What is the primary abnormality on this plain film?
An ill-defined, permeative lesion within the right ileum/acetabulum demonstrating expansion and a wide-zone of transition.
What is the differential now?
The differential is of a malignant lesion within the right hemi-pelvis in a 14 year old girl. Ewing sarcoma is on top of the list, although osteosarcoma should also be considered.
What is the next most helpful test and why?
An MRI of the pelvis will allow further characterisation of the lesion and locate any soft-tissue mass associated with the lesion as well as identifying any other disease within the pelvis.
The right iliac bone demonstrates an ill-defined lytic lesion, with a wide zone of transition. The medial part of the acetabular cortex is poorly seen, potentially representing extension into the joint.
In addition, there is some fullness and increased density of the soft tissues immediately medial to this region within the pelvis, suggesting extension beyond the bone. There are smaller patchy translucencies of the right proximal femur, the inferior pubic ramus and the tuber ischii, as well.