Solution: Transitional cell carcinoma arising in a bladder diverticulum. This is just an interesting case of a bladder diverticulum that developed a transitional cell carcinoma. You can see the enhancing mass in the diverticulum on the earlier image and on the delayed images you can see that the diverticulum fills with excreted contrast, and the filling defect related to the mass is still there. The stasis that occurs in a bladder diverticulum will make this portion of the urothelium more susceptible to forming transitional cell carcinoma, so don\'t forget to look there.