AJCC 8th Edition: Colorectal Cancer

In the 8th edition of the American Joint Committee on Cancer (AJCC) staging manual, the chapter on colorectal cancer provides an extended description of anatomy, followed by rules for clinical and pathologic classification. Although the basic staging structure has remained the same, there have been many updates and clarifications. One of the significant additions in the discussion of pathologic classification is the detailed description of Tis dysplasia. Penetrating into the lamina propria with possible invasion into the muscularis mucosa, Tis lesions are referred to as intramucosal adenocarcinoma. Penetration through the basement membrane at any gastrointestinal site is considered invasive, but in colorectal cancer, only tumors that invade the submucosa metastasize. Due to the potential for sampling errors, Tis lesions are recorded in the cancer registry, while those with other forms of dysplasia, including high-grade, are not. T categories have not changed. As in AJCC 7, T4 is subdivided into T4a and T4b. Tumors that invade the serosal surface (visceral peritoneum) are referred to as T4a. There is further clarification that tumors with perforation, in which the tumor cells are continuous with the serosal surface through inflammation, are also considered T4a. In areas of the colon and rectum without peritoneal covering, such as posterior aspects of the ascending and descending colon and lower rectum, T4a is not applicable. Tumors that directly invade or adhere to adjacent organs or structures are considered T4b. N categories have also not changed; however, there is an extended discussion of isolated tumor cells in lymph nodes and micrometastases. Isolated tumor cells, which generally consist of up to 20 cells within the subcapsular or marginal sinus of a lymph node, are of controversial prognostic value. According to AJCC 8, they should be designated N0 (or N0i?), but their presence does not elevate disease to stage III. Micrometastases are clusters of 20 or more cells or metastases measuring[ 0.2 mm and \ 2 mm in diameter. A recent meta-analysis demonstrated that micrometastases are associated with poor prognosis. Lymph nodes harboring micrometastases should be considered positive and are denoted N1. AJCC 8 clarifies the interpretation of discrete tumor nodules found within the lymph drainage area of a primary colon or rectal carcinoma. Nodules containing no identifiable lymph node tissue or vascular/neural structures should be considered tumor deposits and designated N1c. The shape, contour, and size of the deposit are not considered in these designations. Tumor deposits within a vessel wall should be considered lymphovascular invasion, with the site-specific designations L? for lymphatic or small-vein invasion and V? for deposits in endothelial-cell-lined spaces with associated red blood cells or smooth muscle cells. If tumor nodules are found around a neural structure, they should be categorized as perineural invasion. N1c elevates disease to stage III, even in the absence of nodal metastases. The number of tumor deposits is recorded with site-specific factors but does not influence the designation (i.e. a patient with one tumor deposit and a patient with four tumor deposits are both staged as N1c). The number of tumor deposits is not added to the number of positive lymph nodes. The M category has been expanded, with the addition of M1c for peritoneal metastases (M1a denotes metastases to one distant site or organ, and M1b denotes metastases to more than one). The rationale for the M1c designation is that patients with peritoneal metastases generally fare worse than those with visceral organ metastases. AJCC 8 provides a fuller discussion of proper colorectal cancer resection, including measurement of the distance Society of Surgical Oncology 2018