a surgical lesion indicated for intervention, but the current dilemma is that significant proportion of benign lesions are in this category. The known interobserver variability was proved to be highest among Bosniak II and III masses [19]; however, Bosniak IIF category was not evaluated in that study. Recently Weibl [23] and Quaia et al. [24].showed a high rate of variability between Bosniak II, IIF and III groups.

In some cases, the management of Bosniak III mass may vary from center to center, regardless of the fact the Bosniak III is a surgical mass.

In some specific cases, one may seek for further diagnostic evaluation, such as: mass is indeterminate on CT [Fig. 1], young patients with completely intrarenal mass, which limits the nephron sparing procedure, relatively young patients with solitary kidney or on the contrary patients with short-term survival. In these cases, probably the indication for surgery will not be so straightforward or absolute.

Fig. 1
figure 1

Diagnostic and therapeutic protocol for Bosniak III—potential further evaluations according to the literature data

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Dedicated 4-phase CT scan or MRI is the basic tool for categorizing CRCM according to the Bosniak classification. Biopsy may be contributory especially when the infectious nature of the mass is suspected. However, negative biopsy does not exclude malignancy, that the lesion is not malignant, what is certainly frustrating as well for the urologist as for the patient. Role of biopsy of small renal masses and complex renal cysts is controversial [25], even though some authors recommend biopsy of indeterminate or Bosniak III masses with favorable results [26–28]. To date, there is no general consensus. The results of upcoming biopsy studies with potential oncomarkers may bring some new answers [29]. Even though biopsy or aspiration cytology in borderline cystic renal masses should be considered as one possible variable included in the development of risk calculator or a nomogram. Additionally integration of parameters such as age, ASA score, initial size, enhancement, growth rate and type of progression improves the decision-making process.

Active surveillance of surgical, potentially malignant complex renal cystic lesions is still considered to be experimental. High-risk patients with multiple comorbidities, with short-term overall survival and those who refuse any kind of intervention are potential candidates for this management. Long- and mid-term follow-up data are lacking. Until today, there is no clear consensus about the growth rate that warrants intervention for masses under active surveillance. That is why the growth itself is a not reliable and accurate predictor of malignancy and surgical intervention. As mentioned previously, again combination of more variables should improve the accuracy for intervention (size, radiographic growth, symptoms, type of progression, especially change in internal architecture of the cystic lesion).

Conclusion

Bosniak classification system has been established in 1986, proved its efficiency, but even after more than 20 years of clinical experience has some limitations. Current dilemmas and improvements are needed especially in Bosniak III category, because a significant proportion have benign character. Limitation such as interobserver variability among various readers with different levels of experience could be potentially improved by developing a normogram or a risk calculator. The goal is to specify which variables are the most relevant and accurate to be included in such a development. It is not a rare phenomenon that some urologist and radiologists tend to group some Bosniak II and IIF lesions in one category and observe them regularly. The crucial role in this group of patients is playing the parameters of progression. The radiographic growth itself is not a sufficient factor for intervention. The change of internal architecture and presence of enhancement play the most important role in depicting malignant lesions during the time frame of active surveillance.