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Abstract Background Primary mucinous adenocarcinomas of the kidney are rare and pose a challenge for preoperative diagnosis. The histogenesis of these tumours remains largely unknown, with three proposed theories: chronic irritation, differentiation of celomic epithelium, and kidney maldevelopment. Here, we present two cases of renal mucinous adenocarcinoma in patients with developmental renal anomalies, specifically a duplex collecting system and a horseshoe kidney. Case Presentation First, A 50-year-old male presented with loin pain and jelly-like discharge in urine with a duplex collecting system and gross hydronephrosis of the upper moiety on imaging. The patient underwent upper polar nephrectomy with controlled drainage of 1.5 L of mucinous fluid. Histopathology was suggestive of mucinous borderline cystic neoplasm with invasive microcarcinoma. The patient presented one year later, with a hydronephrotic lower moiety of the left kidney and a rectus abdominis mass. Fine needle aspiration biopsy of the mass revealed papillary adenocarcinoma with histological resemblance to the renal pelvis lesion, establishing it as a metastasis from the primary renal malignancy. Second, A 53-year-old male who had undergone right laparoscopic cyst deroofing for a symptomatic renal cyst, whose postoperative histology revealed findings consistent with mucinous adenocarcinoma, presented with flank pain and palpable retroperitoneal mass. Imaging revealed a horseshoe kidney morphology with a large multilobulated hypodense non-enhancing cystic lesion arising from the right kidney. Cyst excision with right open radical nephrectomy was performed. Gross examination revealed multiple cystic spaces replacing renal parenchyma, filled with gelatinous material. Microscopy was suggestive of recurrent mucinous adenocarcinoma. Conclusions Renal mucinous cystadenocarcinomas can be associated with anomalous kidneys. Definitive diagnosis relies on histopathology, and these tumours are recognized for their aggressive nature. Complete resection is the preferred treatment, but further studies are needed to assess the efficacy of adjuvant treatment, given the poor prognosis and high likelihood of recurrence. Clinical trial number Not applicable.

Background: Primary mucinous adenocarcinomas of the kidney are rare and pose a challenge for preoperative diagnosis. The histogenesis of these tumours remains largely unknown, with three proposed theories: chronic irritation, differentiation of celomic epithelium, and kidney maldevelopment. Here, we present two cases of renal mucinous adenocarcinoma in patients with developmental renal anomalies, specifically a duplex collecting system and a horseshoe kidney. Case Presentation: First, A 50-year-old male presented with loin pain and jelly-like discharge in urine with a duplex collecting system and gross hydronephrosis of the upper moiety on imaging. The patient underwent upper polar nephrectomy with controlled drainage of 1.5 L of mucinous fluid. Histopathology was suggestive of mucinous borderline cystic neoplasm with invasive microcarcinoma. The patient presented one year later, with a hydronephrotic lower moiety of the left kidney and a rectus abdominis mass. Fine needle aspiration biopsy of the mass revealed papillary adenocarcinoma with histological resemblance to the renal pelvis lesion, establishing it as a metastasis from the primary renal malignancy. Second, A 53-year-old male who had undergone right laparoscopic cyst deroofing for a symptomatic renal cyst, whose postoperative histology revealed findings consistent with mucinous adenocarcinoma, presented with flank pain and palpable retroperitoneal mass. Imaging revealed a horseshoe kidney morphology with a large multilobulated hypodense non-enhancing cystic lesion arising from the right kidney. Cyst excision with right open radical nephrectomy was performed. Gross examination revealed multiple cystic spaces replacing renal parenchyma, filled with gelatinous material. Microscopy was suggestive of recurrent mucinous adenocarcinoma. Conclusions: Renal mucinous cystadenocarcinomas can be associated with anomalous kidneys. Definitive diagnosis relies on histopathology, and these tumours are recognized for their aggressive nature. Complete resection is the preferred treatment, but further studies are needed to assess the efficacy of adjuvant treatment, given the poor prognosis and high likelihood of recurrence. Clinical trial number: Not applicable. [ABSTRACT FROM AUTHOR]