The pelvic bones are common sites for metastasis and myeloma, but the changes will rarely be located directly adjacent to the SIJ and simulate sacroiliitis. Primary mesenchymal tumors such as osteochondromas, giant cell tumors and sarcomas will usually imply an expansive osseous lesion and/or a soft tissue extension, but the presence of osteoid osteoma may give rise to primarily osseous edema and simulate sacroiliitis if the tumor nidus is not identified. Clinically the symptoms can also simulate those of sacroiliitis with pain especially during rest at night. Supplementary CT will often be necessary for this differential diagnosis as shown beneath.