Objective To discuss the imaging characteristics of pelvic insufficiency fracture (PIF) in patients with malignant tumors who underwent ¹⁸F-fluorodeoxyglucose (FDG) PET/MRI.

Methods A retrospective cohort study was conducted on the analysis of clinical and imaging data of 44 patients with PIF confirmed by clinical follow-up, who underwent ¹⁸F-FDG PET/MRI examination at Renmin Hospital of Wuhan University between March 2018 and December 2023. The cohort included 8 males and 36 females, with a mean age of (63.3±8.3) years. Clinical data primarily included gender, age, menopausal status of female patients, presence or absence of significant clinical symptoms, history of malignant tumors and clinical staging, history of pelvic radiotherapy, and bone mineral density. Imaging data comprised MRI features, such as lesion location, fracture line, marrow edema, signal changes and morphological characteristics of lesions, enhancement features, and PET features, such as the degree, distribution, and pattern of ¹⁸F-FDG uptake at lesion location.

Results Among the 44 patients, 34 (77.3%) had received radiotherapy for pelvic malignancies, and most of them were elderly (29/44, 65.9%). Postmenopausal women with osteoporosis were more common (35/44, 79.5%). Most patients presented with no significant clinical symptoms (32/44, 72.7%); only 12 patients (12/44, 27.3%) presented with discomfont such as lumbosacral or hip pain. Six patients (6/44, 13.6%) had no history of pelvic malignancy. Involvement of the sacrum and the lower surface of the sacroiliac joint (including the sacral ala and auricular surface of the ilium) is the most common location of PIF lesions (73/111, 65.8%). On PET/MRI, the main imaging findings included strip-like or patchy areas demonstrating T1 weighted imaging (T1WI) hypointensity and T2 weighted imaging hyperintensity within the sacral ala, sacral vertebral bodies, and ilium. The lesions exhibited ill-defined margins, appeared hyperintense on fat-suppressed sequences, and showed enhancement on post-contrast images. Varying degrees of bone marrow edema were observed in the surrounding soft tissues, and some lesions showed "serrated" T1WI low signal fracture line shadows. Notably, no osteolytic destruction or soft tissue mass formation was evident. In the involved regions, ¹⁸F-FDG uptake mildly or moderately increased, with a maximum standardized uptake value of 2.7±1.1 (range: 1.0–5.7). The lesion areas of increased ¹⁸F-FDG uptake were predominantly oriented parallel to the sacroiliac joints.

Conclusions ¹⁸F-FDG PET/MRI can accurately identify the lesion location, characterize ¹⁸F-FDG uptake features, and provide multiparametric MRI information of PIF in patients with malignant tumor. This integrated approach facilitates the diagnosis of PIF and its differentiation from bone metastases.