13 Anal Canal 13

13 Anal Canal

(The classification applies to carcinomas only; melanomas, carcinoid tumors, and sarcomas are not included.)

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SUMMARY OF CHANGES

• The definitions of TNM and the Stage Groupings for this chapter have not changed from the Fifth Edition.

C21.0 Anus, NOS C21.1 Anal canal C21.2 Cloacogenic zone

C21.8 Ovelapping lesion of rectum, anus, and anal canal

INTRODUCTION

The proximal region of the anus encompasses true mucosa of three different histologic types: glandular, transitional, and squamous (proximal to distal, respectively). Distally, the squamous mucosa merges with the perianal skin (true epidermis). This mucocutaneous junction historically has been called the anal verge or margin. Thus, two distinct categories of tumors arise in the anal region.

Tumors that develop from mucosa (of any of the three types) are termed anal canal cancers, whereas those that arise within skin at or distal to the squamous mucocutaneous junction are termed anal margin tumors. The proximal bound- ary of the anal margin is indistinct on macroscopic examination and, anatom- ically, may vary with the patient’s body habitus. A proximal boundary located 5–6 cm from the squamous mucocutaneous junction applies in the majority of adults.

Anal canal tumors are staged using the classification system described and illustrated herein. Anal margin tumors are biologically comparable to other skin tumors and therefore are classified by the schema presented in Chapter 23, Carcinoma of the Skin. However, the regional nodal drainage (relevant to the N category) of the skin of the anal margin is uniquely specific to this anatomic site, as outlined in this section.

Because the primary management of carcinomas of the anal canal has shifted from surgical resection to nonsurgical treatment, they are typically staged clinically according to the size and extent of the primary tumor. Thus, patients with cancer of the anal canal may be staged at the time of presentation by inspec- tion, palpation and biopsy of the mass, palpation (and biopsy as needed) of regional lymph nodes, and radiologic imaging of the chest, abdomen, and pelvis.

ANATOMY

Primary Site. The anatomic subsites of the anal canal are illustrated in Figure

13.1. The anal canal begins where the rectum enters the puborectalis sling at the

apex of the anal sphincter complex (palpable as the anorectal ring on digital

120 American Joint Committee on Cancer • 2006 exam) and ends at the squamous mucocutaneous junction with the perianal skin. The most proximal aspect of the anal canal is lined by colorectal mucosa, and at the dentate line, a narrow zone of transitional mucosa that is similar to urothelium is variably present. The proximal zone (from the top of the pub- orectalis to the dentate line, including the transitional zone) measures approxi- mately 1–2 cm. In the region of the dentate line, anal glands may be found subadjacent to the mucosa, often extending across the internal sphincter to the intersphincteric plane. A proximal boundary located distal to the dentate line and extending to the mucocutaneous junction is a nonkeratinizing squamous epithelium devoid of skin appendages (hair follicles, apocrine glands, and sweat glands).

Carcinomas that overlap the anorectal junction may be problematic. They should be staged as rectal tumors if their epicenter is located more than 2 cm proximal to the dentate line and as anal tumors if their epicenter is 2 cm or less from the dentate line. However, extension of low rectal tumors beyond the dentate line implies risk of metastatic spread to the superficial inguinal lymph nodes.

Regional Lymph Nodes. Lymphatic drainage and nodal involvement of anal cancers depend on the location of the primary tumor. Tumors above the dentate line spread primarily to the anorectal, perirectal, and paravertebral nodes, whereas tumors below the dentate line spread to the superficial inguinal nodes.

The regional lymph nodes are as follows (Figure 13.2):

Perirectal Anorectal Perirectal Lateral sacral

Internal iliac (hypogastric)

Levator ani m.

Internal sphincter m.

Dentate line

External sphincter m.

Anal canal C21.1

Anal margin (skin) C44.5

1. Transitional epithelium

2. Squamous epithelium devoid of hair and glands (not skin) 1

2

FIGURE 13.1. Anatomic subsites of the anal canal.

Inguinal Superficial Deep femoral

All other nodal groups represent sites of distant metastasis.

Metastatic Sites. Cancers of the anus may metastasize to any organs, but the liver and lungs are the distal organs that are most frequently involved. Involve- ment of the abdominal cavity is not unusual.

DEFINITIONS Primary Tumor (T)

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

T1 Tumor 2 cm or less in greatest dimension (Figure 13.3)

T2 Tumor more than 2 cm but not more than 5 cm in greatest dimension (Figure 13.4)

T3 Tumor more than 5 cm in greatest dimension (Figure 13.5)

T4 Tumor of any size invades adjacent organ(s), e.g., vagina, urethra, bladder

(Figure 13.6)

Regional Lymph Nodes (N)

NX Regional lymph nodes cannot be assessed N0 No regional lymph node metastasis

N1 Metastasis in perirectal lymph node(s) (Figure 13.7)

N2 Metastasis in unilateral internal iliac and/or inguinal lymph node(s) (Figures 13.8A, B)

N3 Metastasis in perirectal and inguinal lymph nodes and/or bilateral inter- nal iliac and/or inguinal lymph nodes (Figures 13.9A–C)

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Inguinal lymph nodes Internal iliac

nodes

Perirectal nodes

FIGURE 13.2. Regional lymph nodes of the anal canal.

122 American Joint Committee on Cancer • 2006

£2 cm T1

FIGURE 13.4. Two views of T2 showing tumor more than 2 cm but not more than 5 cm in greatest dimension. On the right side of the diagram, the tumor extends above the dentate line.

>2-5 cm

>2-5 cm

T2 T2

FIGURE 13.3. T1 is defined as tumor 2 cm or less in greatest dimension.

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>5 cm T3

FIGURE 13.5. T3 is defined as tumor more than 5 cm in greatest dimension.

T4

FIGURE 13.6. T4 is defined as tumor of any size invading adjacent organ(s), e.g., vagina (as illustrated), urethra, bladder.

Inguinal lymph nodes Internal iliac

nodes

Perirectal

nodes

N1

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Inguinal lymph nodes Internal iliac

nodes

Perirectal nodes

N2 N2

A

Inguinal lymph nodes Internal iliac

nodes

Perirectal nodes N2

B

FIGURE 13.8. A. Two views of N2, which is defined as metastasis in unilateral

internal iliac (left) and/or inguinal lymph node(s) (right). B. N2: metastases in

unilateral internal iliac and inguinal lymph node(s).

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Inguinal lymph nodes Internal iliac

nodes

Perirectal nodes N3

A FIGURE 13.9. A. N3 is defined as metastasis in perirectal and inguinal lymph nodes (as illustrated) and/or bilateral internal iliac and/or inguinal lymph nodes. B. N3:

metastases in bilateral internal iliac lymph nodes. C. N3:

metastases in bilateral internal iliac and inguinal lymph nodes.

Inguinal lymph nodes Internal iliac

nodes

Perirectal nodes N3

B

Inguinal lymph Internal iliac nodes

Perirectal

nodes

N3

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

II T2 N0 M0

T3 N0 M0

IIIA T1 N1 M0

T2 N1 M0

T3 N1 M0

T4 N0 M0

IIIB T4 N1 M0

Any T N2 M0

Any T N3 M0

IV Any T Any N M1

NOTE

1. Direct invasion of the rectal wall, perirectal skin, subcutaneous tissue, or the sphincter muscle(s) is not classified as T4.

126 American Joint Committee on Cancer • 2006