Primary mucinous adenocarcinoma of the renal pelvis associated with obstructive urolithiasis and giant hydronephrosis

Abstract

Primary mucinous adenocarcinoma of the renal pelvis is an exceptionally rare malignancy that often mimics chronic stone disease or infected hydronephrosis. We report a 55-year-old man presenting with bilateral flank pain, bilateral nephrolithiasis, and giant right hydronephrosis. Ultrasound and contrast-enhanced computed tomography suggested obstructive infected hydronephrosis caused by a large ureteropelvic junction stone. During surgery, abundant gelatinous mucin and thickened friable renal pelvic mucosa raised suspicion for malignancy. Biopsy and final histopathology confirmed moderately differentiated mucinous adenocarcinoma. This case highlights the diagnostic difficulty of this tumor and the importance of intraoperative suspicion and histopathological assessment in longstanding obstructive stone disease.

1. Introduction

Primary adenocarcinoma of the renal pelvis is a very uncommon upper urinary tract malignancy, and the mucinous subtype is among its rarest pathological variants.1, 2, 3 Reported cases suggest that patients usually present with non-specific symptoms such as flank pain, fever, abdominal mass, hydronephrosis, pyonephrosis, or non-functioning kidney, so the lesion is frequently mistaken for severe chronic stone disease or infection.⁴

Although the exact pathogenesis remains uncertain, repeated reports have implicated chronic urothelial irritation and glandular metaplasia related to nephrolithiasis, recurrent infection, hydronephrosis, and longstanding obstruction.⁵ These chronic insults are believed to induce glandular or intestinal metaplasia of the urothelium, which may subsequently progress to dysplasia and, in rare cases, adenocarcinoma.⁶

Because both clinical manifestations and imaging findings are non-specific, preoperative diagnosis remains challenging.^7,8 We report a case of primary mucinous adenocarcinoma of the renal pelvis associated with obstructive urolithiasis and giant hydronephrosis, initially managed as infected stone disease, and discuss the diagnostic clues and surgical implications.

2. Case presentation

A 55-year-old man was admitted to Binh Dan Hospital on 26 January 2026 because of bilateral flank pain. He also reported low-grade fever, although the exact temperature had not been documented. Physical examination showed a soft abdomen with bilateral flank tenderness. No other remarkable systemic findings were recorded. The patient had no previous history of urinary tract surgery or known urothelial malignancy. However, detailed information regarding previous urinary tract infections was not clearly known.

Abdominal ultrasonography demonstrated multiple right renal stones, the largest approximately 30 mm, with grade IV hydronephrosis. The right renal pelvis contained low-echoic fluid and several hypoechoic septated areas, the largest measuring approximately 54 × 36 mm (Fig. 1). The left kidney also contained multiple stones, the largest about 18 mm, with grade III hydronephrosis.

Fig. 1.

Abdominal ultrasonography showing severe right hydronephrosis with low-echoic intrapelvic fluid and internal septation/multiloculated cystic appearance.

Contrast-enhanced abdominal multi-slice computed tomography showed several right renal calculi, the largest measuring approximately 18 × 12 mm, and a large right ureteropelvic junction stone measuring about 44 × 39 × 32 mm, associated with grade IV hydronephrosis. The left kidney showed multiple calculi, including a staghorn stone measuring about 46 × 31 × 18 mm, with grade I–II hydronephrosis (Fig. 2). A low-attenuation lesion of less than 10 HU was identified at the right ureteropelvic junction and along the right ureter. This lesion contained multiple internal calcified septa and did not show appreciable enhancement after contrast administration. However, its margin was difficult to differentiate from the fluid within the dilated collecting system and ureter (Fig. 3). Small low-density hepatic nodules and minimal pericardial fluid were also described, without other definite abdominal abnormalities.

Fig. 2.

Contrast-enhanced abdominal computed tomography demonstrating bilateral nephrolithiasis with giant right hydronephrosis. A: Axial CT image showing marked dilatation of the right collecting system with a large right ureteropelvic junction stone and bilateral renal calculi. B: Coronal CT image showing giant right hydronephrosis and multiple left renal stones, including a staghorn calculus.

Fig. 3.

