Bone metastases in midgut neuroendocrine tumors: imaging characteristics, distribution, and risk factors

Bone metastases (BM) affect 10-30% of patients with small intestine neuroendocrine tumors (siNET), but little descriptive data are available regarding their distribution throughout the skeleton or potential risk factors. Aim of the study is to better describe the imaging characteristics, distribution, and risk factors of siNET bone metastases using 18F-FDOPA PET/CT.

All patients with well-differentiated siNET who underwent an 18F-DOPA PET/CT examination in our institution were retrospectively screened between October 2017 and February 2020. Location, SUVmax and CT density of each BM were collected. Sex, metabolic tumor volume (MTV) excluding bone, and metastatic sites other than bone were studied to determine risk factors of BM.

Among the 69 patients included, 11 patients (15.9%) presented BM on 18F-FDOPA (65 metastases). The most frequently involved sites were: thoracic spine, pelvic bones and ribs. About 64% of patients presented multiple BM. On coupled CT scan, 63% of BM were not visible. Using an optimal threshold of 19.2 ml, MTV was an independent predictor of BM (p = 0.004) with a derived sensitivity of 100% [65.0-100.0] and a specificity of 70.9% [57.7-81.2]. Hepatic metastatic involvement was also a significant predictor of BM (p = 0.044).

The development of BM in siNETs appears to be a late event, occurring in patients with a high tumor burden and hepatic involvement. They are often multiple and predominate in the axial skeleton.