Incidentally discovered osseous metaplasia within high-grade urothelial carcinoma of the bladder.

A 66-year-old male presented with gross hematuria and acute renal failure secondary to bilateral ureteral obstruction. Further work-up revealed muscle invasive urothelial carcinoma. Pathologic examination following radical cystoprostatectomy revealed high grade urothelial carcinoma with focal tumor-associated stromal osseous metaplasia. Reactive bone formation within urothelial carcinoma is a very rare clinical entity. Although typically benign, the presence of mature bone elements warrants thorough examination for sarcomatoid components.