[Radiation-induced osteonecrosis of the pelvic bones vs. bone metastases--a difficult differential diagnosis].

Background: Since the introduction of megavoltage radiation therapy radiation osteitis has become a rare event and may be easily mistaken for bone metastases. A case of radiation osteitis is reported and diagnostic features are discussed.

Case report: A 70 year-old female patient underwent rectum resection for rectum cancer and was given standard adjuvant therapy consisting of irradiation of the tumor site and lymph nodes in the pelvis with 18 MeV photons, boxfield technique up to 50.4 Gy and chemotherapy with 5-FU. Eight months later she complained of severe lower back pain. Plain radiographs and CT revealed osteolytic lesions in the ileosacral joints and os sacrum which appeared as circumscript areas of signal loss in MRI (T1-weighted sequence). A soft tissue mass was not detected. CT-guided biopsy excluded bone metastases.

Conclusion: Characteristic features of radiation osteitis are spongiosa destructions initially in weight-bearing bones within the radiation field, namely the ileosacral joints, and the lack of pathologic soft tissue mass. False treatment, i.e. radiation for bone metastases, should be avoidable.