From radiation osteitis to osteoradionecrosis: incidence and MR morphology of radiation-induced sacral pathologies following pelvic radiotherapy.

OBJECTIVE: 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.
MATERIAL AND METHODS: 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.
RESULTS: 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.
CONCLUSION: 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. KEY POINTS: • Radiation-induced sacral bone marrow changes appear frequently (17.6 %) following pelvic radiotherapy. • Insufficiency fractures are common late effects (43 %). • Radiation osteitis develops early (4 mo), with recovery between 16.5 and 39.5 months. • Definite radiological osteoradionecrosis is proportionately rare (3 %). • A 3-stage classification system simplifies and standardizes the morphological disease staging.