[en] To investigate the incidence rate, time-to-onset and recovery, MRI morphology and occurrence of insufficiency fractures in radiation-induced changes in the sacrum following pelvic radiotherapy. 410 patients with pelvic malignancies treated with radiotherapy were reviewed. Follow-up was 1-124 months (mean 22 months). Serial MRI (average four studies/patient) were analysed using a new semi-quantitative score (Radiation-Induced Sacral Changes=RISC). A size category (I/II/III), a type category for MR signal morphologies (a/b/c) and sacral insufficiency fractures (+/-) were applied. Seventy-two patients (17.6 %) were found to have new pathological signal changes. Radiation osteitis was documented in 83.3 % (60/72, RISC stage a + b), and definite osteonecrosis (stage c) in 12 patients (16.7 %, 12/72). Thirty-one patients (43.1 %) had sacral insufficiency fractures. Initial bone marrow signal changes were found 1-35 months (median 4 months) after radiotherapy. The maximum manifestation of radiation-induced signal changes occurred after 1-35 months (mean 11 months). Fifty-six cases (77.8 %) showed a significant signal recovery within 16.5 months. Radiation-induced bone marrow changes appear with a high incidence at the sacrum with an early onset and frequent recovery. The majority presented a pattern of radiation osteitis, whereas osteoradionecrosis was proportionately rare. (orig.)