Bladder metastasis from gastric cancer presenting as acute post-renal obstruction during immunotherapy: a case report.

Bladder metastasis from gastric cancer is extremely rare and may be misdiagnosed, particularly during immunotherapy, when new urinary symptoms can be attributed to genitourinary immune-related adverse events. We report a case of a 35-year-old man with advanced gastric cancer who initially responded to chemotherapy combined with sintilimab, an anti-Programmed Death-1 (PD-1) inhibitor. After four cycles, he developed acute kidney injury and progressive obstructive uropathy, including bilateral hydronephrosis and diffuse irregular bladder wall thickening, whereas the primary gastric lesion remained stable. Routine urinalysis revealed no such abnormalities. Cystoscopy revealed patchy follicular mucosal elevations with non-visualization of both ureteral orifices. Ureteral stent placement failed, and bilateral percutaneous nephrostomy was performed, resulting in partial recovery of the renal function. Bladder biopsy confirmed a metastatic poorly differentiated adenocarcinoma with signet-ring cell features. Immunohistochemistry was positive for caudal-type homeobox 2 (CDX2), cytokeratin 20 (CK20) and Villin, consistent with gastric origin. Despite urinary diversion, the disease progressed rapidly, leading to multiple organ failure and death in the patient. This case highlights the importance of early cystoscopy and targeted biopsy in patients receiving immunotherapy who develop new urinary symptoms or atypical urinary tract imaging findings to distinguish metastatic disease from immunotherapy-related toxicity.