11 Small Intestine 11

11 Small Intestine

(Lymphomas, carcinoid tumors, and visceral sarcomas are not included.)

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INTRODUCTION

Although the small intestine accounts for one of the largest surface areas in the human body, less than 2% of all malignant tumors of the gastrointestinal tract actually occur in the small bowel. Most cancers occur in the first or second portion of the duodenum and are adenocarcinomas. A variety of other tumor types occur in the small intestine, but approximately 50% of the primary malig- nant tumors are adenocarcinomas. An increased incidence of second malig- nancies has been noted in patients with primary small bowel adenocarcinoma.

At the beginning of the twenty-first century, approximately 5,000 new cases of cancer involving the small intestine are seen annually in the United States. The 1,200 deaths predicted to occur from small intestinal cancer are divided equally between men and women. The patterns of local, regional, and metastatic spread for adenocarcinomas of the small intestine are comparable to those of similar histologic malignancies in other areas of the gastrointestinal tract. The classifi- cation and stage groupings described in this chapter are used for both clinical and pathologic staging of carcinomas of the small bowel and do not apply to other types of malignant small bowel tumors. Although small bowel carcinoid tumors are not traditionally staged using the TNM system, reports from the United States and throughout the world attempt to stage these neuroendocrine tumors using the TNM system.

ANATOMY

Primary Site. This classification applies to carcinomas arising in the duode- num, jejunum, and ileum. These anatomical sites are illustrated in Figure 11.1.

It does not apply to carcinomas arising in the ileocecal valve or to carcinomas that may arise in Meckel’s diverticulum. Carcinomas arising in the ampulla of Vater are staged according to the system described in Chapter 17.

Duodenum. About 25 cm in length, the duodenum extends from the pyloric sphincter of the stomach to the jejunum. It is usually divided anatomically into four parts, with the common bile duct and pancreatic duct opening into the second part at the ampulla of Vater.

C17.0 Duodenum C17.1 Jejunum C17.2 Ileum

C17.8 Overlapping lesion of small intestine

C17.9 Small intestine, NOS

SUMMARY OF CHANGES

• The definitions of TNM and the Stage Grouping for this chapter have not

changed from the Fifth Edition.

Jejunum and Ileum. The jejunum (8 feet in length) and ileum (12 feet in length) extend from the junction with the duodenum proximally to the ileoce- cal valve distally. The division point between the jejunum and the ileum is arbi- trary. As a general rule, the jejunum includes the proximal 40% and the ileum includes the distal 60% of the small intestine, exclusive of the duodenum.

General. The jejunal and ileal portions of the small intestine are supported by the mesentery, which is a fold of the peritoneum containing the blood supply and the regional lymph nodes. The shortest segment, the duodenum, has no real mesentery and is covered by peritoneum only over its anterior surface. The wall of all parts of the small intestine has five layers: mucosal, submucosal, muscu- lar, subserosal, and serosal. A very thin layer of smooth muscle cells, the mus- cularis mucosae, separates the mucosa from the submucosal. The small intestine is entirely ensheathed by peritoneum, except for a narrow strip of bowel that is attached to the mesentery and that part of the duodenum that is located retroperitoneally.

Regional Lymph Nodes. For pN, histologic examination of a regional lym- phadenectomy specimen will ordinarily include a representative number of lymph nodes distributed along the mesenteric vessels extending to the base of the mesentery.

Duodenum:

Duodenal Hepatic

Pancreaticoduodenal Infrapyloric Gastroduodenal Pyloric

Superior mesenteric

Duodenum (C17.0)

Ileum (C17.2)

Jejunum (C17.1)

FIGURE 11.1. Anatomical sites of the small intestine.

Pericholedochal

Regional lymph nodes, NOS Ileum and Jejunum:

Posterior cecal (terminal ileum only) Ileocolic (terminal ileum only) Superior mesenteric

Mesenteric, NOS

Regional lymph nodes, NOS

Metastatic Sites. Cancers of the small intestine can metastasize to most organs, especially the liver, or to the peritoneal surfaces. Involvement of regional lymph nodes and invasion of adjacent structures are most common. Involve- ment of the celiac nodes is considered M1 disease for carcinomas of the duo- denum, jejunum, and ileum. The presence of distant metastases and the presence of residual disease (R) have the most influence on survival.

DEFINITIONS Primary Tumor (T)

TX Primary tumor cannot be assessed T0 No evidence of primary tumor Tis Carcinoma in situ

T1 Tumor invades lamina propria or submucosal (Figure 11.2) T2 Tumor invades muscularis propria (Figure 11.3)

T3 Tumor invades through the muscularis propria into the subserosa (Figure 11.4, top) or into the nonperitonealized perimuscular tissue (mesentery or retroperitoneum) with extension 2 cm or less

(Figures 11.5, top) T4 Tumor perforates the visceral peritoneum (Figure 11.4, bottom) or

directly invades other organs or structures (includes mesentery, or retroperitoneum more than 2 cm [Figure 11.5, bottom], other loops of small intestine [Figure 11.6], and abdominal wall by way of serosa; for duodenum only, invasion of pancreas [Figure 11.7])

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T1

Mucosa

Muscularis mucosae Lamina propria Submucosa Muscularis propria Perimuscular

tissue (mesentery, retroperitoneal

adventitia) Subserosa

Serosa

FIGURE 11.2. Two views of T1: tumor invading lamina propria (left side of figure)

or submucosa (right side of figure).

T2

Muscularis mucosae Lamina propria Submucosa Muscularis propria Subserosa Serosa Adventitia

Mucosa

FIGURE 11.3. T2 is defined as tumor invading muscularis propria.

T4 T3

Serosa Subserosa

Subserosa

Serosa Adventitia

FIGURE 11.4. T3 is defined as tumor invading through the muscularis propria

into the subserosa whereas T4 is defined as tumor that perforates (penetrates) the

visceral peritoneum.

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T3

T4

1 Mucosa 2 Submucosa 3 Muscularis propria

4 Perimuscular tissue (mesentery, retroperitoneal adventitia, or subserosa)

£2 cm

>2 cm 3

2 1

4

3 2 1

4

Adventitia

FIGURE 11.5. T3 is defined as tumor invading into the nonperitonealized perimuscular tissue (mesentery or retroperitoneum) with extension 2 cm or less whereas T4 directly invades other organs or structures (includes, mesentery, or retroperitoneum) more than 2 cm.

T4

FIGURE 11.6. T4 directly invades other organs or structures, including other loops

of small intestine.

Regional Lymph Nodes (N)

NX Regional lymph nodes cannot be assessed N0 No regional lymph node metastasis N1 Regional lymph node metastasis Distant Metastasis (M)

MX Distant metastasis cannot be assessed M0 No distant metastasis

M1 Distant metastasis

STAGE GROUPING

0 Tis N0 M0

I T1 N0 M0

T2 N0 M0

II T3 N0 M0

T4 N0 M0

III Any T N1 M0

IV Any T Any N M1

NOTE

1. The nonperitonealized perimuscular tissue is, for jejunum and ileum, part of the mesentery and, for duodenum in areas where serosa is lacking, part of the retroperitoneum.

T4

FIGURE 11.7. T4 (duodenum only) tumor invades the pancreas.