Primary mucinous adenocarcinoma of the kidney is rare, originating almost entirely from the epithelium of the renal pelvis. Although this type of tumor is located in the renal pelvis, it can also be located in the kidney, ureter, or bladder. To date, approximately 100 cases of mucinous adenocarcinomas of the renal pelvis have been reported. Van Langenhove et al. summarized eight cases of MC of the kidney in the last 10 years after searching the PubMed database [2]. We found only one case of MC of the kidney originating from one pole of the kidney without invading the renal pelvis and ureter, which was resected through partial nephrectomy [3]. In this paper, we report a case of mucinous cystadenocarcinoma (MC) of the kidney that was located at the lower pole. The disease occurs frequently in men, and there are on specific symptoms, such as mucusuria, flank pain, hematuria, or symptoms that are generally caused by kidney stones and pyelonephritis. In addition, the tumor is generally large, and the abdominal mass is palpable [2.4]. The patient only presented symptoms of left lower back pain. Several theories about the histogenesis of MC in the kidney have been proposed to explain the metaplasia of glands in the pluripotent uroepithelium of the collecting system. The three histogenetic theories include chronic stimulation and differentiation of coelomic epithelial and renal dysplasia [5].
Radiological examinations included ultrasound, CT, or magnetic resonance imaging (MRI) to locate and evaluate the nature of the tumor. However, MC has no unique radiological characteristics. Therefore, distinguishing between benign or malignant tumors or determining the tumor origin are difficult using preoperative radiographic images. After summarizing 30 cases of mucinous adenocarcinomas of the renal pelvis, Li et al. found that mucinous adenocarcinomas presented as multiple renal pelvic calculi, severe hydronephrosis, calculous pyonephrosis, or ureteric junction obstruction, with few cases manifesting as a mass or tumor [4]. Although, unique imaging features are lacking to differentiate mucinous adenocarcinomas from common renal tumors, there are also substantial proposals to involve urologists. Most papillary renal cell carcinomas are commonly associated with lymph node metastasis or renal vein infiltration [2]. In our imaging findings, because the tumor secreted mucus, it turned into a low-density mass with a slightly thicker and smoother cyst wall on CT, with a CT value of 5 ± 10 HU and flocculent high-density shadow. Enhanced CT suggested no obvious enhancement in the content and slight enhancement in the cystic wall; the tumor could be classified as a Bosniak III. After consulting with general orthopedic surgeons and performing an improved three-dimensional CT scan of the pancreas, an invasive relationship or serious adhesion between the tumor and surrounding tissues, including the psoas major and pancreas, was not found. Because the imaging findings were not specific, we considered the hematoma to be caused by a history of trauma or retroperitoneal tumor. Apart from imaging, special auxiliary examinations may also help in the diagnosis of mucinous adenocarcinomas of the kidney. Some cases have indicated that urine exfoliative cytology might serve as a useful tool when combined with clinical findings. In addition, several studies have also described the important role of CEA and CA199 plasma levels as useful markers before surgery to diagnose mucinous adenocarcinoma and as independent markers for prognosis and recurrence [6].
Renal MC generally have multiple cysts, and the tumor body is large and contains glue-like substances. Most of the cyst walls are smooth, white, or pink, and the local position may have nodular, granular, or micropapillary structures [7]. According to the pathological examination results of this patient, a gray-white cystic mass measuring 7×6×5 cm and a large amount of mucus in the tumor were observed in the lower pole of the kidney, which was closely related to the ureter. Many mucous lakes and mucoid columnar epithelium were observed under the microscope, which appeared highly similar to the cellular epithelium of colorectal adenocarcinomas. Unlike benign adenomas, we found that the tumor cells showed invasive growth microscopically. As stromal invasion is a definitive marker of malignancy, MC was diagnosed.
Considering that the pathological feature of the tumor is the secretion of a large amount of mucus, we analyzed the tumors from a neoteric perspective. The patient was thought to have either a mucinous tubular and spindle cell carcinoma (MTSCC), mixed epithelial and stomal tumor (MEST), or renal abscess. Particularly, MTSCC cells can secret mucus. Fine et al. categorized MTSCC into different subtypes according to the proportions of mucus, tubular, and fusiform components. One subtype is typical, which combines three components, and the other subtype is mucin-lacking, which has little mucus but is full of spindle cells or tubules [8]. In addition, the tumor is located in the renal parenchyma with expansive growth and a cystic-solid tumor is located in the renal parenchyma with expansive growth and a cystic-solid tumor with a clear boundary as determined by imaging. Plain CT scans are generally isodense or have low density [9]. Enhanced CT showed that the density of the tumor was significantly lower than that of the renal cortex and medulla [10]. However, in this case, there were only mucous matrix and mucous epithelial cells, no tubular or spindle cells were found, and the imaging was not consistent; therefore, we ruled out the diagnosis.
