Midgut Neuroendocrine Tumors: Imaging Assessment for Surgical Resection

Abstract

Midgut neuroendocrine tumors (MNETs) are rare, and the primary tumor is usually small and difficult to visualize at imaging. Patients often present late with extensive liver and nodal metastases and may experience symptoms secondary to the release of active substances by the primary tumor, such as serotonin and its metabolites, which have local and systemic effects. Locally, this causes desmoplasia and vascular encasement and may lead to small bowel obstruction and ischemia, with significant morbidity and mortality. Systemically, the release of active substances into the circulation can cause flushing, diarrhea, and abdominal pain (carcinoid syndrome); these substances can be detected in urine and blood serum and used as markers for diagnosis and treatment follow-up. MNETs retain expression of specific peptide receptors such as somatostatin receptors, which will bind to synthetic somatostatin analogs such as octreotide. This feature is useful for functional imaging of patients with MNETs and for peptide receptor radionuclide therapy using somatostatin analogs. Resection of the primary tumor is advocated, even in patients with extensive metastases, because it may prevent development of local complications, can help control systemic symptoms, and has been shown to confer some survival advantage. Computed tomography and functional imaging are used to identify the primary tumor and assess its resectability. The main factors governing resectability are patient comorbidities (eg, carcinoid heart disease), vascular involvement, and desmoplasia.