Mucinous adenocarcinoma is a rare occurrence on the kidney, commonly arising from the renal pelvis but can also occur within the renal parenchyma [4]. It can be primary or secondary, depending on the tumour’s origin. It is crucial to rule out primary tumours from the pancreas, ovary, and appendix, particularly in the context of pseudomyxoma peritonei [5]. Limited evidence exists for secondary renal mucinous cystadenocarcinoma, with a single reported case of appendicular cystadenocarcinoma presenting as a renal tumor [6]. Both of our cases presented as primary renal mucinous adenocarcinomas, as there were no tumours observed in other organs.

The histogenesis is uncertain. The theory of chronic irritation, often associated with obstructing urinary calculi and recurrent infections, is a commonly proposed explanation in reported cases [7, 8]. Another hypothesis involves the differentiation of celomic epithelium, where mesothelium undergoes mucinous metaplasia [7, 9].

Less commonly, renal mucinous cystadenocarcinoma is found in patients with developmental renal anomalies, supporting the theory of renal maldevelopment [10,11,12]. Our Case 1 represents, to the best of our knowledge, the first documented instance of a duplex collecting system presenting with muconephrosis. It is postulated that sequestration of segment of renal pelvic epithelium within the renal parenchyma during maldevelopment predisposes to the development of these tumours [12]. This explains the presentation in our case 2 who presented as a renal parenchymal cyst. Presently, the association of Renal Mucinous adenocarcinoma and Developmental Renal anomalies remains hypothetical, and further studies focusing on molecular or immunohistochemical characteristics are needed to explore and potentially confirm this link.

Clinical presentation ranges from asymptomatic renal cysts to those presenting with pseudomyxoma peritonei. Mucosuria is a distinctive feature but is seldom observed [13]. Factors raising suspicion include prolonged symptom duration, association with calculi, presence of hydronephrosis, and a preoperative appearance suggestive of an inflammatory condition [14]. Our Case 1 showed mucosuria, indicating potential mucinous tumours. Conversely, Case 2, initially diagnosed as a benign renal cyst, lacked suggestive clinical history.

There are no specific diagnostic characteristics for mucinous cystadenocarcinoma on cross-sectional imaging. Laboratory testing is nonspecific. In some reported cases, primary mucinous adenocarcinomas of the renal pelvis have been linked to elevated levels of CEA or CA19-915. Both our cases did not have any suggestive radiological or laboratory findings. The definitive diagnosis of mucinous cystadenocarcinoma is established through histopathology [16]. Immunohistochemistry studies show positive staining for CDX2, MUC2, and CK20, CK 7, EMA, and CEA [17]. This aligns with our findings in Case 1, which exhibited diffuse CK7, CK20 positivity, and focal CDX2. However, immunohistochemistry was not performed in Case 2 due to financial constraints.

Treatment options range from partial to radical nephrectomy [9, 16]. Some recommendations also include additional ureterectomy with a bladder cuff, similar to the approach for upper tract urothelial carcinoma (UTUC) [9, 10, 16]. Some authors have explored the role of adjuvant chemotherapy and radiotherapy [15]. However, the impact on prognosis remains uncertain. In Case 1 the patient experienced tumor recurrence in the lower moiety with uncertainty about it being metachronous or resulting from tumor implantation from spillover of contents during the initial procedure possibly explaining the metastatic lesion in the rectus abdominis. In Case 2 tumor recurrence post cyst deroofing suggests tumor cell implantation from the cyst fluid spillover. Therefore in any surgical intervention involving a cystic renal lesion efforts must be made to avoid cyst fluid spillage to reduce the risk of recurrence. This also highlights the potential need for adjuvant therapy to prevent tumour recurrence.

Generally, the prognosis appears to be poor, with overall survival of 2–5 years [9]. Recent literature suggests a disease-free follow-up of 28 months [18]. However, both of our cases exhibited recurrence within 12 months, underscoring the aggressive nature of these tumours.