The TNM-8 System for Lung Cancer Staging: An Overview
—Among lung cancer care teams, an understanding of the TNM-8 clinical staging system is critical for prognosis and treatment selection. A newly published report lays out the key criteria, special considerations, and some potential future revisions.
In a new report published in Clinics in Chest Medicine, clinicians at several academic medical centers in the U.S. and Canada describe the TNM-8 (tumor, node, and metastasis) staging system and review special considerations, as well as new studies that may warrant attention prior to future revisions by the International Association for the Study of Lung Cancer (IASLC).1
“Accurate clinical staging of lung cancer is essential to stratify patients into groups that determine prognosis, appropriately guide therapy, and predict survival,” the authors wrote. “Thorough knowledge of the TNM-8 staging system, as well as the strengths and weaknesses of discussed imaging modalities, will aid in optimizing lung cancer staging.”1
Here are the main criteria for each category:
T: tumor descriptor
The main criteria for the T stage are tumor size and invasion. Solid tumors, or the solid portion of part- solid tumors, are measured to the nearest 1 mm on the plane with the longest diameter. The highest category level is used if a tumor has more than 1 category.
The category TX is used when the tumor is not visualized by imaging or bronchoscopy but malignant cells are found in sputum or bronchial washings, or when the tumor can’t be assessed. For T0, there’s no evidence of a primary tumor. The category is Tis (carcinoma in situ) for a pure ground-glass nodule measuring 0.6 to 3.0 cm. If a ground-glass nodule is ≤5 mm, it’s considered an atypical adenomatous hyperplasia.
Tumors ≤3 cm in diameter without lobar or central bronchi involvement are category T1. T2 tumors are >3 cm up to 5 cm or have invasion into the main bronchus or visceral pleura, or result in pneumonitis or atelectasis. The T1 and T2 categories are further subdivided by size.
T3 tumors are >5 cm up to 7 cm or tumors that have a satellite nodule or nodules in the same lobe or invasion into the parietal pleura, parietal pericardium, chest wall, or phrenic nerve.
T4 tumors are >7 cm. Smaller tumors are T4 if they invade nearby organs or have 1 or more satellite nodules in a different lobe of the same lung. Magnetic resonance imaging (MRI) can be helpful to image potential invasion into the chest wall or diaphragm if computed tomography (CT) imaging is unclear.
Satellite nodules should have the same morphologic characteristics as the primary tumor. If not, the patient may have multiple primary lung cancers.
N: nodal descriptor
The nodal descriptor describes the involvement of 14 regional lymph nodes. The authors of the current report note that positron emission tomography/CT (PET/CT) is preferable to CT for nodal evaluation because of superior sensitivity, but infectious or inflammatory nodal disease may cause false positives. PET/CT is less sensitive for small lymph nodes ≤1 cm.
Currently, the lymph node location is the only staging component for the N descriptor. The authors suggest that future revisions should consider classification criteria that include the number of lymph nodes involved, as they say other studies have proposed.
The authors also note that emerging techniques, such as mathematical modeling to correct blurry PET images and artificial neural networks to assess PET/CT data, may improve lymph node imaging and evaluation.
For categorization, NX is used when lymph node involvement can’t be determined. When there is no spread to the regional lymph nodes, the category is N0.
The specific lymph nodes involved define the other categories. Spread to ipsilateral peribronchial, ipsilateral hilar, or intrapulmonary lymph nodes is classified as N1. N2 is defined as spread to ipsilateral mediastinal or subcarinal lymph nodes.
M: metastasis descriptor
The metastasis descriptor, M, categorizes metastasis to distant sites. The authors note that PET/CT is beneficial for detecting distant metastases. For example, they cite studies that have shown that PET/CT correctly upstaged more patients than conventional staging, reducing the number of thoracotomies and futile thoracotomies.
PET/CT is also helpful for assessing metastases in the adrenal glands. For bone metastases, PET/CT is more accurate than 99mTc-methylene diphosphonate bone scintigraphy.2
Other techniques may be useful for the detection of liver or brain metastases. MRI is more accurate than CT for differentiating liver metastases from benign findings.3 The authors also say that contrast-enhanced CT for stage II non-small cell lung cancer (NSCLC) and contrast-enhanced MRI for patients with stage III NSCLC or any patient with neurologic symptoms are recommended by National Institute for Health and Care Excellence guidelines when the goal is treatment with curative intent.1,4
If there is no distant metastasis, the category is M0.
The presence of metastasis is M1, with 3 subcategories based on survival data. M1a is used for nodules in a contralateral lobe or pleural or pericardial effusions if the effusions are judged to be related to the tumor. Pleural fluid should be evaluated if imaging for pleural effusion is ambiguous.
A single extrathoracic metastasis in a single organ is category M1b, and ≥2 extrathoracic metastases in a single organ or multiple organs are category M1c.
Approaches when there are multiple pulmonary sites
The IASLC depicted 4 patterns of tumors at multiple pulmonary sites. First, satellite nodules with the same morphologic characteristics as the primary tumor are M1a if in the contralateral lung, T3 if in the same lobe, or T4 if in a different lobe of the same lung, as described in the TNM system.
Second, if ≥2 tumors are detected with different characteristics, they’re considered multiple primary lung cancers and should be staged separately within the TNM staging system. According to the IASLC subcommittee, clinical, imaging, and histopathologic features should be considered when classifying cancers as multiple primary lung cancers.
Third, the subcommittee defines tumors as multifocal ground-glass or lepidic adenocarcinomas when there are multiple subsolid nodules with features of cancer. The solid portion is considered to be the invasive part of the tumor. The solid portions are staged based on the lesion with the highest T descriptor and by the number of lesions. Pure ground-glass nodules <5 mm are common in these patients and thought to be benign. They’re not considered in the staging descriptors.
Finally, some tumors may also have areas of consolidation without central bronchial obstruction. These are usually invasive mucinous adenocarcinomas without metastases. When multiple sites are present, they’re staged as if they were nodules. The category is T3 if present in a single lobe, T4 if found in the same lung, and M1a if present in both lungs.
Some final thoughts
The TNM-8 staging system classifies lung cancer into resectable versus unresectable disease. The authors of the new report explained that tumors staged as T4, N3, or M1 are typically considered unresectable and treated with other therapies. They also pointed out that this isn’t always true.
“It should be noted, however, that at tertiary cancer centers, selected unresectable patients may be surgical candidates under a multidisciplinary approach,” they wrote.1
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