Primary retroperitoneal mucinous cystic tumour of borderline malignancy mimicking kidney duplicate: cases report and literature review

Primary retroperitoneal mucinous cystic tumour with borderline malignancy (PRMC-BM) is an extremely rare disease [1]. Only 23 cases have been reported, and only one exhibited metastasis [1, 2]. No PRMC-BM mimicking duplicated kidneys have been reported in the literature. In this report, we present two cases of PRMC-BM mimicking duplicated kidneys. We also provide a literature review and discuss the outcomes of different surgical procedures.

Mucinous cystadenomas are common in the ovaries, pancreas, and appendices. It is thought to be a PRMC when located in the retroperitoneum with a normal visceral organ[3]. According to its pathology, there are three types of PRMC: mucinous cystadenoma, mucinous cystic tumour of borderline malignancy, and mucinous cystadenocarcinoma[4]. Using Medline, we performed a literature review since 1966, and only 23 PRMC-BM cases have been reported (Table 1) [1, 2, 4‐20].

Preoperative diagnosis of PRMC is very difficult. Imaging is helpful in the detection of retroperitoneal cystic masses, but it is difficult to differentiate PRMC from a variety of retroperitoneal cystic diseases because of the lack of typical imaging features[3]. In the 23 cases reported in the literature, the age of the patients ranged from 22 to 68 years. Most patients presented with local swelling, pain, and self-examination of the mass, similar to our two cases. As shown in Table 1, most cysts were located in the pelvis or near the colon. Although it is close to the kidney, the cystic mass often appears as squeezing the kidney and is easily differentiated from hydronephrosis. Here, we report the first case of PRMC-BM mimicking kidney duplication. In our first case, a huge retroperitoneal cystic mass located close to the kidney was misdiagnosed as malformation of a duplicate kidney, and a robot-assisted laparoscopic partial nephrectomy for a duplex kidney was scheduled. We found that ultrasound-guided puncture of the cyst may be useful in the differential diagnosis when a duplicate kidney is suspected. In our second case, the fluid in the cyst was very viscous, and hydronephrosis was excluded. Percutaneous cyst puncture of the cystic mass and cytological examination of cystic fluid were performed in some cases[6, 21], similar to our second case, which did not seem to increase the risk of tumour implantation and recurrence.

The occurrence of PRMC is most commonly seen in female patients, but its histogenesis remains unclear. The main theories are as follows: seeding of ectopic ovarian tissue[22], monodermal variant of teratomas, enterogenic duplication of cysts, and coelomic metaplasia[23]. Retroperitoneal cystectomy is considered an effective treatment for PRMC. Both open and laparoscopic surgical approaches have been reported for the treatment of PRMC-BM, and the outcome is generally favourable [4]. Surgeons prefer the open approach. The laparoscopic surgical approach is thought to have the advantage of being minimally invasive[21], and cyst aspiration and fragment removal do not affect prognosis[6]. We recommend an open surgical approach for this type of tumour. First, the cystic mass is often large and needs to be aspirated when using a laparoscopic surgical approach, which carries the risk of tumour implantation. In our first case, the patient experienced tumour recurrence six months after cystectomy using a robot-assisted laparoscopic approach, which may be related to the opening of the cyst. Moreover, the cystic fluid could be viscous and difficult to aspirate which may greatly increase the difficulty of the operation. Second, the ureter encapsulated by the mass can be touched by open surgery and, thus, can be safely retained. In our second case, the cystic mass was large and half-encapsulated in the kidneys and ureters. A double J was placed in the left ureter before tumour removal. We found that it was very easy to find the location of the ureter and thus avoid ureteral injury. Third, the open surgical approach had a shorter operation time and less intraoperative bleeding, suggesting that the open surgical approach is safer.

Furthermore, PRMC-BM is a potential malignant type of PRMC. However, we reviewed other reports and found that the recurrence rate in such cases was very small. We tried to analyse the imaging features of the cystic lesions, multilocular, enhanced, and solid nodules but failed to reveal any regularity in the recurrence rate. Chemotherapy is recommended for mucinous cystadenocarcinoma[1]. However, in PRMC-BM, only close follow-up is required.

In conclusion, PRMC-BM is a rare disease that may mimic urinary tract-related diseases, and urologists need to be aware of it. Ultrasound-guided puncture of the cyst may be useful in differential diagnosis before surgery. Since cystic masses can be malignant, careful protection of the cyst wall and prevention of cyst rupture can reduce the risk of tumour recurrence, and an open surgical approach may be more appropriate.