TNM Staging System for Mesothelioma

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James Stevenson, M.D. Thoracic Medical Oncologist

The TNM staging system classifies the advancement of cancer, using stages 1 – 4. The TNM system is commonly used to stage pleural mesothelioma. TNM staging assesses tumor extent, lymph node involvement and metastasis. Doctors use the staging system to help estimate a patient’s prognosis and treatment options.

01. TNM Staging System Overview

What Is the Tumor Node Metastasis (TNM) Staging System?

The TNM staging system is a method of categorizing how advanced a patient’s cancer is. It is commonly used for pleural mesothelioma. The TNM system is also used to stage several other cancers, including lung cancer and breast cancer.

The TNM classification system uses three components to identify cancer stage:

  • Tumor extent (T): The location and size of the primary tumor site
  • Lymph node involvement (N): Whether the cancer has spread to nearby lymph nodes
  • Metastasis (M): Whether the cancer is localized or has spread to distant areas of the body

A doctor scores each component and assigns a value. A combination of component scores establishes the mesothelioma stage, which is a number between 1 and 4.

The National Comprehensive Cancer Network (NCCN) recommends using the TNM staging system developed by the International Mesothelioma Interest Group (IMIG). The American Joint Committee on Cancer (AJCC) also approves the system.

Other Mesothelioma Cancer Staging Systems

In 1994, the International Mesothelioma Interest Group (IMIG) published the first widely accepted TNM classification for malignant pleural mesothelioma. Today, doctors generally use the TNM system, but used other systems in the past.

Older staging systems for mesothelioma are:

  • The Butchart staging system
  • The Brigham staging system

In some cases, doctors use a modified TNM system to stage peritoneal mesothelioma. More commonly, the Peritoneal Cancer Index (PCI) is used to stage peritoneal mesothelioma.

Peritoneal Cancer Index (PCI) vs. TNM

The Peritoneal Cancer Index (PCI) is a system for staging cancers. It is used for many abdominal cancers, including peritoneal mesothelioma.

The PCI scores cancers on a scale from 1 to 39, with the higher the number identifying a more advanced cancer. The system separates the abdomen into 13 different regions and scores each region from 1 to 3. The combined score determines the overall stage.

02. Understanding TNM Classification

How Doctors Use the TNM System to Classify Cancer

Doctors use the TNM system to classify cancer spread and involvement of other organs. The letters T, N and M stand for tumor extent, lymph node involvement and metastasis. Doctors assess these factors and assign a score to each. The combined scores indicate a patient’s overall cancer stage.

Clinicians may score each factor using physical exams, imaging tests and biopsy results. This is called clinical staging. Doctors often tell mesothelioma patients their clinical stage as part of a diagnosis.

In some cases, doctors may update staging information after surgical removal of tumors. This is called surgical staging.

Surgery allows physicians to more directly visualize and assess tumor size and spread. Therefore, the surgical stage may differ slightly from the clinical stage.

Classifying Tumor Extent (T)

Doctors assign a designation of T1, T2, T3 or T4. The designation depends on the size of the primary tumor site and the extent of its spread to nearby tissues.

Pleural Mesothelioma Tumor Classifications

Tx: At this classification, doctors cannot determine the original, or primary, tumor.

T0: There is no evidence of the primary tumor.

T1: The tumors are located on either the left or right side of the outer layer of the lining around the lung, called the parietal pleura. Tumors may also be on the inside of the lining around the lung, called the visceral pleura.

T2: Tumors are located on both layers of the lining around the lung (the pleura) on one side of the body. Tumors are also found in at least one of the following areas:

  • Covering the entire outer surface of the visceral pleura (the inside layer of the lining around the lung)
  • On the diaphragm muscle
  • Within the lung tissue

T3: Tumors are located on both layers of the pleura on one side of the body. Tumors are also in at least one of the following places:

  • Between the parietal pleura and the ribs (endothoracic fascia)
  • Extending into the fatty tissue in the space between the lungs (the mediastinum)
  • In the chest wall (but surgically removable)
  • In or on the lining around the heart (pericardium), but not extending all the way through it

T4: Tumors are located on both layers of the pleura on one side of the body. The patient also has at least one of the following:

  • Multiple tumors within the soft tissues of the chest wall that cannot be surgically removed
  • Tumors in the heart, trachea or esophagus
  • Tumors on the left and right sides of the pleura
  • Tumors in or on the ribs
  • Tumors in the spine or the nerves in the shoulder (brachial plexus)
  • Tumors extending through all layers of the pericardium
  • Tumors in the heart muscle

Although tumor size and extent may provide an indication of prognosis, it does not classify cancer stage on its own. It must be considered in combination with lymph node involvement and metastasis to identify a patient’s TNM cancer stage.

Classifying Lymph Node Involvement (N)

Lymph node involvement is the spread of cancerous cells from the primary tumor site to nearby lymph nodes.

