Radiation-induced insufficiency fractures of the sacrum: evaluation with MR imaging.

In patients with malignant pelvic tumors, radiation-induced insufficiency fractures of the sacrum may be mistaken for metastasis. Magnetic resonance (MR) imaging was performed in 18 patients with this condition, and the findings were studied retrospectively. The diagnosis was confirmed with at least one additional imaging modality in 16 patients, whereas the other two patients underwent clinical follow-up at 13 and 20 months without evidence of metastasis. In 17 patients, the lesions were consistent with edema and had diffuse low signal intensity on T1-weighted images and diffuse high signal intensity on short-inversion-time inversion-recovery images, thereby simulating metastasis. In one patient, bilateral sacral wing fractures were demonstrated directly as fairly well-defined linear zones. The alae sacri were involved in all patients; 16 patients had bilateral lesions. Awareness of this clinical entity and of the high sensitivity of MR imaging for demonstrating edema caused by the fracture should prevent confusion with metastatic disease and inappropriate treatment.