Imaging description

Pelvic insufficiency fractures are a form of stress fracture in which the physiologic load of weight bearing is sufficient to cause pelvic fractures in bone that is weakened by demineralization and decreased elastic resistance. Pelvic radiation is one cause of bone weakening that may lead to insufficiency fracture. At CT, pelvic insufficiency fractures appear as linear sclerotic lesions with or without cortical discontinuity in the sacral body parallel to the sacro-iliac joints. This results in the characteristic H or Honda sign in the sacrum at bone scintigraphy. MRI shows reduced T1 and increased T2 signal intensity. The pubic rami adjacent to the symphysis pubis and the acetabulum can also be affected. PET findings are variable and likely related to timing; that is, in the acute phase FDG uptake may be increased but it may decrease as the fracture heals (Figures 97.1–97.4) [1–4].

Importance

Postradiation pelvic insufficiency fractures are often misdiagnosed as bony metastases because of the combination of a known prior malignancy and the detection of a new bone lesion. Negative consequences of such an erroneous diagnosis include unnecessary biopsy and unwarranted further irradiation or chemotherapy.

Typical clinical scenario

The reported frequency of insufficiency fracture after pelvic radiation varies between publications, likely reflecting selection bias and different methodologies. Despite this variation, it is clearly a common occurrence. In a study of 18 patients with advanced cervical cancer studied by serial MRI before and after radiation therapy, 16 (89%) developed pelvic insufficiency fractures [5]. Fractures typically developed in the first year after treatment and many showed evidence of healing by 30 months. In a larger study of 510 patients undergoing irradiation for cervical cancer, the 5-year cumulative prevalence of pelvic insufficiency fracture was 45%, with a median interval between radiation and diagnosis of 17 months [6].