In early stages, cancer has either not spread to the lymph nodes at all or has only spread to nearby lymph nodes. In later stages, distant lymph nodes are affected.

Lymph node involvement is graded as NX, N0, N1 or N2.

Lymph Node Classification for Mesothelioma

NX: Regional lymph nodes can’t be assessed.

N0: Cancer cells have not spread to nearby lymph nodes.

N1: The cancer cells have spread to nearby lymph nodes on the same side as the primary tumor, but they have not spread to lymph nodes above the clavicle.

N2: The cancer cells have spread to nearby lymph nodes on the opposite side from the primary tumor or to the lymph nodes above the clavicle on either side of the body.

Lymph node involvement is an important factor for staging mesothelioma. If cancer cells spread to the lymph nodes, it increases the likelihood of distant metastasis in the body. As cancer spreads, patients may face more limited treatment options and a less favorable prognosis.

Classifying Metastasis (M)

Cancer metastasis is the spread of cancer from its origin. Metastasis can lead to tumors growing beyond the primary tumor site in distant areas of the body.

Mesothelioma metastasis plays an important role in TNM classification. It is scored as either M0 or M1.

Metastasis Classification for Mesothelioma

M0: This classification means the cancer has not spread. The cancer is either stage 1, stage 2 or stage 3.

M1: At this classification, the cancer is no longer localized and has spread to distant areas of the body. The cancer is at stage 4 regardless of T or N classification.

In pleural mesothelioma cases, metastasis may indicate cancer has spread to the opposite side of the chest and abdominal lining.

03. Determining Mesothelioma Stage

Determining Mesothelioma Stage Using the TNM System

After each factor in the TNM system is scored, the doctor combines the information to identify the patient’s cancer stage. In the TNM system, physicians classify the mesothelioma cancer as:

  • Stage 1A or 1B
  • Stage 2
  • Stage 3A or 3B
  • Stage 4

In the earlier stages, mesothelioma is considered less advanced and patients may be eligible for more aggressive treatment options, such as surgery. Later stages may signify mesothelioma cells have spread to distant parts of the body, which may limit a patient’s treatment options.

Doctors may use a table similar to the one below.

To understand how doctors use the table, consider fictional Patient One, who has been diagnosed with pleural mesothelioma. Doctors have found tumors on the inside surface of the lining of their right lung. These tumors have grown into the lung and on the diaphragm. Biopsy testing shows some cancer has spread to lymph nodes on the same side as the main tumor, but doctors find no additional spreading. Doctors then classify Patient One with T2 | N1 | M0, which translates to stage 2 pleural mesothelioma.

Tumor, Lymph Nodes and Metastasis by TNM Stage

Stage T N M
1A T1 N0 M0
1B T2, T3 N0 M0
2 T1, T2 N1 M0
3A T3 N1 M0
3B T1-3 N2 M0
4 T4 N0-2 M0
4 Any T Any N M1

Identifying the stage allows mesothelioma doctors to provide patients with a more accurate prognosis. Typically, patients diagnosed with mesothelioma in stage 1 and stage 2 have better survival rates than those diagnosed in stage 3 or stage 4.

What Other Factors Impact Mesothelioma Prognosis Besides Stage?

The TNM system helps doctors assess mesothelioma prognosis. However, additional patient characteristics may impact prognosis and mesothelioma life expectancy.

Factors that may impact mesothelioma survival rate include:

Physicians will assess prognosis based on the TNM stage and unique patient characteristics. These factors also help clinicians determine the best way to move forward with treatment.

04. Common Questions

Common Questions About TNM Staging

What are the different stages of mesothelioma and what do they mean for treatment options?

Mesothelioma has four stages:

Earlier mesothelioma stages may have more aggressive treatment options. For example, a patient with stage 2 pleural mesothelioma may qualify for surgery and a heated chemotherapy treatment called HITHOC. Some late-stage patients may also be eligible for this treatment. If not, many late-stage patients benefit from chemotherapy and immune checkpoint inhibitors.

Can a patient’s cancer stage change over time?

A patient’s cancer stage does not change over time. In fact, doctors usually refer to a cancer using the stage it was assigned at diagnosis, even if the cancer spreads. Sometimes, a doctor may restage a patient’s cancer if it comes back after remission, but this is not common. This type of stage may be referred to as a recurrence or retreatment stage.

Why isn’t there an official staging system for peritoneal mesothelioma?

Peritoneal mesothelioma does not have an official staging system because it is rare and its characteristics do not fit well into a staging system like the TNM system. The TNM system relies on a primary tumor and the cancer’s spread to other parts of the body. Peritoneal mesothelioma has no clear primary tumor and does not usually spread outside the abdomen.

Why do doctors use cancer staging systems?

Doctors use cancer staging systems to help them understand how serious their patients’ cancer is. The stage helps doctors estimate the cancer prognosis, plan the best treatment and identify which clinical trials may benefit their patients.