| 要約: | Introduction As an aggressive adenocarcinoma phenotype, primary signet ring cell carcinoma of the urinary bladder is an extremely rare variant. The prognosis of metastatic signet ring cell carcinoma of the urinary bladder is extremely poor and the clinical course for its specific pathogenesis remains unelucidated. Case presentation A 64‐year‐old Japanese male patient was diagnosed with invasive urothelial carcinoma with glandular differentiation of a signet ring cell–type with pT4aN0M0, and he was eventually diagnosed with metastatic signet ring cell carcinoma of the urinary bladder. He was initially responsive to systemic combination induction chemotherapy of S‐1 and cisplatin followed by avelumab switch maintenance therapy; however, signet ring cell carcinoma of the urinary bladder relapse occurred in the pathological findings of a biopsy from the right thigh. Immunohistochemical analysis of this specimen identified strong positive staining for nectin‐4 and, following enfortumab–vedotin treatment, the patient showed a good response. Conclusion We thus describe a rare case of metastatic signet ring cell carcinoma of the urinary bladder with nectin‐4 expression diagnosed by a biopsy of a metastatic site.
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