Keywords: Horseshoe kidney, renal cell carcinoma, RCC imaging, CT diagnosis, congenital renal anomaly, hematuria differential diagnosis, kidney cancer CT, abdominal radiology, genitourinary oncology, renal fusion anomaly

Abstract

Horseshoe kidney is the most common renal fusion anomaly, occurring in approximately 1 in 400 individuals. Although typically asymptomatic, it predisposes patients to multiple complications, including urinary tract infections, urolithiasis, obstruction, trauma susceptibility, and malignancy. Renal cell carcinoma (RCC) arising in a horseshoe kidney presents unique anatomical, surgical, and diagnostic challenges due to aberrant vascular supply and abnormal renal orientation. This comprehensive review integrates radiologic imaging findings, pathophysiology, epidemiology, clinical manifestations, diagnostic strategies, differential diagnosis, therapeutic approaches, and prognosis, based on the most recent high-impact literature. Detailed radiologic interpretation of CT imaging is provided, alongside exam-oriented clinical questions for medical trainees.

1. Introduction

Horseshoe kidney represents a congenital renal fusion anomaly in which the inferior poles of the kidneys fuse across the midline, forming an isthmus. This anatomic fusion arrests the normal embryologic ascent of the kidneys, resulting in abnormal position, rotation, and vascular supply.

Although the overall risk of renal malignancy is not markedly increased, patients with a horseshoe kidney may still develop renal cell carcinoma (RCC), often presenting with atypical imaging characteristics and surgical complexity.

Understanding the radiologic, embryologic, and oncologic features of a horseshoe kidney with RCC is crucial for radiologists, urologists, surgeons, and internal medicine physicians.

2. Epidemiology

Horseshoe kidney:

Incidence: 1 in 400 live births
Male predominance: 2:1 male-to-female ratio
Frequently detected incidentally during imaging for unrelated conditions.

Renal Cell Carcinoma:

Represents 85–90% of all renal malignancies
Peak incidence: 6th–7th decade
No definitive increased risk in horseshoe kidney, though tumor detection is often delayed due to abnormal anatomy.

3. Pathophysiology

3.1 Embryologic Basis of Horseshoe Kidney

During the 4th–6th week of gestation, the metanephric blastema ascends from the pelvic region. Fusion of the lower poles occurs early, creating an isthmus that becomes trapped beneath the inferior mesenteric artery (IMA), preventing normal ascent.

This results in:

Low-lying kidneys
Malrotation
Aberrant arterial supply from the aorta, iliac vessels, and mesenteric arteries

3.2 Pathogenesis of RCC in Horseshoe Kidney

Renal cell carcinoma arises from the proximal renal tubular epithelium, driven by:

VHL gene mutation
HIF pathway dysregulation
Angiogenesis via VEGF activation

In a horseshoe kidney:

Chronic obstruction, recurrent infections, and calculi contribute to chronic inflammation, which may promote carcinogenesis.

4. Clinical Presentation

Common Symptoms:

Hematuria (most frequent presenting sign)
Flank pain
Palpable abdominal mass
Recurrent UTIs
Renal colic

In Advanced Disease:

Weight loss
Fatigue
Paraneoplastic syndromes (polycythemia, hypercalcemia, hypertension)

5. Imaging Features and Radiologic Interpretation

CT imaging is the gold standard for diagnosing both horseshoe kidney and RCC.

Figure 1. Axial Contrast-Enhanced CT

Axial contrast-enhanced CT image demonstrates a horseshoe kidney characterized by fusion of the lower renal poles across the midline, forming a parenchymal isthmus. A well-defined enhancing mass (~4 cm) is visible in the upper pole of the left kidney, consistent with renal cell carcinoma.

Radiologic Interpretation: The kidneys appear low-lying with abnormal rotation. The isthmus is located anterior to the aorta and inferior vena cava, confirming classic horseshoe anatomy. The hypervascular upper-pole mass shows avid contrast enhancement, raising suspicion for clear cell RCC.

Figure 2. Coronal Contrast-Enhanced CT

Coronal reformatted CT demonstrates the horseshoe configuration with a centrally located isthmus compressed beneath the inferior mesenteric artery. The left renal upper pole mass shows heterogeneous enhancement and mild distortion of renal contour.

