Lung cancer remains one of the most prevalent and fatal malignancies globally, accounting for a significant proportion of cancer-related morbidity and mortality. Accurate staging is critical in determining prognosis, guiding treatment decisions and ensuring a coordinated approach across multidisciplinary teams. The tumour-node-metastasis (TNM) classification system, established by the American Joint Committee on Cancer (AJCC) and the Union for International Cancer Control (UICC), provides a standardised framework for assessing the extent of disease. Since its introduction, the TNM system has undergone multiple revisions to reflect emerging evidence and advances in cancer treatment.
The 9th edition of the TNM classification, which took effect in January 2025, introduces several refinements aimed at improving the precision of lung cancer staging. These updates focus primarily on the classification of nodal (N) and metastatic (M) disease, introducing new subcategories that distinguish between different levels of involvement. These changes carry significant implications for radiologists, who play a crucial role in evaluating tumour extent and communicating findings that influence clinical decision-making. The adoption of the 9th TNM edition ensures more accurate patient stratification, which in turn supports more tailored therapeutic approaches and improved patient outcomes.
Refinements in the Nodal (N) Classification
One of the most significant modifications in the 9th TNM classification is the reorganisation of the N2 category. Previously, N2 encompassed all cases of ipsilateral mediastinal and subcarinal lymph node involvement without further distinction. The updated system introduces two distinct subcategories: N2a, which refers to single-station involvement, and N2b, which denotes metastases in multiple nodal stations. This refinement is based on evidence suggesting that patients with single-station N2 disease have a more favourable prognosis compared to those with multiple-station involvement.
For radiologists, this change underscores the importance of detailed lymph node assessment in staging lung cancer. Advanced imaging modalities such as PET-CT and contrast-enhanced CT play a crucial role in distinguishing between single- and multiple-station nodal involvement. The 9th TNM classification emphasises the need to report the number of affected lymph node stations rather than the total number of involved lymph nodes. This distinction is essential, as the revised classification could result in stage migration for certain patients, potentially altering their treatment options.
From a clinical perspective, the N2a/N2b distinction has direct implications for treatment planning. Patients with single-station N2 disease may now be considered for more aggressive local therapies, including surgery, in conjunction with systemic treatments. This refinement highlights the evolving role of multimodal therapy in lung cancer management, further reinforcing the need for precise radiological evaluation.
Reclassification of Metastatic Disease (M Category)
In addition to changes in nodal staging, the 9th TNM classification introduces an important update to the M1c category, which previously encompassed all cases of multiple extrathoracic metastases. The revised system divides this category into two subgroups: M1c1, which refers to multiple metastases confined to a single organ system, and M1c2, which describes metastases involving multiple organ systems. This distinction is designed to reflect differences in prognosis and response to treatment, as the extent of metastatic dissemination influences therapeutic decision-making.
From a radiological standpoint, these refinements necessitate meticulous evaluation of metastatic burden. PET-CT and MRI remain essential imaging tools for assessing the distribution of metastases and ensuring that findings align with the new classification criteria. The 9th TNM definition considers certain organ systems, such as the skeleton, as a single entity, regardless of whether metastases are localised to a specific bone or widely distributed. This aspect must be carefully considered when interpreting imaging studies and assigning the appropriate M1c1 or M1c2 designation.
These updates in metastatic classification contribute to a more nuanced approach to staging and treatment. Patients with M1c1 disease, for example, may be eligible for targeted interventions aimed at controlling localised metastases, whereas those with M1c2 disease may require more extensive systemic therapy. The increasing complexity of metastatic classification underscores the necessity for radiologists to provide precise and comprehensive reporting, ensuring that oncologists and other specialists have the necessary information to determine the most appropriate treatment strategy.
Impact on Stage Classification and Treatment Strategies
The changes introduced in the 9th TNM classification have a direct impact on overall stage classification, particularly in stages IIA, IIB, IIIA and IIIB. In some cases, tumours that would have been classified as stage IIIA under the previous system are now downstaged to IIB, reflecting a more refined stratification of disease extent. This reorganisation is significant, as it expands potential treatment options for certain patient groups.
The updated classification aligns with advancements in lung cancer treatment, which increasingly incorporate multimodal strategies. The integration of systemic therapies, including immunotherapy and targeted treatments, alongside local interventions such as surgery and radiotherapy, has reshaped the landscape of lung cancer management. The ability to more precisely delineate disease extent allows for improved patient selection for specific treatment approaches, maximising the potential for favourable outcomes.
For radiologists, the implementation of the 9th TNM edition necessitates a thorough understanding of its implications for staging and treatment planning. Accurate interpretation of imaging findings, combined with clear and detailed reporting, is essential to ensure that patients receive the most appropriate and effective care. As treatment paradigms continue to evolve, the role of radiologists in lung cancer management becomes increasingly vital, reinforcing the importance of adapting to the latest classification standards.
The 9th edition of the TNM classification represents a significant step forward in lung cancer staging, introducing refinements that enhance prognostic accuracy and inform treatment decision-making. The reclassification of nodal and metastatic disease ensures a more precise delineation of tumour extent, directly influencing patient management. For radiologists, integrating these updates into routine imaging assessment is crucial for maintaining standardised and effective communication across multidisciplinary teams.
As lung cancer treatment continues to evolve, the TNM system must adapt to reflect emerging clinical evidence and therapeutic advancements. While anatomical staging remains fundamental, non-anatomical factors, such as genetic mutations and biomarker expression, are becoming increasingly relevant in guiding treatment selection. Future updates to the TNM system may incorporate these additional variables, further refining lung cancer classification. In the meantime, radiologists must remain at the forefront of staging accuracy, ensuring that the latest classification updates are effectively implemented to support optimal patient care.
Source: European Radiology
Image Credit: iStock
