Evaluation of Bosniak category IIF complex renal cysts

The patient was assessed while supine using a CT scanner (SOMATOM Sensation 4, Siemens Medical Solutions, Germany). Patients were orally administrated 500 ml water 30 min prior to the examination. The scan range field of view (FoV), including only the kidneys and adrenal glands, was at the skin level. Before contrast injection an unenhanced CT was performed. A bolus of 90 ml iodine contrast (Visipaque® 320 mg/ml, Iodixanol, GE Healthcare) was injected intravenously followed by a 20 ml saline solution (0.9 % NaCl) flush with an injection rate of 4 ml/s. To obtain correct and reproducible contrast phases, the arterial contrast concentration was monitored with the region of interest (ROI) in the abdominal aorta at diaphragm level with a trigger level at 100 Hounsfield units (HU). Corticomedullary and nephrographic phase images were performed 10 and 65 s after the trigger. Scanning parameters were kept the same: 2.5 mm collimation × 4; table speed 12.5 mm/s (pitch, 1.25 mm); 165 mAs; 120 kV; 512 × 512 matrix. All images were reconstructed with a soft algorithm and WW/WL at 400/40 in transversal, coronal and sagittal orientation in 3-mm slice thickness with an overlap of 2 mm.

Moderately and more complex renal cysts of category BIIF and above are quite rare, and consequently data with corresponding pathological findings are sparse. When BIIF lesions progress in terms of enhancement, changes in internal architecture by developing irregular, thick enhancing septa, solid components or multilocular characteristics, they are upgraded, and interventions are recommended. Published series with more than 20 complex renal cysts with pathological findings are presented in Table 2. In the BIIF group, the 12 % malignancy rate represents BIIF lesions with radiological progression in complexity over time and subsequent upgrading. Compared to the distribution of malignancy in each Bosniak category in publications before the introduction of BIIF, the accuracy of predicting malignancy in BIII lesions has improved from 51 % to 61 % after the introduction of BIIF, which means that by introducing this category approximately 10 % could be spared surgical intervention. Surprisingly, the malignancy rate in the BII category is still unacceptably high (12 %), even in the series published after the introduction of BIIF.

We found 44 BIIF moderately complex renal cysts among 8,402 CT examinations of the kidneys in a 7-year period. During the follow-up 16 % (5/32) of the lesions progressed radiologically, which corresponds well with the literature (Table 2). Unfortunately, we do not have histological specimens of all five BIIF lesions that progressed radiologically because of co-morbidity of the patients. Two of three BIIF lesions with proven histology were malignant, indicating that radiological progression is indicative of malignancy. On the other hand, the two patients for whom histology was not available did not progress further radiologically during a mean follow-up of more than 3 years, suggesting that these lesions are either benign or low-grade aggressive.

Several studies report that the chance of malignancy increases to 40–80 % when enhancement is observed within a renal cystic lesion [5, 10, 12, 17, 23]. It is therefore of upmost importance that categorisation of complex cystic changes in the kidney is performed with high-quality CT in several phases. To detect enhancement in small areas such as a septum or the wall of a cyst, one cannot rely on split bolus CT examinations that present both corticomedullary and excretory phases in one examination. RCCs are often hypervascular and therefore have a characteristic contrast enhancement pattern characterised by washout in the venous phase [33]. Usually protocols for characterising complex renal cysts consist of a CT without contrast (non-contrast CT, NCCT) and a minimum of two CT single-bolus phases with contrast-enhanced CT and CCT in an arterial and venous phase, respectively [2, 5]. Most radiologists would agree that pre- and post-contrast CT examinations are mandatory, but the number of post-contrast phases is debatable.

In most centres a multidetector CT (MDCT) is available with the possibility of performing isotropic multiplanar reconstruction in high quality. Technically, there are different software opportunities for CT—including software algorithms—to help assess iodine content in a lesion or dual-energy modes to gain additional/alternative information regarding complex renal cystic masses. Preliminary data are available in the literature regarding these issues [34, 35] but further studies are needed.

