Andrea Lopez Sanmiguel1, Dilara Akbulut1, Jeff Chen1, Yash S. Khandwala1, Paul Russo1†
Affiliations:
1 Department of Surgery, Urology Service, Memorial Sloan Kettering Cancer Center, New York, NY
Abstract
Horseshoe kidney is the most common renal fusion anomaly and is associated with an elevated risk of renal malignancy, including primary renal neuroendocrine tumors. Surgical management of renal tumors in this setting is technically demanding due to the anatomical complexity of these patients. We report a case of a 50-year-old male with an incidentally discovered renal mass in the right moiety of a horseshoe kidney, identified on imaging obtained during workup for gallstone pancreatitis. Further evaluation with dedicated computed tomography renal angiography, magnetic resonance imaging, and renal ultrasound revealed a 4.6 × 3.1 cm solid, enhancing cortical neoplasm with protrusion into the sinus fat, arising lateral to the isthmic junction and accompanied by a supernumerary cross-fused ectopic left kidney. He underwent an open partial nephrectomy with intraoperative ultrasound for mapping. Three distinct right renal arteries were identified and clamped, with a total cold ischemia time of approximately 32 minutes (31 minutes and 39 seconds). Final pathology demonstrated a well-differentiated neuroendocrine tumor. This case highlights the importance of comprehensive preoperative imaging and individualized surgical planning in patients with horseshoe kidney and renal neoplasms.
Introduction
Horseshoe kidney (HSK) is the most common renal fusion anomaly, with a reported incidence of 1 in 400 to 1 in 1,800 of the general population.1-3 These cases are often characterized by isthmic fusion, malrotation, ectopia, and a highly variable arterial supply, which has been classified into six anatomical patterns as described by Boatman et al.2-4
Patients with HSK also carry an elevated risk of renal malignancy, with a reported prevalence of approximately 12%.1,5-8 A recent European multicenter retrospective cohort study on HSK tumors reported a median age of 62 years at diagnosis, male predominance, and a majority of stage I tumors. Histologically, nearly 50% were clear cell RCC, 19% papillary RCC, and 9% chromophobe RCC.7 An association between HSK and primary renal neuroendocrine tumors (NETs) has also been reported, although data on prognosis remain limited and variable.8-11 Renal NETs have a reported median overall survival of 84 months, and limited evidence suggests a more favorable prognosis in patients with HSK.8,11
Surgical management of renal tumors in HSK remains technically challenging due to anatomical complexity and vascular variability.1,4,6,12 The transperitoneal approach has most frequently been employed, and both open and minimally invasive techniques have been reported in the literature.6,12,13 Partial nephrectomy, while shown to have satisfactory oncologic outcomes, has been associated with longer operative times and greater estimated blood loss compared to non-HSK cases.1,6,12 Regardless of approach, preoperative planning is critical. Imaging tools such as the three-dimensional multi-slice helical CT have proven valuable in delineating tumor characteristics and vascular anatomy.1,11,13,14
We present a case of a 50-year-old man with an incidentally discovered renal mass in the lower pole of the right moiety of a horseshoe kidney, who underwent open partial nephrectomy for a well-differentiated neuroendocrine tumor.
Case Presentation
A middle-aged male patient initially presented for care in October 2025 with gallstone pancreatitis. He had a medical history also notable for chronic obstructive pulmonary disease and a surgical history of an upper abdominal hernia repair and tonsillectomy. His father and paternal grandfather both had prostate cancer, though he denied any other urologic or oncologic family history. The patient lived with his wife and 2 children and denied smoking or extensive alcohol use.
In early October, he presented to a local emergency room with exquisite epigastric right upper quadrant pain with emesis. He was diagnosed with gallstone pancreatitis and underwent ERCP with sphincterotomy and gallstone removal. On his diagnostic CT abdomen/pelvis with IV contrast, he was also found to have a horseshoe kidney with a supernumerary cross-fused left kidney. There was an indeterminate mass arising from the right aspect of the horseshoe kidney concerning for possible renal cell carcinoma.
The patient was counseled to undergo a combined robot-assisted partial nephrectomy along with a cholecystectomy. At this point, he sought a second opinion at our institution and was seen on October 21, 2025 by both Urology and Hepato-Pancreato-Biliary teams. He appeared cachectic and had lost 30 pounds since his pancreatitis episode. He was able to tolerate only 1000 calories per day without nausea and felt physically deconditioned. An updated CT scan revealed a 4.6 x 3.1 cm solid, enhancing cortical neoplasm arising from the lower pole of the right sided moiety of the horseshoe kidney with inner margin of the cortical neoplasm protruding into the sinus fat. There was also increased peripancreatic fluid that was tracking into the left side of the retroperitoneum and down the right lobe of liver into the paracolic gutter.
The patient subsequently underwent a dedicated CT renal angiogram, MR Renal, and renal ultrasound to further delineate vascular anatomy (Figure 1). We advised that he first undergo cholecystectomy, recover fully and then proceed to partial nephrectomy as a secondary operation at a later date. He elected this approach and underwent an uncomplicated robot-assisted cholecystectomy. Over the next several months he completely recovered from his pancreatit is, normalized his nutritional status and recovered from his previous deconditioned status. His pre-operative creatinine was 0.9 mg/dL, while his liver function markers had normalized.
