Simple diagrammatic approach to delineate duodenum on a radiotherapy planning CT scan

Abstract

In recent years, there has been increasing application of intensity-modulated radiotherapy and stereotactic body radiotherapy for the treatment of abdominal malignancies (stomach, pancreas, liver, spinal metastases). This warrants accurate delineation of organs at risk, especially the duodenum. The tortuous and curvy anatomy of duodenum often indistinguishable from adjoining organs is a practical challenge. Radiation Therapy Oncology Group (RTOG) has already published upper abdominal normal structure contouring guidelines to ease the delineation process. This pictorial essay following the RTOG guideline elaborates the step-by-step identification of the different parts of duodenum in relation to the adjoining important structures.

Figure 1.

Axial CT slices showing liver, stomach, spleen and aorta and inferior vena cava. Duodenum yet to begin.

Figure 2.

Stomach has narrowed down into pylorus and lumbar vertebrae are yet to begin. The approximate next slice would begin as the duodenum's first part.

Figure 3.

The first part like a bulb (duodenal cap) can be seen even when part of pyloric stomach continues. It is a part above lumbar (L1) vertebrae and always to the right.

Figure 4.

Approximate end of the duodenum first part (D1) where superiorly pylorus and towards left body of pancreas appears. The D1 takes a sharp vertical turn almost like a C-loop, and this is where the second part is about to begin. It is between the right and left lobes of liver, and both kidneys can be appreciated.

Figure 5.

The duodenum second part continues and pancreatic head almost overlaps it on the left side, also inferior vena cava is seen in close proximity. Another landmark is both kidney's corticomedullary structures identified.

Figure 6.

The duodenum first part (D1) is almost over and the duodenum second part (D2) continues. Important is liver lobes are about to end. Body of stomach, D1 and D2 are completely covering pancreatic head.

Figure 7.

The duodenum second part seen as vertical roundish structure with pancreas on upper medial aspect.

Figure 8.

The duodenum second part starts moving upward and medial turn and the duodenum fourth part (D4) can be seen. D4 remains very difficult to identify and standard guideline being inferior to the pancreatic body.

Figure 9.

With aorta exactly in midline, liver almost disappearing, the duodenum second part (D2) and the duodenum fourth part (D4) are distinctively identified. D2 sandwiched between the right kidney and remaining parts of pancreas while D4 seen as a hollow structure superolateral to the aorta.

Figure 10.

With the duodenum fourth part being a very small segment (around 2–2.5 cm, on a 3-mm CT cuts for sections), the further downward part would be the duodenum third part. It would be superolateral to the aorta.

Figure 11.

The duodenum second part (D2) becomes more vertical while the duodenum third part (D3) remains in midline in close contact to the aorta. The fat plane almost disappears between D2 and D3. Further CT slices would have D3 alone.

Figure 12.

The duodenum third part stays in front of both the inferior vena cava and aorta and occupies midline.

Figure 13.

The duodenum third part almost towards the end but remains in midline only, anterior to the inferior vena cava and aorta.

Figure 14.

(a, b) Coronal view of all four parts of the duodenum.

Figure 15.

(a, b) Sagittal view of all four parts of the duodenum.