The mass is a soft tissue mass of the pelvis which is somewhat inhomogeneous, eroding the sacrum, enlarging multiple neural foramen and surrounding the thecal sac.

There is no fluid component.    

 The primary consideration is such a lesion should be a neurogenic tumor.

Due to the young age of the patient, neuroblastoma must be a prime consideration.

The spinal canal erosion and smooth contours of the lesion would cause one to consider more benign lesions such as ganglioneuroma, ganglioneuroblastoma and even plexiform neurofibroma(in a patient with neurofibromatosis.)    

Other pelvic neoplasms which occur in children and invade the spinal canal include lymphoma and rhabdomyosarcoma.    

 Although sacrococcygeal teratomas are a prime consideration in pelvic masses in young children, this mass does not arise in the coccyx.

Another pelvic mass which is excluded is chordoma as the epicenter of the mass is not in the bone.