Axial contrast-enhanced CT image showing a low-attenuation lesion at the right ureteropelvic junction/proximal ureter with internal calcified septa (arrows), difficult to distinguish from collecting-system fluid.

Based on the clinical and imaging findings, the patient was initially diagnosed with infected right hydronephrosis caused by ureteral stone in the setting of bilateral nephrolithiasis. Because infected obstructive hydronephrosis was suspected, the patient underwent open surgical exploration and drainage of the right collecting system rather than a percutaneous or retrograde endoscopic procedure. During the operation, more than 1000 mL of turbid urine was aspirated from the collecting system. Exploration revealed a thickened, friable ureteropelvic junction and renal pelvic mucosa. The collecting system was filled with abundant gelatinous mucin. Mucin was sent for cytological examination, and a biopsy sample was taken from the renal pelvis for histopathological assessment. Stones were removed, the collecting system was irrigated, and a 7-Fr double-J stent was placed.

Histopathological examination of the biopsy specimen showed gland-forming epithelial tumor cells with hyperchromatic, moderately pleomorphic nuclei and mucin production, consistent with moderately differentiated mucinous adenocarcinoma (Fig. 4).

Fig. 4.

Histopathological examination of the biopsy specimen. A: Low-power microscopic image showing mucin-producing malignant epithelial cells within a mucinous background. B: Higher-power microscopic image showing atypical gland-forming epithelial tumor cells with hyperchromatic nuclei, moderate pleomorphism, and mucin production, consistent with moderately differentiated mucinous adenocarcinoma.

After discharge, the patient was readmitted on 23 February 2026 for definitive surgical treatment of the right renal pelvic tumor. On 27 February 2026, he underwent right nephrectomy through a flank approach. Intraoperatively, the right kidney was markedly hydronephrotic and thin-walled, with mild inflammatory adhesions to surrounding tissue. The renal pelvis was adherent to the psoas muscle, raising concern for local invasion. The ureter was dissected distally, the old double-J stent was removed, and nephrectomy was completed. Estimated blood loss was 100 mL.

Gross pathological examination of the nephrectomy specimen showed a kidney measuring 14 × 7 × 4 cm, with attached ureter approximately 9 cm in length. On sectioning, the renal parenchyma was thinned and the renal pelvis was dilated. The renal pelvic mucosa was covered by multiple blood clots with focal friable mucinous excrescences. The ureteropelvic junction contained membranous and friable brown lesions with mucinous material, and the tumor extended into the upper one-third of the ureter, spanning approximately 3 cm.

Microscopically, the tumor consisted of atypical gland-forming cells with hyperchromatic nuclei, moderate pleomorphism, and moderate mitotic activity, arranged in clusters and elongated structures with focal glandular lumina. The final diagnosis was moderately differentiated mucinous adenocarcinoma involving the ureteropelvic junction and ureter, without invasion into the renal parenchyma. Lymphovascular invasion and perineural invasion were absent. The ureteral/vascular hilar margin was negative for malignancy.

The left kidney was planned for staged management of nephrolithiasis after recovery from right nephrectomy and completion of oncological assessment. Follow-up imaging and renal function monitoring were recommended, with further urological intervention to be considered according to symptoms, renal function, infection status, and stone burden. Serum CEA and CA19-9 were not measured before diagnosis because the initial clinical impression was infected obstructive hydronephrosis related to stone disease. Given previous reports of elevated CEA and/or CA19-9 in renal pelvic mucinous adenocarcinoma, postoperative baseline measurement and serial monitoring of these tumor markers were recommended for follow-up and recurrence surveillance.

3. Discussion

Primary mucinous adenocarcinoma of the renal pelvis is one of the rarest malignancies of the upper urinary tract.^1,3,6 Published reports consistently emphasize its rarity, non-specific presentation, and frequent association with long-standing nephrolithiasis, infection, pyonephrosis, or hydronephrosis.^3,4,6,7

Our case illustrates several classic diagnostic pitfalls. First, the patient presented with bilateral flank pain and fever-like symptoms, while imaging demonstrated giant right hydronephrosis, a large ureteropelvic junction stone, and bilateral renal calculi. These findings strongly supported a working diagnosis of complicated obstructive stone disease.^9,10 Second, the low-attenuation lesion in the right renal pelvis and ureter was difficult to distinguish from collecting-system fluid, even on contrast-enhanced CT. Similar diagnostic confusion has been described in prior case reports, where renal pelvic mucinous adenocarcinoma was initially interpreted as hydronephrosis, pyonephrosis, or a non-functioning stone-bearing kidney.4, 5, 6, 7.

