The renal cystic lesion is frequently encountered in clinical practice; it may include benign cysts, precancerous lesions, or malignant lesions[13]. Among these lesions, the most common is simple renal cortical cysts, followed by renal cell carcinoma with the cystic change, and atypical renal cyst, which is much less encountered. By definition, atypical renal cysts are those cystic masses of the kidney that bear an epithelial lining “piled up” or stratified and/or have short papillary projections or tufting. The epithelial cells are cytologically bland with small nuclei and a variable amount of clear or eosinophilic cytoplasm. The stroma or septa should be free of nests/cords of epithelial cells and often coexist with other diseases[1]. A review of the English literature at PubMed (www.ncbi.nlm.nih.gov) shows that 14 papers with a total of 42 cases had relevant information and clinical records. Due to the rarity of this renal cyst, we discuss the case and make a literature review of previous cases (all cases were summarized in Table 1) to enhance the understanding and differential diagnosis of the atypical renal cyst to alert pathologists to the precancerous lesion and to avoid unnecessary or excessive treatment. The age of the patients was between 17 and 80 years old. There was no significant gender difference. Atypical renal cysts often occur in unilateral kidneys, and multilocular cysts are slightly more than unilocular cysts. The cysts vary in size with a diameter from 0.8cm to 10cm. The overall survival of atypical renal cysts is good, if there are no combined other primary diseases. The patient recovered well after surgery, without any sign of malignant progression. Among the reviewed cases, only one patient died of postoperative tumour recurrence. One patient died of metastatic lung cancer. Two patients died of sepsis, one of who was associated with respiratory failure and multiple metabolic disorders. One patient died of unknown causes. Atypical renal cysts usually coexist with other diseases, such as renal cell carcinoma (RCC) and VHLD. 34 of the 42 cases has accompanied other conditions, mainly of which are renal cell carcinoma (13/34), and seven cases of which are accompanied by VHLD (7/34), and others are ADPKD, angiomyolipoma (AML), tuberous sclerosis (TS) and so on.
Renal cystic lesions are relatively common, and the CT manifestations of the atypical renal cyst are complex and lack of signs of typical renal cyst, which may easily lead to misdiagnosis. Therefore, it is necessary to differentiate from other renal cystic diseases. The ultrasonography of simple renal cysts was smooth and without separation. They are common and require no clinical follow-up or treatment because they do not pose any risk for malignant transformation[14]. Unless the cyst is large, it compresses other kidney tissue or surrounding organs. At this time, surgical treatment needs to be considered. We must admit the coexistence of renal cell carcinoma and atypical renal cysts do exist in clinical practice[15, 16]. Ibrahim et al.[6] found that atypical renal cysts exist in renal parenchyma close to the renal carcinomas. The characteristics of atypical cells lining the cysts are similar to those found in the close well-differentiated renal cell carcinomas[17]. DNA quantitative studies show that both the renal cell carcinomas and the atypical cyst lining cells have the same DNA indices and are essentially DNA euploid[6]. And it was reported that atypical renal cysts associated with end-stage renal disease had been considered to be precursors of malignant renal tumours[9]. These descriptions suggest that atypical renal cysts may be associated with renal carcinoma. CT and ultrasound have a high diagnostic value for cystic renal carcinoma[19, 20]. The Bosniak classification system, created in 1986[21], established morphological CT criteria to differentiate between benign cysts and maliganat cysts. Studies have shown that Bosniak criteria combined with ultrasound can greatly improve the diagnostic of benign and malignant renal cystic lesions[22]. If the shape of the renal cyst found by ultrasound examination does not have the characteristics of a simple renal cyst and the boundary with the surrounding renal tissue is unclear, the inner wall is not smooth, or there are nodules and separation, which most likely cystic kidney cancer or a complex renal cyst. At this time, CT or enhanced CT examination is needed to eliminate the possibility of renal malignancy further. But exact diagnosis is difficult at times because the imaging features between CRCC and benign renal cystic diseases are similar[23]. Therefore, if necessary, an ultrasound-guided biopsy should be performed. There are multiple cysts of different sizes in the cancer tissue, the cyst wall is lined with transparent cancer cells, and the transparent cancer cells in the capsule membrane are the main pathological features of cystic renal cell carcinoma. Multilocular cystic renal cell carcinoma (MCRCC), also known as multilocular clear cell renal cell carcinoma or multicystic clear cell carcinoma, is a rare type of renal cell carcinoma (RCC) with a relatively better outcome. And the diagnostic criteria for MCRCC are relatively strict, limited to RCCs with a malignant clear cell lining and small collections of cells without the formation of a malignant and expansive nodule. Histologic examination of MCRCC will reveal cysts lined by occasionally flattened cuboidal clear cells and septa containing aggregates of epithelial cells with clear cytoplasm[24]. But in the case, the morphological finding shows that the lining epithelium is “piled up” or stratified and/or with short papillary projections or tufting, and computed tomography (CT) scanning shows that there is lacking nests/cords of epithelial cells in the stroma or septa and without a solid mural nodule. Therefore, we excluded renal cancer accompanied by cystic change and diagnosed it as an atypical renal cyst. Also, to reduce the misdiagnosis of cystic renal carcinoma and improve the understanding of cystic renal carcinoma and atypical renal cyst, we should comprehensively analyze the patient's medical history, symptoms, signs and imaging examinations to make the diagnosis correct and provide reliable clinical support. For patients with atypical renal cysts, the possibility of cystic renal cancer should be considered before surgeries to avoid misdiagnosis.
Autosomal dominant polycystic kidney disease (ADPKD) is the most prevalent inherited nephropathy, which due to mutations in the PKD1 or PKD2 gene[25, 26]. Autosomal dominant polycystic kidney disease (ADPKD) is characterized by progressive enlargement of kidney cysts leading to chronic kidney disease (CKD) and end-stage renal disease (ESRD) [27–29]. Characteristic pathological changes and imaging tests are the primary diagnostic basis of ADPKD. Commonly used imaging examinations mainly include ultrasound and magnetic resonance imaging (MRI)[30]. In this case, the patient had no typical pathological features and family genetic history. Therefore, the genetic polycystic kidney is not considered first.
Here, we reported a rare case of atypical renal cyst (clear cell type). Atypical renal cyst is extremely rare among the routine renal pathological biopsies. Based on its histopathologic features, it might be over diagnosed as renal cell carcinoma with cystic change and caused unnecessary treatment and excessive psychological burden.