He ultimately underwent a right open partial nephrectomy with intraoperative ultrasound on January 26, 2026. The patient was positioned supine and a midline incision was utilized to obtain access to the medialized renal hilum as well as to fully mobilize the right lateral moiety of the horseshoe kidney. Three right renal arteries were identified and clamped for a total of 31 minutes and 39 seconds of cold ischemia time. The resected tumor was found just lateral to the isthmus of the horseshoe kidney and was anterior and partially exophytic (Figure 2).
The patient recovered from surgery without complications. His diet was gradually advanced and his Foley catheter was discontinued on post-operative day 1. His pain was initially managed with a Dilaudid patient-controlled analgesia pump (PCA) though he was transitioned to oral pain medications as soon as he was tolerating a regular diet on post-operative day 1. He was discharged on post-operative day 3 with his creatinine remaining stable at 1.0 mg/dL.
Histopathologic examination demonstrated a well-differentiated neuroendocrine tumor (historically named as “carcinoid”) measuring 3.2 cm in greatest dimension. The Ki-67 proliferation index was 2%, consistent with a low proliferative rate. The tumor was associated with lymphohistiocytic inflammation, and coagulative necrosis. Immunohistochemical analysis of the tumor revealed diffuse positivity for synaptophysin, INSM1, and cytokeratin AE1/AE3, with patchy positivity for chromogranin and S100, supporting the diagnosis. The tumor cells were negative for GATA-3, p63, uroplakin-2/3, WT-1, TTF-1, and CDX-2. Surgical resection margins were free of tumor. A pseudocystic structure lined by urothelial epithelium was present adjacent to the tumor. This feature is most consistent with a residual upper urinary tract remnant and provides additional support for the tumor’s primary renal origin.(Figure 3).
The patient remains well and his incision has healed well. He suffered no complications from the operation and his creatinine one month after surgery was 1.2 mg/dL. He is being followed by the Gastrointestinal Oncology team for surveillance of his resected Grade 1 neuroendocrine tumor and is undergoing surveillance with cross-sectional CT imaging of his chest, abdomen, and pelvis (including triphasic liver evaluation) every 6 months for 3 years. He will then transition to annual imaging for a total of 10 years of surveillance.
Discussion
A horseshoe kidney (HSK) occurs in approximately 0.25% of the population, and its increased risk of renal malignancy has been well documented.1,5,6 HSK also carries a recognized association with primary renal neuroendocrine tumors (NET).8-10 Well-differentiated primary renal NETs are generally regarded as low-grade malignancies with indolent behavior, and tumor grade is determined by the Ki-67 proliferation index, adapted from the lung and gastrointestinal counterparts which are more frequent, and correlates with the risk of disease progression. Metastatic potential does exist, with late recurrences reported up to 7 years after surgery, warranting long-term surveillance.8,10,11 Metastasis, when present, is most commonly observed within the regional lymph nodes (36.4%) and liver (34.5%).10
Surgical management of these tumors can be challenging due to variations in the anatomy from case to case. HSK is associated with incomplete renal ascent, malrotation with anteriorly facing pelvises, and highly variable vascular supply.2-4 Consequently, detailed preoperative imaging evaluation with three-dimensional reconstruction is required for operative planning.1,11,13,14 In the present case, three distinct renal arteries were identified, reflecting the vascular variability of HSK.
Open surgery continues to represent the most reported approach accounting for 80% of cases.1,6,12,13 The type of surgical approach is determined by tumor location and the underlying anatomical complexity. When technically feasible, partial nephrectomy is preferred to maximize nephron preservation; however, one must attempt to preserve both the contralateral renal moiety and functional isthmic parenchyma which may have intertwined vascular supply.1,10-13
The spectrum of tumors reported in HSK is broad. Renal cell carcinoma is the most frequently encountered malignancy, with clear cell representing the predominant histology (46.5%), followed by papillary (19.1%) and chromophobe (9.5%).6,7 Other reported neoplasms include upper tract urothelial carcinoma (4.8%), Wilms tumor, and primary renal NET, which has been reported to arise in HSK in 17.8% of all primary renal NET cases.4,8-11 While renal NETs account for a small minority of HSK-associated tumors, HSK has been reported to confer a significantly elevated relative risk of primary renal NET compared to the general population.⁹
Given the rarity of renal NETs, postoperative surveillance lacks disease-specific guidelines. Current NET consensus frameworks focus primarily on gastroenteropancreatic tumors, where functional imaging and biochemical markers such as serum chromogranin A and urinary 5-HIAA play an established role; however, their utility in primary renal NETs remains undefined.8,10 Surveillance is therefore primarily imaging-based, guided by existing RCC frameworks, with cross-sectional imaging every 3–6 months for the first 2 years, followed by annual imaging for 10 years.8,10,11
This case highlights that renal NETs, though uncommon, must be included in the differential diagnosis of renal masses arising in HSK. It further highlights that after comprehensive preoperative imaging and careful surgical planning, open partial nephrectomy with intraoperative ultrasound guidance represents a safe and oncologically sound approach in this setting.
Declaration of interest
The authors declare no conflicts of interest related to this study.
Key words
Carcinoid Tumor, Horseshoe Kidney, Nephrectomy, Kidney
References
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