The presumed pathogenesis in this setting is chronic urothelial irritation. Recurrent inflammation and long-term stone burden may induce glandular or intestinal metaplasia of the urothelium, which can then progress through dysplasia to invasive adenocarcinoma.¹¹ This metaplasia-dysplasia-carcinoma sequence has been proposed repeatedly in the literature and is supported by recent pathological descriptions of intestinal-type differentiation in renal pelvic mucinous tumors.^2,12

In the present patient, the combination of giant obstructive stone disease, severe hydronephrosis, thickened friable pelvic mucosa, and abundant gelatinous intraluminal mucin was highly suspicious intraoperatively. The operative finding of copious mucin is especially important because it is unusual for ordinary infected hydronephrosis and should prompt immediate biopsy and thorough pathological assessment. In practice, a definitive diagnosis is often established only after intraoperative frozen-section assessment or final histopathologic examination of the nephrectomy specimen, as occurred in our patient.^5,6,13

Because no standardized treatment protocol has been established, surgery—most commonly radical nephrectomy or nephroureterectomy—remains the principal treatment reported in the literature.^1,6,10,14 Because many cases are diagnosed after the kidney has already become non-functioning or massively hydronephrotic, nephrectomy is commonly performed. Some published cases also describe ureteral involvement, which is relevant because tumor extension beyond the renal pelvis can influence operative planning and pathological evaluation.^2,7,10 In our case, the final specimen confirmed involvement of both the ureteropelvic junction and ureter, while the renal parenchyma itself was not invaded and the margin was negative.

The prognosis of this tumor remains uncertain because available evidence is limited mainly to isolated case reports and small case-based discussions.^1,3, 4, 5 Nonetheless, delayed diagnosis is a recurring concern, especially when the lesion is masked by longstanding calculous disease.^1,4,6,15 Our case reinforces the need for heightened suspicion in patients with unusual mucinous contents in the collecting system or indeterminate intrapelvic low-density lesions associated with chronic stone disease.

This report has limitations inherent to a single case. Immunohistochemical data were not available in the source records, and long-term oncological follow-up had not yet been documented. In addition, photographs of the gross nephrectomy specimen and intraoperative mucinous material were unavailable. Serum CEA and CA19-9 were not measured before diagnosis because the initial presentation suggested infected obstructive hydronephrosis due to stone disease; however, postoperative baseline testing and serial monitoring were recommended. Even so, the case remains informative because it captures the full diagnostic sequence from misleading imaging to intraoperative suspicion and definitive histopathological confirmation.

4. Conclusion

Primary mucinous adenocarcinoma of the renal pelvis is a rare but important differential diagnosis in patients with longstanding nephrolithiasis, obstructive hydronephrosis, and atypical mucinous or low-attenuation lesions within the collecting system. Imaging may suggest only complicated stone disease, and diagnosis often depends on intraoperative recognition and histopathological confirmation. Early suspicion and complete pathological assessment are essential to avoid missed or delayed diagnosis.

CRediT authorship contribution statement

Quang Huy Huynh: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Chi Phong Nguyen: Writing – review & editing, Writing – original draft, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Thanh Luan Bui Thi: Writing – review & editing, Writing – original draft, Methodology, Funding acquisition, Formal analysis, Data curation, Conceptualization. Hung Lenh Do: Writing – review & editing, Writing – original draft, Visualization, Validation, Resources, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

Consent

Written informed consent for publication of this case report and accompanying images was obtained from the patient or the patient's legal representative. A copy of the written consent should be available for review by the Editor-in-Chief on request.

Ethical approval

Ethical approval was not required for this single case report according to our institutional policy. All patient information was anonymized before manuscript preparation.

Sources of funding

None.

Conflict of interest

The authors declare no conflicts of interest.

Data availability

Data supporting this case report are contained within the article. Additional de-identified details may be available from the corresponding author on reasonable request, subject to institutional approval.