Differential diagnosis of pediatric pelvic mass

• Kidney – pelvic kidney
• Bladder – due to bladder outlet obstruction (posterior urethral valves, genitourinary rhabdomyosarcoma)
• Uterus – hydometrocolpos
• Ovary – neonatal ovarian cyst, ovarian tumor, ovarian torsion
• Supravesical – urachal remnant
• Presacral mass – sacrococcygeal teratoma, anterior meningocele, neuroblastoma, rectal duplication, abscess, chordoma

Approach to the differential diagnosis of pelvic mass:

• Ultrasound should be used to determine the organ of origin
• Always check the renal fossa to exclude ectopic kidney
• If the bladder is distended, consider causes of bladder outlet obstruction
• A cystic pelvic mass in the midline in association with the umbilicus is often an urachal remant
• A cystic pelvic mass in a female that is not in the midline is ovarian in origin until proven otherwise
• A cystic pelvic mass in a female in the midline posterior to the bladder is hydrometrocolpos
• Presacral masses will need further imaging with CT or MRI