What is the differential diagnosis?
Metastases, lymphoma or myeloma/plasmacytoma would be the top three.
What would be a useful next test?
Nuclear medicine bone scan - if multiple then differential is between metastatic disease or multiple myeloma. A skeletal survey would also be a reasonable choice.
Lytic destructive lesion of the right iliac wing extending to the sacroiliac joint and the right sacral ala. Associated soft tissue mass.