Furthermore, MEST has a mucous matrix composition. It is a complex cystic and solid mass characterized by the presence of stromal components similar to the stroma of the ovary (composed of fusiform cells with full nuclei and rich cytoplasm) and epithelial component of cysts with an epithelial lining [12]. MEST is a complex tumor composed of large cysts, microcapsules, and tubules. The largest cysts consist of columnar and cuboidal epithelia, which occasionally forms small papillary masses. The mucous stroma and fascicular area of the smooth muscle cells may protrude [13]. Chu et al. reported a case of a borderline MEST-secreting myxoid matrix [14]. The imaging manifestations are not specific. In a typical MEST, an expansive multilocular cystic mass may protrude into the renal pelvis with varying sizes and a thick cystic septum. The tumor does not have a thick fibrous wall, but the cystic septum is thicker than a typical cystic nephroma [13]. In our case, there were no special structures observed under the microscope, except for the mucinous epithelium; therefore, MEST was excluded.
Lastly, renal abscesses had similar imaging findings. Renal abscesses are characterized by complex renal cysts with fluid density, uneven intensity, and thick and irregular walls. Owing to the presence of viscous pus, the liquid components show strong and uneven diffusion limitations on diffusion-weighted imaging. Contrast-enhanced CT showed well-defined, low-density round masses, often with thick edges or halos. The abscess had extended around the kidney. MRI showed inhomogeneous thick-edge lesions with low signal intensity on T1-weighted images and high signal intensity on T2-weighted images with limited diffusion [11.12].
Renal MC can be primary or secondary. However, because primary mucinous adenocarcinoma of the kidney is relatively rare, metastasis from other primary origins, including the pancreas, ovary, colorectum, appendix, should first be excluded. The immunohistochemical results, CDX-2(+), CK20(+), CEA(+), MUC2(+), MUC5AC(+), Villin(+), and CK7(-), were almost similar between the intestinal and ovarian phenotypes. However, combined with the imaging findings of the patient, the origin of the primary lesion could not be determined. Only one case of low-grade cystadenocarcinoma of the appendix presenting as a renal tumor was previously published by Gómez-Román et al. in 1995 [15]. The three intestinal tumor markers, CDX2(+), CK20(+), and CK7(-), were similar to the immunophenotype of colorectal adenocarcinoma. Colorectal adenocarcinoma is characterized by CK7(-) and CK20(+) [16]. Both the CK7-/CK20 + phenotype and CDX2 antibody expression are highly specific and sensitive markers of the origin of colorectal cancer, and the specificity of CK7-/CK20 + is 97.6% [17]. Therefore, it is necessary to consider whether the tumor is of intestinal origin. CDX2 can also be expressed in ovarian mucinous adenocarcinoma, but most express CK7(+) and CK20(-). In addition, MUC2 and MUC5AC are secretory mucins, and the biological significance of their positive expression is mainly in the formation of high-viscosity gel-like mucus. These immunostaining results support another differentiation theory of the histogenesis of the coelomic epithelium in MC in which the peritoneum (mesothelial) undergoes mucinous metaplasia and mucinous cystadenoma [3].
Radical nephrectomy is generally used in the surgical treatment of renal MC; however, Chung et al. reported the use of a partial nephrectomy [3]. However, partial resection may be associated with the risk of recurrence and tumor rupture, leading to peritonitis. Some reports have also suggested an additional ureterectomy to improve prognosis, namely conventional nephroureterectomy with a bladder cuff, in case of mucusuria to prevent implantation of tumor cells, due to its development in areas such as the bladder and ureter where the urothelium was present [4]. In this case, we performed robot-assisted laparoscopic radical resection of a left renal tumor, and the operation was completed using a transperitoneal approach because the relationship between the tumor and surrounding tissue was unclear and adherent. This was the first robot-assisted laparoscopic radical nephrectomy of MC of the kidney. We considered partial resection of the tumor; however, in view of the situation during the operation and after communicating with the family, we performed robot-assisted laparoscopic radical nephrectomy, and the tumor was completely removed. Compared with laparoscopic radical nephrectomy, robotic radical nephrectomy has obvious advantages, such as providing 3D visual effects with a wider and clearer range of observation and reducing the interference of the instrument, so that the operator can accurately view the anatomy for accurate suture and tremor of the surgeon's hand can be eliminated [18]. Autorino et al. Concluded that patients who underwent robotic radical nephrectomy had a higher histological grade and pathological stage and shorter hospital stay, but no difference in operative complications were observed. These results suggest that robotic radical nephrectomy could treat patients with a more advanced and challenging MC [19]. Similarly, this case adopted a transperitoneal approach, which is not common in modern urological surgery. The retroperitoneal approach is associated with an earlier recovery of intestinal function, shorter hospital stay, earlier recovery, shorter operation time, and shorter renal hilar vascular control time. However, the transperitoneal approach has a wider workspace and anatomical landmarks and locations can be identified easily [20]. Considering the larger tumor size, risk of adhesion with other tissues, and practical needs, we chose transperitoneal access. In summary, we have provided a new method to treat renal MC. The patient recovered quickly postoperatively. The catheter was removed on the second postoperative day, and the drainage tube was removed one week after the operation.
The prognosis of primary adenocarcinoma of the renal pelvis is generally poor, with most patients dying during the 2–5 year follow-up period. The longest reported follow-up period with good prognosis was 79 months. [21]. Chemotherapy, radiotherapy, and chemoradiotherapy has been recommended for the treatment of mucinous colorectal cancer and mucinous ovarian carcinoma [4]. Therefore, we have adopted adjuvant therapy, such as chemotherapy after surgery, to prolong the survival time of patients.