Case Report: Solitary right thigh skin metastasis of bladder cancer misdiagnosed as erysipelas with literature review

Background Cutaneous metastasis from bladder carcinoma is an uncommon clinical manifestation and may be misdiagnosed as a benign inflammatory dermatosis, such as erysipelas, causing diagnostic delay and missed opportunities for timely systemic treatment. Case presentation We report a case of a 70-year-old man with a history of muscle-invasive bladder carcinoma treated with radical cystectomy and bilateral cutaneous ureterostomies, who presented 6 years later with a maculopapular rash on his right thigh. The lesion was initially misdiagnosed and treated as erysipelas for over 2 months without improvement. A subsequent skin biopsy confirmed metastatic urothelial carcinoma, with immunohistochemistry positive for GATA-3, CK7, and CK20. The tumor also exhibited Human epidermal growth factor receptor 2 (HER2) (2+) expression and high Programmed death-ligand 1 (PD-L1) (Combined Positive Score (CPS) = 20). Based on prior intolerance to cisplatin-based chemotherapy and the tumor biomarker profile, combination therapy with disitamab vedotin and tislelizumab was initiated. This regimen led to complete resolution of pain by cycle 6, and achieved complete clinical and radiological remission of the skin metastasis after 18 cycles of treatment. However, the patient later developed grade 3 immune-related adrenal insufficiency, necessitating hormone replacement and permanent discontinuation of immunotherapy. Conclusion Persistent inflammatory-appearing skin lesions in patients with a history of bladder cancer should prompt early biopsy, particularly when they are refractory to antibiotic therapy. Molecular profiling may help identify actionable targets and guide systemic treatment. This case highlights the diagnostic challenge of erysipelas-like cutaneous metastasis and suggests the potential value of biomarker-guided HER2-directed antibody–drug conjugate plus Programmed cell death protein 1 (PD-1) inhibitor therapy in selected patients with metastatic urothelial carcinoma, while emphasizing the need for careful monitoring of immune-related toxicity.