FIGURE 116.22 Ovarian dermoids Note calcification (arrowhead ) in superior aspect of a large pelvic mass in a 12-year-old girl Malignant Intra-Abdominal Masses About 50% of the solid malignant tumors seen in children occur within the abdominal cavity Most solid masses occur in the retroperitoneum The most common is neuroblastoma, followed by Wilms tumor and rhabdomyosarcoma Other unusual tumors, such as embryonal cell carcinomas (yolk sac tumor) and lymphosarcoma, also occur in young children (see Chapter 98 Oncologic Emergencies ) As with most malignant tumors, early diagnosis and treatment provide the best prospects for a cure Patients with newly diagnosed abdominal tumors generally require admission for coordinated approach by the surgeon and oncologists Neuroblastoma Neuroblastoma most often occurs as a tumor arising from the adrenal gland, but it can develop anywhere along the sympathetic chain or in the pelvis It can grow extensively, often crossing the midline of the abdomen and enveloping key vascular and visceral structures The best cure rates are generally in children who