A Rare Presentation in a Common Renal Anomaly: Metastatic Adenocarcinoma in a Horseshoe Kidney

Horseshoe kidney is the most common renal fusion anomaly, occurring in approximately 1 in 500 individuals. While renal cell carcinoma (RCC) is the most frequently reported malignancy in horseshoe kidneys, primary adenocarcinoma of the renal pelvis is exceedingly rare, accounting for less than 1% of all renal malignancies. Due to its rarity and nonspecific presentation, diagnosis is often delayed, leading to poor prognosis. We report the case of a 35-year-old male who presented with progressive abdominal pain and backache for three months. Initial imaging, including radiography, ultrasound, and contrast-enhanced computed tomography (CT), revealed a horseshoe kidney with a heterogeneously enhancing mass arising from the left renal pelvis, containing coarse calcifications. Further evaluation with magnetic resonance imaging (MRI) and fluorodeoxyglucose (FDG) positron emission tomography (PET) demonstrated metastatic involvement of the dorsal vertebrae (D4, D8) and first lumbar vertebra (L1), along with pulmonary metastases. CT-guided biopsy confirmed primary adenocarcinoma of the renal pelvis. Next-generation sequencing (NGS) did not reveal actionable mutations, limiting the role of targeted therapy. The patient was managed with systemic chemotherapy and palliative radiotherapy for bone metastases. This case underscores the importance of multimodal imaging for accurate diagnosis and staging, as well as the role of molecular profiling in treatment planning. Given the poor prognosis associated with metastatic disease, further research into the molecular characteristics and targeted therapies for this rare malignancy is warranted.