Aims: Neuroendocrine tumors (NETs) are known to overexpress somatostatin receptors (SSTR), which can be visualized by DOTATOC-PET. Reduced SSTR expression on the other hand may indicate dedifferentiation. The aim of this retrospective study was to assess, if conventional PET parameters and textural features (TF) derived from simultaneous PET and MRI including apparent diffusion coefficient (ADC) are associated with the proliferative activity of NETs, potentially allowing non-invasive tumor grading.
Methods: Our institutional database was screened for patients with NET and liver metastases >1 cm. We assessed conventional PET parameters, such as maximum and mean standardized uptake value and more elaborate TF parameters from PET and ADC-MRI (including entropy and homogeneity) from up to the five largest liver lesions per patient. The association of proliferative activity as measured by Ki67-/MIB1-index with the aforementioned parameters was analyzed.
Results: One hundred patients with NET/NECs were eligible with a Ki67-index ranging from <1% to 30%. Overall, 304 liver lesions were analyzed. Conventional PET parameters, entropy, homogeneity of PET and ADC maps differed significantly between G1 and G2 NETs. However, Spearman's test showed a weak association (r = -0.23 to 0.31).
Discussion: In our study cohort, conventional PET parameters and TF of PET and ADC-MRI showed only a weak correlation with Ki67. This indicates that in patients with a Ki67-index of up to 30% TF analysis of combined PET/MRI may not be reliably used for accurate non-invasive tumor grading. On the other hand, DOTATOC-PET might be a suitable staging tool in some higher grade NET/NECs.