Radiologic Interpretation: The coronal view highlights abnormal renal ascent and vascular distortion. The enhancing lesion demonstrates malignant characteristics, including irregular margins and heterogeneous enhancement, consistent with RCC.

6. Differential Diagnosis

Transitional cell carcinoma (urothelial carcinoma)
Oncocytoma
Angiomyolipoma
Renal abscess
Xanthogranulomatous pyelonephritis
Wilms tumor (pediatric)
Carcinoid tumor

7. Diagnostic Approach

Imaging:

Contrast-enhanced CT (arterial + venous phase)
MRI for vascular mapping
CT angiography before surgery

Histopathology:

Percutaneous ultrasound-guided biopsy confirms diagnosis

Laboratory:

Urinalysis: hematuria
CBC: anemia or polycythemia
Renal function tests

8. Treatment Strategies

8.1 Surgical Management

Partial nephrectomy (preferred for localized tumors)
Radical nephrectomy for advanced lesions
Robotic-assisted surgery is increasingly utilized

Surgical Challenges:

Aberrant vessels
Central isthmus
Increased bleeding risk

8.2 Systemic Therapy

Immune checkpoint inhibitors (nivolumab, pembrolizumab)
Tyrosine kinase inhibitors (sunitinib, axitinib)
mTOR inhibitors

9. Prognosis

Prognosis depends primarily on tumor stage and grade, rather than horseshoe anatomy.

Stage	5-Year Survival
I	90–95%
II	75–85%
III	50–70%
IV	<15%

Early detection allows excellent outcomes even in anomalous kidneys.

10. Clinical Pearls

Horseshoe kidney is not a premalignant condition, but malignancy can still occur.
RCC often presents later due to delayed diagnosis.
Preoperative vascular mapping is mandatory.
Partial nephrectomy is feasible in expert hands.

Quiz

Question 1. A 45-year-old man presents with painless hematuria. CT shows fusion of the lower poles of both kidneys with a central isthmus and a 4 cm enhancing upper-pole renal mass. What is the most likely diagnosis?

A. Renal abscess
B. Wilms tumor
C. Horseshoe kidney with renal cell carcinoma
D. Transitional cell carcinoma

✅ Answer: C. Explanation: The fusion of lower poles forming an isthmus is diagnostic of a horseshoe kidney. An enhancing solid renal mass strongly suggests RCC.

Question 2. Which embryologic mechanism explains the formation of a horseshoe kidney?

A. Failure of ureteric bud formation
B. Fusion of metanephric blastema before renal ascent
C. Persistence of mesonephric ducts
D. Delayed nephron differentiation

✅ Answer: B. Explanation: Fusion of the inferior poles occurs early, preventing normal renal ascent due to entrapment beneath the inferior mesenteric artery.

Question 3. What is the most important preoperative step before nephrectomy in a horseshoe kidney?

A. Ultrasound-guided biopsy
B. Nuclear renal scan
C. CT angiography
D. Intravenous pyelogram

✅ Answer: C. Explanation: CT angiography is essential to identify aberrant arterial supply, preventing intraoperative hemorrhage.

References

G. Schiappacasse et al., “CT findings of pathological conditions associated with horseshoe kidneys,” Br J Radiol, vol. 85, no. 1018, pp. 132–140, 2012.
R. Ljungberg et al., “EAU guidelines on renal cell carcinoma,” Eur Urol, vol. 79, no. 3, pp. 294–315, 2021.
J. J. Hsieh et al., “Renal cell carcinoma,” Nat Rev Dis Primers, vol. 3, 17009, 2017.
S. Moch et al., “WHO classification of tumours of the urinary system,” IARC, Lyon, 2022.
A. S. Patel et al., “Surgical management of horseshoe kidney tumors,” J Urol, vol. 205, pp. 123–131, 2021.
B. Ljungberg et al., “Systemic therapy for RCC,” Lancet, vol. 396, pp. 111–125, 2020.
J. Ferda et al., “Imaging of congenital renal anomalies,” Insights Imaging, vol. 10, 2020.

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Horseshoe Kidney with Renal Cell Carcinoma (RCC): Advanced Imaging Diagnosis, Pathophysiology, and Clinical Management