Objective:

To report a rare case of bladder metastasis from gastric cancer presenting as acute post-renal obstruction during immunotherapy.

Approach:

• Case Presentation: A 35-year-old man with advanced gastric cancer developed acute kidney injury and obstructive uropathy after responding to immunotherapy.
• Diagnostic Procedures: Cystoscopy and bladder biopsy confirmed metastatic poorly differentiated adenocarcinoma with signet-ring cell features.
• Interventions: Bilateral percutaneous nephrostomy was performed after failed ureteral stent placement, leading to partial recovery of renal function.

Key Findings:

• Bladder metastasis from gastric cancer is extremely rare and can be misdiagnosed during immunotherapy.
• Routine urinalysis may not reveal abnormalities despite significant urinary tract issues.
• Early cystoscopy and targeted biopsy are crucial for distinguishing between immunotherapy-related toxicity and metastatic disease.

Interpretation:

This case illustrates the diagnostic challenges in patients receiving immunotherapy who present with new urinary symptoms.

Limitations:

• The rarity of bladder metastasis from gastric cancer limits generalizability.
• The case study design does not allow for broader conclusions about treatment protocols.

Conclusion:

This case highlights the importance of vigilance in diagnosing urinary symptoms in patients undergoing immunotherapy for gastric cancer.