It is estimated that the risk of getting a fatal cancer from the ionised radiation of CT scans is approximately 1/1,000 for every 10 mSv received, assuming an effective dose of 10 mSv per scan and a risk of 5 % per Sievert [36]. One study suggests that 2 % of all future cancers may be due to radiation exposure from CT examinations [37]. This statement has been widely criticised and the numbers are probably lower [38, 39]. Depending on patient size and body habitus, radiation doses from CT are often quite high—in the range of and even above 20–30 mSv in case of 64-channel CT scanners. In our study the mean radiation dose was 12.8 mSv (7–30 mSv) on our four-channel CT. It is very important to perform radiological imaging according to the ALARA principle. This means that patients, regardless of age and disease, should be examined selectively with radiation doses “as low as reasonably achievably” according to the clinical problem in question. The Bosniak classification was initially developed solely on the basis of CT findings [1, 2]. Subsequently, a few studies have suggested that MR [31, 40] and CEUS [4, 8, 41‐46] may be equally useful for characterisation of complex renal cystic masses. Results from these studies are promising. However, both MR [31] and CEUS [8, 45, 46] have a tendency to upgrade lesions, probably because of the superior contrast resolution compared to CT. The literature is still unequivocal on whether the higher contrast resolution of MR and CEUS will result in earlier detection of potential malignant lesions or inappropriate higher false-positive rates. Further research regarding the role of MR and CEUS are needed and forthcoming.

Recommended follow-up protocols for BIIF cystic lesions vary in the literature. To detect potential progression, a multiphase contrast CT is needed. M.A. Bosniak proposed the first control CT to be done after half a year, and if the lesion was unchanged, a control CT should be repeated annually for a minimum of 5 years [6]. Other authors recommend close monitoring in the first year with CT scans after 3, 6 and 12 months. At our institution patients with complex Bosniak IIF cysts used to be followed according to the latter protocol and in cases without progression annually for 5 years. It should, however, be noted that very narrow follow-up periods may reduce detection rates if the changes in the cysts from examination to examination are very small. As a consequence of these considerations and the results of this study, we have dropped the early 3-month follow-up. However, due to the lack of prospective series, the most appropriate follow-up regimen in these patients is still debatable [9].

Bosniak IIF lesions vary in appearance: (1) BIIF lesions with minimal findings close to category II in complexity and (2) BIIF lesions that are closer to category III in complexity. M.A. Bosniak has recently suggested that BIIF cysts with minimal findings only need follow-up for 1–2-years, whereas more complex BIIF cysts should be followed for a longer period (e.g., 3–4 years or longer) [1]. This seems to be a reasonable compromise taking into consideration the high, cumulated radiation dose of repeated examinations. CT is still considered the gold standard when categorising complex renal cystic masses. For BIIF lesions an early switch to MRI/CEUS with a baseline examination immediately following the CT examination is now used at many institutions. Long-term data on such a regime may change clinical management in the future. Caution must be made for lesions ≤2 cm [1]. We have chosen to stop follow-up if no progression is seen after 5 years, suggesting that the BIIF lesion was benign. To our knowledge this has only been debated very sparsely in the literature, and studies on the most appropriate follow-up regimen in these patients are, as mentioned, warranted [1, 2, 5, 47].

Biopsy of moderate complex cystic masses in the kidney has recently come into focus and the trend seems to be a more aggressive approach [2, 9, 15, 30, 48, 49]. This is presumably to avoid follow-up imaging of those BIIF lesions that are close to BIII and undoubtedly due to the possibility of using local nephron-sparing treatment (ablation with cryo/radiofrequency therapy and/or laparoscopic resection). Whether biopsy and aspiration of the cyst fluid should be used routinely is controversial. It may be problematic to accept a negative biopsy result since the renal cysts usually are very inhomogeneous. In general, it is recommended that Bosniak category III and IV lesions be treated surgically or with minimally invasive ablation measures. The treatment strategy depends on the size and localisation of the lesion and the patient’s general condition. It should be noted, however, that the indication for minimally invasive treatment in cases with suspected malignancy is still regarded as being in the exploratory phase. Routine use of these minimally invasive methods should therefore be protocolised and results published.

Although our numbers are small, the data seem to be consistent in the regard that if progression occurs radiologically, it corresponds to pathological results. The corresponding aggressiveness of the lesions needs further evaluation in prospective studies, however. Our cases were analysed in consensus, so interobserver variability could not be evaluated. Like M.A. Bosniak, we believe that, if the complexity of a category II lesion is in question, the lesion should be placed in category IIF. In cases of disagreement among the radiologists, the lesion should always be placed in the highest category [5, 13, 47, 50]. Furthermore, decisions on the intervention may be easily influenced by clinical parameters, as in our series, in which surgical resection was omitted because of co-morbidity. Other comparable studies have the same limitation with regard to accordance between radiological and pathological progression, since not all category IIF lesions that progressed were resected [6, 9, 10].