The small bowel is a long and mobile stretch of the digestive system. It rarely develops malignancy and is therefore not a major topic of interest for radiation oncologists. It is, however, radiosensitive and there are several guidelines on contouring and dose limits. The duodenum in particular is an important landmark for pancreatic cancer and upper abdominal nodal volumes.
The small bowel consists of the duodenum, jejenum and ilium. The duodenum is retroperitoneal, and is usually the most posterior of the small bowel loops seen on CT.
In contrast, the jejenum and ilium are suspended within the peritoneal cavity on The Mesentery. This allows them to be highly mobile within the cavity, which is relevant for pelvic treatments as they can often be found resting on the pelvic organs or in the pouches between them.
Duodenum
The duodenum is the shortest and most proximal part of the small bowel. It is retroperitoneal as opposed to the following segments. It is closely related to the pancreas, kidneys, ureters and liver.
Structure
The duodenum is conventionally divided into four parts that run in different vectors.
- The first part runs from the pylorus in a right lateral direction. It is related anteriorly to the peritoneal cavity. The greater and lesser omentum attach the the inferior and superior part of the duodenum. Posteriorly the pancreatoduodenal artery runs superior to inferior and may be involved by duodenal peptic ulcers.
- The second part runs inferiorly and is not in contact with the peritoneum. The common bile duct and pancreatic duct empty into the duodenum's medial wall at the ampulla of Vater. The transverse colon passes over the anterior aspect of this part.
- The third part runs anterior to the aorta and inferior vena cava and posterior to the superior mesenteric artery (a useful landmark)[1].
- The fourth part runs superiorly for a short distance before transforming into the jejenum.
Jejunum and Ileum
This is a continuous length of peritonealised small bowel attached via the mesentry to the posterior abdominal wall. Contouring guidelines can be found with Gay et al[2].
