My cytology found a rare urothelial cell fragment and rare granular casts. My urologist and nephrologist say nothing is wrong. Should I still worry?

Cast in urine. These findings suggest minor nephron irritation. Ibuprofen does this. You are at risk.
Rare means few. Your cytology report says rare cells. They do not mean rare like unusual. They mean to say a few cells are seen. It is only describing a number. The english terms are often misunderstood. From what you describe you should not worry at all. Please always talk to your doctor before you leave and bring a family member to help clarify any findings. Hope this helps.