Diagnosis by Resolution: Ultrasound Follow-Up of Neonatal Polypoid Cystitis Mimicking Botryoid Rhabdomyosarcoma.

Bladder masses in neonates are rare and often raise concern for rhabdomyosarcoma; benign inflammatory conditions may mimic malignant diseases on ultrasound. A 16-day-old male presented with hematuria and blood clots, no clinical symptoms. Urinalysis showed pyuria, and urine culture was positive for Escherichia coli. Ultrasound revealed bladder-wall thickening and an 8-mm polypoid lesion, suspicious for botryoid-rhabdomyosarcoma. Given the patient's condition, a conservative approach with antibiotics and ultrasound follow-up was adopted. At 12 days, ultrasound demonstrated complete bladder normalization. In selected stable neonates, short-interval US follow-up may support diagnosis through resolution, helping to distinguish inflammatory conditions from malignancy. This approach may reduce unnecessary procedures.