APPENDIX (ICD‐O‐3 C18.1)

Rules for Classification

There should be histological confirmation of the disease and separation of carcinomas into mucinous and non‐mucinous adenocarcinomas. Goblet cell adenocarcinomas are classified according to the carcinoma scheme. Grading is of particular importance for mucinous tumours.

Anatomical Site

Appendix (C18.1)

Regional Lymph Nodes

The ileocolic are the regional lymph nodes (Fig. 162).

Carcinoma

TNM Clinical Classification

T – Primary Tumour

TX	Primary tumour cannot be assessed
T0	No evidence of primary tumour
Tis	Carcinoma in situ: intraepithelial or invasion of lamina propria¹
Tis (LAMN)	Low‐grade appendiceal mucinous neoplasm confined to the appendix (defined as involvement by acellular mucin or mucinous epithelium that may extend into muscularis propria)
T1	Tumour invades submucosa (Fig. 163)
T2	Tumour invades muscularis propria (Fig. 164)
T3	Tumour invades subserosa or mesoappendix (Fig. 165)
T4	Tumour perforates visceral peritoneum, including mucinous peritoneal tumour within the right lower quadrant and/or directly invades other organs or structures^2,^3,⁴ (Figs. 166, 167)
	T4a	Tumour perforates visceral peritoneum, including mucinous peritoneal tumour within the right lower quadrant (Fig. 166)
	T4b	Tumour directly invades other organs or structures (Figs. 167, 168, 169)

Photo depicts the ileocolic are the regional lymph nodes. — **Fig. 162**

Schematic illustration of tumour invades submucosa. — **Fig. 163**

Schematic illustration of tumour invades muscularis propria. — **Fig. 164**

Schematic illustration of tumour invades subserosa or mesoappendix. — **Fig. 165**

Schematic illustration of tumour perforates visceral peritoneum, including mucinous
peritoneal tumour within the right lower quadrant. — **Fig. 166**

Notes

¹ Tis includes cancer cells confined within the glandular basement membrane (intraepithelial) or lamina propria (intramucosal) with no extension through muscularis mucosae into submucosa.

² Direct invasion in T4 includes invasion of other intestinal segments by way of the serosa, e.g., invasion of ileum.

³ Tumour that is adherent to other organs or structures, macroscopically, is classified as T4b. However, if no tumour is present in the adhesion, microscopically, the classification should be pT1–3.

⁴ LAMN with involvement of the subserosa or the serosal surface should be classified as T3 or T4a respectively.

N – Regional Lymph Nodes

NX	Regional lymph nodes cannot be assessed
N0	No regional lymph node metastasis
N1	Metastasis in 1 to 3 regional lymph nodes (Fig. 168)
	N1a	Metastases in 1 regional lymph node
	N1b	Metastases in 2 to 3 regional lymph nodes
	N1c	Tumour deposit(s), i.e. satellites,^* in the subserosa, or in non‐peritonealized pericolic or perirectal soft tissue without regional lymph node metastasis
N2	Metastasis in 4 or more regional lymph nodes (Fig. 169)

Note

^* Tumour deposits (satellites) are discrete macroscopic or microscopic nodules of cancer in the pericolorectal adipose tissue’s lymph drainage area of a primary carcinoma that are discontinuous from the primary and without histological evidence of residual lymph node or identifiable vascular or neural structures. If a vessel wall is identifiable on H&E, elastic or other stains, it should be classified as venous invasion (V1/2) or lymphatic invasion (L1). Similarly, if neural structures are identifiable, the lesion should be classified as perineural invasion (Pn1).

M – Distant Metastasis

M0	No distant metastasis
M1	Distant metastasis
	M1a	Intraperitoneal acellular mucin only
	M1bM1c	Intraperitoneal metastasis onlyNon‐peritoneal metastasis

pTNM Pathological Classification

The pT and pN categories correspond to the T and N categories.

pM1	Distant metastasis microscopically confirmed

Note

pM0 and pMX are not valid categories.

pN0

Histological examination of a regional lymphadenectomy specimen will ordinarily include 12 or more lymph nodes. If the lymph nodes are negative, but the number ordinarily examined is not met, classify as pN0.