Insufficiency fracture of the supra-acetabulum that required differentiation from a pathological fracture secondary to a malignant bone tumor: a case report

BACKGROUND: The supra-acetabulum is a common site for malignant bone tumors, which can be difficult to differentiate from insufficiency fractures. We report a rare case of a stress fracture of the supra-acetabulum that required differentiation from a malignant bone tumor. CASE PRESENTATION: A 74-year-old Japanese man presented to the hospital because of right hip joint pain. X-rays showed no obvious abnormalities. Magnetic resonance imaging showed an abnormality in the right supra-acetabulum, and he was referred to our department. A linear, low-signal region and its surrounding equal signal region were observed at the same site in the T1-weighted image, and a linear low-signal region and high signal region were observed in the surrounding area in the T2-weighted image. On the contrast-enhanced magnetic resonance imaging, the lesion was still unclear and the whole area was gradually enhanced. A computed tomography-guided needle biopsy was performed, but no tumor cells were observed, therefore the lesion was presumed to be a fracture healing. The bone density was 66% for the lumbar spine (young adult mean, L2–4), and blood biochemistry showed an increase in alkaline phosphatase and total type I procollagen N-terminal propeptide. CONCLUSION: This case was diagnosed as an insufficiency fracture of the supra-acetabulum in a male patient with primary osteoporosis by biopsy specimen. Initially, a pathological fracture associated with a malignant lesion was considered. On magnetic resonance imaging, the boundary around the fracture line was unclear and a signal change that was gradually enhanced by gadolinium was observed. This is likely to be bone marrow edema associated with the stress fracture, and we believe this to be a useful finding that may help in differentiating a stress fracture from a pathological fracture secondary to a malignant lesion.