Well-Differentiated Jejunoileal Neuroendocrine Tumors and Corresponding Liver Metastases: Mesenteric Fibrogenesis and Extramural Vascular Invasion in Tumor Progression

Background: Patients with jejunoileal neuroendocrine tumors (JINETs) can live for many years despite liver metastases. Evidence suggests that tumor heterogeneity is prognostically important, hence the selection of Ki67 hotspots for tumor grading. According to the stepwise metastasis model, clonal hotspots should predominate in the metastases. However, an alternative view holds that the polyclonality of metastases is consistent with origin from genetically heterogeneous clusters of disseminated cells. The shortcomings of Ki67 grading are also being recognized, thus renewing the search for other prognostic parameters. Methods: A 20-year retrospective study that paired JINETs and hepatic metastases was conducted by analyzing them for various parameters. Results: There were 43 patients (mean follow-up of 7.234 years); 14 were dead due to the disease, 22 were alive with the disease, and 7 were alive with no evidence of the disease. Most JI NETs (22/30) were grade 1, eight were grade 2, and none were grade 3. Tumor grades for both the primaries and liver metastases were not prognostic (p-values = 0.1260 and 0.2566, respectively). Seventeen of the 41 JI NETs showed mesenteric fibrogenesis (MF), and 18 had EMVI, with a high level of agreement between these parameters (92.68%) (kappa value 0.85), and both were strongly associated with poor outcomes. Conclusions: JINETs and their liver metastases tend to have low proliferation rates. However, an important mechanism in the metastatic cascade appears to be mesenteric fibrogenesis. It encases vessels, which enhances extramural vascular invasion, thereby conveying clusters of tumor cells to the liver. This supports the polyclonal nature of tumor progression rather than origin from hotspot aberrant clones.