Langerhans cell histiocytosis of the urinary bladder in a patient with bladder cancer previously treated with intravesical Bacillus Calmette–Guérin therapy

We report an extremely rare case of Langerhans cell histiocytosis (LCH) of the urinary bladder. A 68-year-old man presented with gross hematuria. Cystoscopy showed multiple papillary tumors in the urinary bladder, and transurethral resection was performed. Pathological diagnosis was high-grade papillary urothelial carcinoma with lamina propria invasion. The patient received six treatments with intravesical Bacillus Calmette–Guérin (BCG) therapy. Seven months after surgery, follow-up cystoscopy showed three elevated lesions in the urinary bladder, two of which were identified histologically as recurrent urothelial carcinoma. Microscopic examination of the lesion at the anterior wall revealed diffuse infiltration of medium to large histiocytoid cells in the lamina propria, many of which had distorted nuclei and nuclear grooves. Dense eosinophilic infiltration was also observed. Immunohistochemically, the histiocytoid cells were diffusely positive for S-100 and CD1a, but negative for cytokeratin AE1/AE3 and melanosome-associated antigen recognized by HMB-45. Based on the histological and immunohistochemical features, we diagnosed the lesion as LCH of the urinary bladder. There was no evidence of recurrence of either bladder cancer or LCH after an 18-month follow-up. To avoid misdiagnosis, urologists and pathologists should be aware that LCH may develop in the urinary bladder after intravesical BCG therapy for bladder cancer.