The study is designed to investigate the safety and efficacy of central D3 lymphadenectomy in cases of small bowel tumors. Such dissection is under debate; consensus guidelines are vague when it comes to surgical techniques and practice is highly variable.

Cancer of the small intestine is rare. Consensus practice on surgical technique have been difficult to reach, both on lymph node dissection level and on strategic choices according to tumor localization and -type. Evidence is strong for systematic and radical lymphadenectomy for neuroendocrine tumors (NET) and adenocarcinomas. This study includes a series of prospective and consecutive patients operated with central (D3) lymphadenectomy. Key points are preoperative mapping of vascular anatomy to facilitate personalized surgery with radical lymphadenectomy to the mesenterial root, both anterior and posterior to the superior mesenteric vessels. Three different surgical techniques (plus one subgroup) are used, and will be reported on, according to tumor localization and -type. Complications, perioperative morbidity and mortality, operating time and length of hospital stay will be noted, as well as tumor types, number of tumors, mesenteric mass size, resection types and margins, and the anatomical distribution of tumors. Patient outcome during 2- and 5-year follow up will be reported. We will investigate the accuracy of the preoperative vascular anatomy reconstructions and eventual procedure-specific complications. Still, the main outcome measures are the lymph node yield: number of lymph nodes gained in the D2 and D3 areas.

Patients are included in the study "Safe extended (D3) mesenterectomy for small bowel tumors" - REK number 19898. Patients are given an informed consent formula. Inclusion criteria are patients with small bowel tumor(s) of probable or confirmed neoplastic nature capable of consent and without general inoperability properties. The ENETS Neuroendocrine Tumor Centre of Excellence at Oslo University Hospital approves and recommends surgery for patients with NET. All patients shall have their mesenterial vascular anatomy reconstructed in 3D preoperatively, for both arteries and veins. The reconstruction is made by manual segmentation based on biphasic CT scans of their abdomens.

The surgical-oncological aim is the same for both open and minimally invasive access surgery: an intact and continuous specimen with tumor and mesentery in one piece including unbroken and correct anatomical tissue planes. We define the D3 volume to include all lipolymphatic tissue anterior and posterior to the superior mesenteric vessels limited by arterial and venous branches to and from the tumor-bearing segment of bowel. Dissection is made along the blood vessels cranially and caudally. Mesenteric nodal masses and fibrosis and desmoplasia are carefully dissected from the underlying blood vessels to be saved by exposing and dividing the vessel sheets

Inclusion Criteria:

• Patients included must be able to fill in an informed, written consent and to understand its implications and contents and to participate in the follow-up
• Radiologically/scintigraphically/histologically verified extraduodenal tumor(-s) in the small bowel and/or in the mesentery of the small bowel
• No signs of inoperability
• Fit for general anesthetics

Exclusion Criteria:

• Extraduodenal small bowel tumors verified as GIST or benign tumor
• Widespread lymphoma