The TNM system describes a cancerous tumor’s involvement at the primary site (T), as well as spread to regional lymph nodes (N) and distant metastasis (M).
Objectives of Staging System
The TNM staging system was created to describe cancer in a uniform fashion and provide physicians a common language to discuss the disease. This allows for a better understanding of prognosis and accurate patient counseling. Treatment protocols can be devised based on treatment results of similar tumors. Finally, it is useful for stratifying cancers for clinical research and for measuring outcomes to various treatment options.
The American Joint Committee on Cancer (AJCC) was formed in 1959, unifying previous classification systems and providing a foundation for our current staging system. Since then, the AJCC has continued to update a stage classification system for all anatomic sites and subsites. The most recent eighth edition was published in 2017.
Definitions of TNM Categories
T describes the extent of the primary tumor. There are seven categories: TX (primary tumor cannot be assessed), T0 (no evidence of primary tumor), Tis (tumor in situ), T1, T2, T3, and T4. Within the head and neck, the size of the tumor generally defines the T stage. Notable exceptions include vocal fold mobility in larynx cancer. Depth of invasion is now included within the AJCC eighth edition staging system for oral cavity cancer and is considered a separate entity to tumor thickness. Modifiers “a” (less severe) and “b” (more severe) can be used within some T categories to further describe the tumor.
N describes spread of the cancer to regional lymph nodes in five categories: NX (regional lymph nodes cannot be assessed), N0, N1, N2, and N3. This is basically described by size of the lymph nodes and is modified by location of the involved nodes. Evaluation of surgically excised lymph nodes by a pathologist can further affect N stage. Specifically, the eighth edition of the AJCC includes the additional modifier of extranodal extension (ENE) to denote spread of tumor outside of the capsule of a lymph node on histopathologic sectioning.
M describes the presence of distant metastasis, either as MX, M0, or M1. A patient with a metastasis beyond the regional lymph nodes has M1 disease. M0 describes no evidence of metastasis after an appropriate evaluation. MX designates that a metastatic workup has not been completed, but the likelihood of metastasis is low.
Prefix modifiers are also used to further describe the staging. The “c” prefix refers to staging based on clinical examination. The “p” prefix refers to staging based on pathological examination after surgical resection.
The current staging system, including sites and subsites, is summarized in the following tables (detailed descriptions can be found in the updated AJCC manual). These ...