Renal Fungal Ball Mimicking Recurrent Urothelial Carcinoma in a Patient Following Radical Cystectomy: an Imaging Challenge

Background:
Renal fungal ball (fungus ball, mycetoma) is a rare cause of an intraluminal filling defect within the renal collecting system, typically occurring in patients with urinary diversion, indwelling catheters, or recurrent urinary tract infection. Because it can present as a discrete mass with associated hydronephrosis, it may closely mimic urothelial carcinoma on imaging, a distinction that becomes especially challenging in patients with known bladder malignancy, where any new filling defect is naturally suspected to represent tumor recurrence.

Case Presentation:
An 80-year-old male with a history of muscle-invasive bladder cancer treated by radical cystectomy with ileal conduit urinary diversion, complicated postoperatively by recurrent E. coli urinary tract infection, was readmitted with fever, chills, decreased appetite, weight loss, nausea, and diarrhea. Laboratory studies showed leukocytosis with neutrophilia, elevated CRP, and metabolic acidosis.
Contrast-enhanced CT demonstrated new left perinephric infiltration.
Renal ultrasound showed the left kidney enlarged relative to the right with moderate hydronephrosis, and hyperechoic content within the lower calyces without posterior acoustic shadowing or internal vascularity on color Doppler, features favoring a fungal ball over a calculus, which typically shadows, or a urothelial tumor, which typically shows vascularity. Given the patient`s oncologic history, recurrent urothelial carcinoma and calculus disease were raised as key differentials. Additional findings included punctate hyperechoic parenchymal foci and left ureteral dilation up to 7 mm. The right kidney was normal.

Conclusion:
This case highlights renal fungal ball as an important mimicker of both recurrent urothelial carcinoma and calculus disease on imaging, particularly in patients with prior bladder malignancy and urinary diversion. Recognizing its characteristic sonographic appearance, an echogenic, non-shadowing, avascular intraluminal mass with associated hydronephrosis, together with correlation of urine culture and clinical context, is essential to avoid unnecessary intervention and ensure timely antifungal therapy.