Bone marrow changes adjacent to the sacroiliac joints after pelvic radiotherapy mimicking metastases on MRI

Radiation-induced changes in the sacroiliac joints mimicking metastases on MR images were evaluated. Twelve patients who received radiotherapy to the pelvic region due to pelvic malignancy were included in the study. All patients had undergone external beam radiation therapy to the pelvic region, and 2 patients received supplementary internal radiation. The changes in the sacroiliac joints were evaluated. Computed-tomography-guided core bone biopsy from the bone marrow was taken from their corresponding MR sections in 5 of the patients. T1 hypointense and T2 hyperintense areas with ill-defined margins in the bone marrow adjacent to the sacroiliac joints were observed in all patients. On bone scintigraphy all the lesions demonstrated increased activity. Other radiological modalities excluded fracture, soft tissue mass, and osseous destruction. Bone biopsies demonstrated peritrabecular fibrosis and inflammatory cell infiltration. Patients receiving radiotherapy to the pelvis may demonstrate T1 hypointense/T2 hyperintense, ill-defined postradiotherapeutic benign changes in the sacroiliac joints. In the absence of any other signs of disease progression and when the imaging pattern is typical, close radiological follow-up should be sufficient to rule out metastases.