The purpose of this study is to investigate the utility of contrast-enhanced magnetic resonance imaging (CE-MRI), diffusion-weighted MRI (DW-MRI), and ⁶⁸Ga-DOTATOC positron emission tomography/computer tomography (⁶⁸Ga-DOTATOC PET/CT) in the assessment of response to loco-regional peptide receptor radiotherapy (PRRT) with ⁹⁰Y-/¹⁷⁷Lu-DOTATOC in patients with hepatic metastases from gastro-entero-pancreatic neuroendocrine tumors (GEP-NET).

CE-MRI, DW-MRI, and ⁶⁸Ga-DOTATOC-PET/CT images were acquired before and 3 months after one to two cycles of intra-arterial ⁹⁰Y-/¹⁷⁷Lu-DOTATOC therapy in 14 patients (nine female, five male; mean age, 54 ± 9 years; range, 41–69 years) with hepatic metastases from GEP-NET. A total of 38 liver metastases were defined as target lesions for which the longest diameter, mean apparent diffusion coefficient (ADC_mean) and maximum standardized uptake value (SUV_max) were assessed. Based on changes in size on follow-up imaging, target lesions were classified as responding (RL) or nonresponding (NRL). Relative changes in tumor size, ADC_mean, and SUV_max were compared between the two subgroups.

A total of 27 responding and 11 nonresponding lesions were successfully evaluated. Mean ADC_mean increased significantly in RL (p = 0.011) as well as NRL (p = 0.025). A significant correlation was found between baseline ADC_mean and both the percent ADC_mean change (p = 0.033) and decrease in lesion size after therapy (diameter p = 0.006; volume p = 0.002). SUV_max of RL declined significantly by 24.1 % (p = 0.014) and remained nearly unchanged in NRL. The change of SUV_max correlated significantly with the pretreatment SUV_max (p < 0.001) and the change in lesion diameter (p = 0.009). NRL with an ADC_mean change >0.31 × 10⁻³ mm²/s on first follow-up imaging showed a decrease in size in the long-term course.

These results suggest that both DW-MRI and DOTATOC-PET imaging provide potential biomarkers for early assessment of treatment and stratification of therapy response, but that DW-MRI should be interpreted only in combination with SSTR expression and morphologic changes.