Renal metastasis of adenocarcinoma of the gastrointestinal tract with unknown primary site: a case report and review of the literature

Cancer of unknown primary (CUP) is an aggressive malignancy with a persistently poor prognosis. Renal metastases from an occult primary adenocarcinoma are exceptionally rare and diagnostically challenging, frequently mimicking primary kidney cancer. This case report details a rare instance of metastatic adenocarcinoma to the kidney from an occult primary origin, initially misclassified as primary renal cell carcinoma. A 60-year-old male patient was admitted to our department for further evaluation of a cystic-solid right renal mass. Staging revealed synchronous extrarenal metastases. Subsequent immunohistochemical analysis indicated gastrointestinal differentiation, yet exhaustive diagnostic workup failed to identify the primary tumor site. The patient underwent radical nephrectomy followed by biomarker-guided FLOT (fluorouracil, leucovorin, oxaliplatin, and docetaxel) chemotherapy combined with camrelizumab immunotherapy, achieving a progression-free survival of approximately 7 months, accompanied by a temporary decline in tumor markers. Tumor recurrence manifested at 11 months postoperatively as pulmonary progression, at which point retrospective molecular profiling of the archived specimen revealed an ERBB2 amplification, providing a molecular rationale for subsequent targeted therapy. This case highlights the crucial roles of multidisciplinary collaboration and molecular technology in resolving diagnostic dilemmas and identifying actionable targets in CUP. It offers two key clinical takeaways: diagnostically, a solitary kidney mass with other metastases should prompt consideration of a renal metastasis, even though primary kidney cancer is more common. Therapeutically, while the identified biomarker provided a rationale for precision therapy, the resulting clinical benefit was limited and transient. This suggests that the aggressive biology and spatial heterogeneity typical of CUP can attenuate the long-term efficacy of biomarker-guided interventions.