Something old something new-introduction to the ninth edition TNM classification of lung cancer

The tumour, node, and metastasis (TNM) classification system is fundamental for describing the anatomical extent of lung cancer, encompassing the primary tumour (T), lymph node involvement (N), and distant metastases (M). It is crucial for patient stratification, treatment planning, and survival prognosis. Clinical staging (cTNM) relies on imaging and physical exams, while pathological staging (pTNM) uses surgical specimens. Advances in tumour biology, imaging, surgery, and treatments necessitate periodic updates to ensure the system reflects current knowledge and practices effectively. The International Association for the Study of Lung Cancer (IASLC), alongside the American Joint Committee on Cancer (AJCC) and Union for International Cancer Control (UICC), updates the system through a global, multidisciplinary approach supported by international data and statistical analysis. The ninth edition of the TNM classification, effective January 1, 2025, introduces revisions to the N and M categories while the T categories remain identical. N2 (ipsilateral mediastinal nodal disease) is now divided into N2a (single lymph node station involvement) and N2b (multiple N2 stations involvement). Similarly, M1c category is split into M1c1 (metastases confined to 1 organ system) and M1c2 (metastases involving multiple organ systems). These updates aim to improve the accuracy and utility of lung cancer staging in clinical practice and research.